Edited by
Robert A. Freitas, Jr. and William P. Gilbreath

Proceedings of the 1980 NASA/ASEE Summer Study
Held at the University of Santa Clara
June 23-August 29, 1980

NASA Conference Publication 2255

5.1 Introduction

As the cost of fossil-fuel energy continues to escalate and supplies of readily accessible high-grade ores and minerals gradually become depleted, the utilization of nonterrestrial sources of energy and materials and the development of a nonterrestrial industrial capacity become increasingly desirable. The Moon offers plentiful supplies of important minerals and has a number of advantages for manufacturing which make it an attractive candidate factory site compared to Earth. Given the expense and danger associated with the use of human workers in such a remote location, the production environment of a lunar manufacturing facility should be automated to the highest degree feasible. The facility ought also to be flexible, so that its product stream is easily modified by remote control and requires a minimum of human tending. However, sooner or later the factory must exhaust local mineral resources and fall into disrepair or become obsolete or unsuitable for changing human requirements. This will necessitate either replacement or overhaul, again requiring the presence of human construction workers with the associated high costs and physical hazards of such work.

The Replicating Systems Concepts Team proposes that this cycle of repeated construction may possibly be largely eliminated by designing the factory as an automated, multiproduct, remotely controlled, reprogrammable Lunar Manufacturing Facility (LMF) capable of constructing duplicates of itself which would themselves be capable of further replication. Successive new systems need not be exact copies of the original, but could, by remote design and control, be improved, reorganized, or enlarged so as to reflect changing human requirements. A few of the benefits of a replicative growing lunar manufacturing facility (discussed at greater length in secs. 5.4 and 5.5) include:

(I) The process of LMF design will lead to the development of highly sophisticated automated processing and assembly technologies. These could be used on Earth to further enhance human productivity and could lead to the emergence of novel forms of large-scale industrial organization and control.

(2) The self-replicating LMF can augment global industrial production without adding to the burden on Earth's limited energy and natural resources.

(3) An autonomous, growing LMF could, unaided, construct additional production machinery, thus increasing its own output capacity. By replicating, it enlarges these capabilities at an increasing rate since new production machinery as well as machines to make new machines can be constructed.

(4) The initial LMF may be viewed as the first step in a demonstration-development scenario leading to an indefinite process of automated exploration and utilization of nonterrestrial resources. (See fig. 5.1.) Replicating factories should be able to achieve a very general manufacturing capability including such products as space probes, planetary landers, and transportable "seed" factories for siting on the surfaces of other worlds. A major benefit of replicating systems is that they will permit extensive exploration and utilization of space without straining Earth's resources.

Figure 5.1. - Automated space exploration and industrialization using self-replicating systems.

5.1.1 Summary of Chapter Contents

The history of the concept of machine replication is reviewed in section 5.2. This theoretical background is largely a consideration of the work of John von Neumann—in particular, his kinematic and cellular models of automata self-reproduction. Post-von Neumann research is reviewed next, noting particularly the established theoretical capabilities of machines in the realm of general construction, inspection, and repair strategies. Such strategies may prove useful, even vital, to the successful design, realization, and operation of actual replicating systems.

Section 5.3 deals with the engineering feasibility of the concept of self-replicating systems (SRS). An attempt is made to confront two important general problems in creating a lunar replicating factory:

• Given that in theory, machines can construct duplicates of themselves, how might systems designers and engineers identify all functions which must be carried out to achieve machine replication and also develop the technological means by which to implement these functions?

• Given the constraints obtaining in the lunar environment, particularly in terms of the inventory of known kinds and quantities of naturally occurring raw materials and the existing repertoire of materials processing technologies, can all machine functions required both for production and for replication and growth be implemented?

To attack the first of these problems - identification of necessary functions for practical machine replication—the team proposes a specific phased demonstrationdevelopment scenario, described in section 5.3. For the second problem—establishing that machine replication can feasibly take place in the actual lunar environment - a strawman mission concept was employed. In this scenario, a 100-ton initial "seed'~ factory is planted on the Moon with access only to local resources and established materials processing techniques. The initial system should be able to successfully develop into an expanded machine system capable of conducting all functions necessary for autonomous replication, growth, and automated production and manufacturing.

The problem of "closure" is also considered at length in section 5.3. The issue of closure is whether autonomous manufacturing and construction systems can make available to themselves all of the materials, parts, and assembly techniques required for all internal operations. An iterative strategy is presented for detecting and eliminating closure gaps, and for optimizing the resulting augmented system.

Section 5.4 deals with possible applications of the SRS concept. Applications of replication technology include enormous gains in terrestrial industrial productivity (automation and computer-aided design and manufacturing), utilization of Solar System resources, orbital and planetary opportunities, and the possibility of interstellar exploration on a grand scale. Indefinitely large masses can be organized in extraterrestrial environments using self-replicating systems.

Section 5.5 deals with just a few of the many implications of SRS. The advantages of space-based replicathe manufacturing are considered, together with possible political, social, economic, cultural, and psychological consequences of the proposed SRS development program.

Section 5.6 sets forth in some detail how NASA can take action at once toward the achievement of the ultimate goal of establishing a replicating manufacturing facility. Suggested statements of work (SOWs) and a listing of institutions that might undertake the tasks outlined in the work statements are included. A series of specific conclusions and recommendations generated by the Replicating Systems Concepts Team are presented in section 5.7.

5.2 Theoretical Background

The notion of a machine reproducing itself has great intrinsic interest and invariably elicits a considerable range of responses—some directed toward proving the impossibility of the process, others claiming that it can be carried out, but almost all of them indicating an unwillingness to subject the question to a thorough examination. In discussing self-replication by automata it is essential to establish early rather important ground rules for the discussion. According to Kemeny (1955), "1f [by 'reproduction'] we mean the creation of an object like the original out of nothing, then no machine can reproduce - but neither can a human being....The characteristic feature of the reproduction of life is that the living organism can create a new organism like itself out of inert matter surrounding it."

Often it is asserted that only biological organisms can reproduce themselves. Thus, by definition, machines cannot carry out the process. On the other hand, others argue that all living organisms are machines and thus the proof of machine reproduction is the biosphere of Earth. Also, sometimes it is claimed that although machines can produce other machines, they can only produce machines less complex than themselves. This "necessary degeneracy" of the machine construction process implies that a machine can never make a machine as good as itself. An automated assembly line can make an automobile, it is said, but no number of automobiles will ever be able to construct an assembly line.

Another common argument is that for a machine to make a duplicate copy it must employ a description of itself. This description, being a part of the original machine, must itself be described and contained within the original machine, and so on, until it is apparent we are forced into an infinite regress. A variant of this is the contention that a machine not possessing such a description of itself would have to use itself for a description, thus must have the means to perceive itself to obtain the description. But then what about the part of the machine that does the perceiving? It cannot perceive itself, hence could never complete the inspection needed to acquire a complete description. (A simple counter is that the original machine might possess multiple perceiving organs, so that the perceiving could be shared.) Yet another related objection is that for the process to be carried out, the machine must come to "comprehend" itself - at which point it is said to be well known that "the part cannot possibly comprehend the whole." These disputations suggest that there is a very deep-seated resistance to the notion of machines reproducing themselves, as well as an admittedly strong fascination with the concept.

The Hungarian-American mathematician John von Neumann (1966), who first seriously came to grips with the problem of machine reproduction, once noted that it would be easy to make the whole problem go away. One could, for example, make the elementary parts of which the offspring machine was to be composed so complex as to render the-problem of replication trivial. In one example of this considered by the team, a robot required only to insert a fuse in another similar robot to make a duplicate of itself would find "reproduction" very simple (see sec. 5.2.3). As von Neumann also pointed out, it is equally useless to go to the other extreme and try to account for the placement of every atomic particle in the system—one would quickly become mired in incomprehensible detail. Even most lifeforms do not have DNA-encoded instructions for reproduction to this fantastic level of detail—their descriptions are largely at the molecular level.

As will be demonstrated presently, although reproduction may be transparently trivialized or intractably complexified, there appear to be no fundamental inconsistencies or insoluble paradoxes associated with the concept of self-replicating machines.

5.2.1 Von Neutnann's Contributions and Subsequent Research

John von Neumann began studying automata replication because he was interested in very complex machines and their behaviors. The early history of the theory of reproducing machines is basically the history of von Neumann's thinking on the matter, and this is reviewed below.

Von Neumann had a tremendous range of interests—he contributed to the logical foundations of quantum theory, was the co-inventor of the theory of games, and he worked on the Manhattan Project (contributing to the design of the implosion mechanism for the plutonium bomb). It is believed that his participation in the Manhattan Project and the tremendous volume of calculations necessary for bomb design led him into automatic computing. Hearing of the ENIAC computer project at the Moore School of Electrical Engineering at the University of Pennsylvania, von Neumann was fascinated by the potential of a computer very much faster than any of the devices that had previously been produced. In the early 1940s there existed only simple relay machines and analog devices such as the differential analyzer. But the new electronic machines that interested von Neumann promised to be perhaps millions of times faster than relay machines.

So von Neumann immersed himself in the ENIAC project, the first electronic computer program where some actual useful computing was produced. Late in 1945 and early 1946, the first problems that were put on ENIAC are believed to have been calculations involving the feasibility of a hydrogen bomb. Von Neumann, although he remained very much interested in nuclear energy and was appointed a member of the Atomic Energy Commission, became fascinated with the idea of large and complex computing machines. He devised the organization employed today in almost all general purpose computational machines—the so-called von Neumann concept of serial processing storedprogram or the "von Neumann machine." After that work was completed he began thinking seriously about the problems of extremely large machines—their reliability, programming, design, how to understand what they do— and he became involved with the many possible analogies to the complex behavior of living systems.

Von Neumann set for himself the goal of showing what the logical organization of a self-reproducing machine might be. He had in mind a full range of self-replicating machine models which he intended to explore, including the (a) kinematic machine, (b) cellular machine, (c) neurontype machine, (d) continuous machine, and (e) probabilistic machine. As it turned out, he ultimately was only able to produce a very informal description of the kinematic machine. Although he wrote a great deal on the cellular machine, his magt2um opus on the subject was left in the form of unfinished notes at the time of his death. Almost no work was done on the other three kinds of selfreproducing machines. For this reason, only the postulated workings of the kinematic and cellular machines are presented below, with brief comments on the other three types. For an additional review of these two models of reproduction, see Burks (1970).

In dealing with machines that could reproduce, von Neumann concluded that the following characteristics and capabilities should be demonstrable for each:

(1) Logical universality—the ability to function as a general-purpose computing machine able to simulate a universal Turing machine (Turing, 1936). This was necessary because SRS must be able to read instructions to carry out complex computations.

(2) Construction capability — to self-replicate, a machine must be capable of manipulating information, energy, and materials of the same sort of which it itself is composed.

(3) Constructional universality - in parallel to logical universality, constructional universality implies the ability to manufacture any of the finitely sized machines which can be formed from specific kinds of parts, given a finite number of different kinds of parts but an indefinitely large supply of parts of each kind.

(4) Self-reproduction —follows immediately from the above, since the universal constructor must be constructable from the set of manufacturable parts. If the original machine is made of these parts, and it is a constructable machine, and the universal constructor is given a description of itself, it ought to be able to make more copies of itself.

Von Neumann formally demonstrated that his cellular model of reproduction possessed these four properties.

Not much was done on a fifth property also believed to be important—evolution—though there have been some more recent results in this area. If one has a machine, and it makes a machine, which then itself makes a machine, is there any proof that the line of machines can become successively "better" in some fashion — for instance more efficient, or able to do more things? Could they evolve to higher and higher forms? This problem raises issues in learning, adaptation, and so forth, and was left largely untouched by von Neumann.

The kinematic machine. The kinetic machine is the one people hear about most often in connection with von Neumann's work on self-reproducing machines, probably because it received the earliest attention and publicity. John Kemeny (1955) produced a paper for the popular publication Scientifc American detailing this model, and a further description appeared in a paper by von Neumann (1 9s 1).

The notion of kinematic machine self-reproduction was dealt with by von Neumann only informally. The mathematician envisioned a machine residing in a "sea" of spare parts. The machine has a memory tape which instructs it to go through certain mechanical procedures. Using a manipulative appendage and the ability to move around in its environment, the device can assimilate and connect parts. The tape-program first instructs the machine to reach out and pick up a part, then to go through an identification routine to determine whether the part selected is or is not the specific one called for by the instruction tape. If not, the component is thrown back into the "sea" and another is withdrawn for similar testing, and so on, until the correct one is found. Having identified a required part the device searches in like manner for the next, then joins the two together in accordance with instructions.

The machine continues following the instructions to make something, without really understanding what it is doing. When it finishes it has produced a physical duplicate of itself. Still, the second machine does not yet have any instructions so the parent machine copies its own memory tape onto the blank of its offspring. The last instruction on the parent machine's tape is to activate the tape of its progeny .

Von Neumann's logical organization for a kinematic machine is not the only one possible, but probably is the simplest way to achieve machine self-replication. In its logic it is very close to the way living organisms seem to reproduce themselves (Dyson, 1979). One conceptual problem with the model is that the parts involved are supplied free to the machine, and those parts are of a relatively high order. The machine dwells in a universe which supplies precisely the sorts of things it needs as a kinematic device to make a duplicate of itself. This raises the issue of closure, a problem which is discussed and conceptually resolved in section 5.3.

The cellular model. Von Neumann evidently was dissatisfied with his original kinematic model because of its seemingly mathematical inelegance. This model of machine self-reproduction, while qualitatively sound, appeared not easily susceptible to mathematically rigorous treatment and so might not serve to convince a determined skeptic.

Stan Ulam, a Polish-American mathematician who had also worked on the Manhattan Project, suggested to von Neumann that the notion of a self-reproducing machine would be amenable to rigorous treatment if it could be described in a "cell space" format—a geometrical grid or tessellation, regular in all dimensions. Within each cell of this system resides a finite state automaton. These cell automata can only be affected by certain of their neighbors, and only in very specific ways. In the model von Neumann finally conceived, a checkerboard system is employed with an identical finite state automaton in each square (fig. 5.2). In this system, as it evolved with subsequent research, the cell-automata can be in one of 29 possible different states (fig. 5.3). Each automaton can communicate with its four cardinal direction neighbors. The state of a cell-automaton is determined by its own state and by the states of its cardinal direction neighbors.

Figure 5.2.- Finite state automation cellular space.




Figure 5.3.- Twenty-r...~ von Neumann 's cellular automata.

At the beginning of operation, all but a finite number of the cell automata are in a "U" or "unexcitable" state. If a given cell is in the "U" state, and all its neighbors also are in the "U" state, then at the next moment of time, the given cell remains in the "U" state. Thus the "U" states can be viewed as representing undifferentiated, passive underlying substrate. Their passivity implies that they may in some cases serve as "insulation" surrounding more active cells in the system.

Then there are "ordinary transmission" cell states. These are states which direct their activity in each of the four cardinal directions. Each of these may be in an excited or quiescent mode, so there is a total of eight different kinds of ordinary transmission states. In addition, there are eight "special transmission states," similar to the ordinary states in that they also point in each of the cardinal directions and can be in excited or quiescent modes. The two basic kinds of transmission states—ordinary and special—differ in that the primary intended role of ordinary transmission states is the routing of informational signals, whereas the primary role of special states is to inject transforming signals into cell locations and thereby convert "U" cells into active elements (or, if need be, convert active elements back into "U" cells).

The system also has four "confluent" states. They are activated if they receive signals from all cells in their neighborhood which are directed toward them. If activation occurs, then after two moments of time they emit signals outward toward any cell in their neighborhood which does not have a transmission directed toward it. Thus, confluent cells can serve as "and" gates, and as wire branching elements. Since they do not emit their output until two moments of time have elapsed, the confluent cells can also be employed to create time delays in the transmission of signals. The eight remaining cell states of the 29 originally employed by von Neumann are of less importance. These are temporary cell states which arise only as the operational states are being created from "U" cells.

Von Neumann first showed how to design a general purpose computing machine in his cell space system. He did this by showing the design of various basic organs — "pulsers" to emit any desired finite train of pulses upon activation, "periodic pulsers" to emit repeated trains of desired pulses after activation until signaled to stop, "decoders" to detect the presence of certain patterns of pulses, and the like. Using these organs, von Neumann developed a design for the control portion of a computing machine in one region of the cell space. He then showed how to organize an adjacent but indefinitely extendable portion of the cell space into a memory or information storage unit, which could be accessed by the control unit.

For the process of construction, von Neumann designed a construction unit, which, taking instructions from the memory unit, could send out a constructing arm (by cteating an active pathway of transmission cells into a region of "U" cells) and at the end of the arm, convert "U" cells to the cell types specified in memory (see fig. 5.4). He showed that this constructor could create any pattern of passive cells whatsoever. Thus, he had designed with mathematical rigor a universal constructor, relative to all possible passive configurations of cells in the cell space.

Since the parent machine itself can be created in passive form, it can make a duplicate of itself by the following process. The parent machine is supplied initially with instructions to make a duplicate of its control, construction and memory units (the memory unit initially is empty). After it completes this major construction phase, the instructions call for the parent machine to make a copy of the instructions in its memory and to feed into the memory unit of the newly constructed machine. Then the parent machine activates the heretofore passive offspring machine, and withdraws the constructing arm. At that moment the offspring is a duplicate, in all respects, of the parent at the time the original machine commenced its reproducthe activities.

Figure 5.4.- Universal construction in the cellular model of machine self-reproduction.

Critique of the cellular model. Although the 29-state von Neumann cellular array system permits a more elegant mathematical approach to the problem of machine construction and self-reproduction, it is more difficult to envision an actual useful physical implementation of the process (compared, say, to the kinematic model of replication). The entire cell space enterprise proceeds in a highly constrained artificial environment, one which is very special despite some features relating in a general way to the natural world. For example, the movement of objects in space, a ubiquitous and familiar phenomenon in the real world, becomes a complex process of deletion of cell states at one location and re-creation of these states at some other location.

There is also an assumption of synchronous behavior throughout the system. All cells, no matter how distant, are subject to change of state at the same instant, a property which would be difficult to implement in any practical large cell space. Indeed, the requirement of a source of clocking pulses violates the array symmetry which makes the cell space notion an attractive object for mathematical treatment.

It is also very difficult to design machines of interest which can be embedded in the cell array format. To make design and embedding easier, a higher-level machine design language would have to be created. It is likely that, rather than undertake that task, one would first redesign the underlying cell space properties to rid the system of the deficiencies already noted.

For instance, one might wish to introduce a new primitive cell state in the system to permit signals to cross without interference. A "wire-crossing" organ can be devised using only the original von Neumann primitive cell types, but this introduces an unnecessary complexity into the machine design process since the organ contains initially active cell states whose creation involves considerable extra care to avoid the propagation of spurious signals. This extra care is especially critical because the cell system, as von Neumann originally constituted it, is highly susceptible to signal errors. (He undoubtedly intended his probabilistic machine model to mitigate this sensitivity and fragility.)

The cell space system has very limited capacity to detect the states of cells. It has some capacity to detect states, for this is required in the operation of the memory unit. But a machine cannot analyze an arbitrary encountered cell to determine what state it is in, thus cannot "read" the states of an encountered machine. This inability severely restricts the capacity of cell-space machines to repair other machines or to attempt self-repair. Such limitations also are evident in the construction process, where the constructing machine must assume that the region in which a new machine is to be created consists entirely of elementary quiescent cells. Should this not be the case, there is no systematic and complete way to detect it. A machine can send destruction signals into cells to reduce them to the quiescent form. Unfortunately, in some cases one must know the state of the cell ahead of time in order to determine what destructive signal must be sent to destroy it.

Finally, all machines that can be produced in von Neumann's cell space system are essentially information transactional devices. Even construction is, in this context, a form of information processing. Physical construction and material transformations can possibly be viewed as informational processes but, in a practical sense, the cell-space notion is far from providing a readily useful paradigm of actual manipulation and transformation of physical materials.

Von Neumann's other self-reproducing machine concepts. In addition to his kinematic and cellular models, von Neumann planned to examine three other models of self-reproducing machines. These were to be a neuronal or "excitation-threshold-fatigue" model, a continuous model, and a probabilistic model. Von Neumann is not known to have left any completed work whatsoever on these models at the time of his death, so his intentions are almost entirely a matter of conjecture.

Following Burks' speculations on this matter (von Neumann, 1966), we can guess that von Neumann's neuronal system might have been a version of the cell-space model in which the individual cell automata in the space were to be constructed of neuron-like elements. This would have been a rather straightforward process, as it is well known that idealized neurons of the McCulloch-Pitts (1943) variety can be employed to implement the kinds of logical gatings and delays called for in the 29-state cell automaton system. The reason for employing neuron-like elements seems mainly an attempt to recast the model in a more "biological" vocabulary.

Von Neumann's postulated continuous model might have been an attempt to comprehend machine reproduction in an even more biological format. The usual mathematical tools for handling actual neuron activity are differential equations expressing the electrochemical flows through and along neuron soma and axons. Thus the actions of cell automata (implemented with neurons) could be expressed by sets of differential equations. In this way the more highly developed tools of mathematical analysis might be employed in representing the behavior of the machine system, in contrast to the use of combinatorics which von Neumann himself characterized as one of the most intractable of mathematical specialties.

Finally, in his proposed probabilistic model von Neumann perhaps intended to consider using whole congeries of neuron-like elements in implementing the behaviors of what in the neuronal model could be carried out by single neurons. By employing redundancy techniques similar to those described in his classic paper on reliability, von Neumann (1956) may finally have hoped to design a reliable, biologically oriented, self-reproducing machine characterizable by differential equations. We can only guess.

Alternative cell array systems. Worlc on cellffpace automata systems in the period following von Neumann's contributions has taken several research directions. The underlying cell-space notion of a homogeneous medium with a local transition function that detemlines global properties has been employed in numerous modeling and simulation projects. For example, weather simulations use the idea of connected cells, the changes of each cell state described by a set of differential equations. Studies of the flow of excitation in heart tissue, the dispersal of medicinal drugs, and pattern recognition all have employed the cellspace concept. Cell spaces also have been investigated as abstract mathematical objects where, for instance, one tries to determine whether from every mathematical pattern all other pattems can be attained, and whether there are some patterns not attainable at all by means of the transition function, and various other specialized questions.

Some work in cellular automata has attempted to carry forth the von Neumann program of machine construction and self-reproduction. For instance, Codd (1968) recapitulated the von Neumann results in a simpler cell space requiring only 8 states rather than 29. This produced a machine design recognizably closer to that of present-day computing machines. Myhill (1970), trying to mitigate the artificiality of the indefinitely extended pre-existing cell space, designed a system in which componentry was drawn into a cell-grid system and was then employed as machine constituents somewhat as biological cell constituents might be drawn through a membrane to be used at an intracellular work site. Arbib (1966), attempting to make the movement of cell machines a less cumbersome matter, designed a cell-space system in which cells and blocks of cells might be joined together by a "welding" operation, thus becoming "co-moving" configurations.

Smith (1970) and Banks (1970) introduced additional simplifications to the cell-space notion, showing that the von Neumann program could be recapitulated in underlying cell spaces of an extremely elementary sort. Indeed, the so-called "Game of Life" designed by Conway (Gardner, 1971) is a cell-space system which, despite its very simple transition rules, has been claimed to be capable of expressing both universal computation and construction. (The game involves a checkerboard cell array with cells in one of two states, "0" or "1." A point whose state is "0" will change to state "1" if exactly three of its eight neighbors are in state "1." A point whose state is "1" will remain in that state if two or three of its neighbors are also in state "1." In all other cases, the state becomes or remains "O.")

Later research on self-reproducing automata. By the late 1960s, the original von Neumann program of machine construction and reproduction had been largly abandoned, although investigation of cell-space systems as abstract mathematical entities or as vehicles for "spatial" modeling and simulation has persisted. Indeed, research in the latter field has been especially vigorous and prolific - one recent author lists over 100 references for cell-space imaging applications (Preston et al.,1979).

Von Neumann's kinematic machine construction system appears to have had no intellectual progeny whatsoever. This is somewhat misleading, since practical application of computers to manufacturing and the persistent human interest in and investigation of robot mechanisms have, without explicit connection to von Neumann's earlier work, prepared the ground for a possible implementation of a hybrid computer/kinematic model of machine construction and reproduction.

The theoretical work of this later period, explicitly derived from von Neumann's research effort, has focused mainly on the molecular biological analogies that can be drawn. For example, in a series of papers Laing (1975, 1976, 1977, 1978, 1979) employs a hybrid cellularkinematic model of machine construction and shows that neither existing natural nor artificial machines need be bound to follow the "classical" reproductive paradigm. In the classical paradigm, a program (DNA in living systems) is first interpreted to construct a machine (protein synthesis in lifeforms) and then is read a second time to make a copy of the program for insertion into the newly constructed duplicate machine (DNA replication in living cells). The principal contribution of Laing is to suggest reproductive strategies other than direct analogues to the known biological process. In this new conception, a machine is able to identify all of the components of which machine systems consist (not merely a subset as in the von Neumann cell system) and can access all of an existing machine structure without requiring dismantling of the system (as would be required in the von Neumann model).

Once this and other similar advanced concepts are brought to bear on the problems of machine reproduction, many alternative reproduction strategies become immediately apparent. A selected few of these are reviewed in the following section.

5.2.2 Alternative Replication Strategies

A number of alternative automata reproduction strategies have been suggested in the decades following the completion of von Neumann's work. Major strides have been made in the scientific understanding of the processes of biological reproduction at the molecular or biochemical level. Recent research has demonstrated the theoretical possibility of inferring structure and achieving selfreplication without first possessing a complete selfdescription. This suggests an enormous range of new machine capabilities which possibly may be technologically exploited in the future, according to specific rules and multiplication strategies for optimal deployment.

Biological reproduction. Biological reproduction is thought to obey the following underlying logical paradigm. The basic genetic program (encoded in the genetic DNA) is employed to make a copy of the same information in a slightly different medium (RNA). This modified form of the genetic program is transported to a work site within the cell where, with the aid of cellular enzymatic machinery, the RNA is interpreted as coding for amino acid strings (proteins). The protein produced plays two major roles: (1) it constitutes the basic structural material of living organisms, and (2) certain smaller and variably active proteins (enzymes) control the metabolic, interpretive, and constructive actions of the system.

When the genetic code embodied in the RNA has been read and acted upon, the machinery construction phase is complete. The cell must then undertake the copying of original genetic material (the DNA) to provide offspring organisms with the necessary instructions. This copying process is the well-known DNA replication phase, in which DNA (in most cases a twisted pair of complementary DNA molecules) untwists to permit new nucleotides to match with existing separated strands to form two twisted pairs of DNA. Reproduction is completed when the newly produced and original organism machineries are divided up, one DNA program remaining with each.

This highly simplified description of biological reproduction is offered only to illustrate the underlying logical strategies: (I) follow instructions to make machinery, (2) copy the instructions, (3) divide the machinery, providing a sufficient set in each half, (4) assign a set of instructions to each half, and (5) complete the physical separatiOn.

Von Neumann 's automata reproduction. Von Neumann's automata reproductive process closely mirrors the biological one. In the original model, instructions exist in two copies. One of the copies is read and acted upon to construct another machine, sans instructions. The second copy is then read and copied twice, and this double copy is inserted into the passive constructed offspring machine which is then turned on and released, thus completing the act of reproduction.

There is no logical necessity for having two sets of identical instructions. Von Neumann employed two copies of the instructions because it eliminated the criticism that the instructions might, in the first (construction) phase, become corrupted and so not be able to transmit a true version for the use of offspring machine. Also von Neumann feared that there might seem to be a paradox in the program acting upon itself to make a copy of itself. There are, however, ways by which a program can successfully be made to make a copy of itself, and indeed many such programs, though exceedingly simple, have already been written (Burger, Brill, and Machi, 1980; Hay, 1980). Another solution is to provide the machine proper with an automatic "wired-in" copy routine which the program calls for at the proper time.

Simplifed von Neumann automata reproduction. Consider a single instruction tape, and a constructor machine which reads the instructions once to build the offspring machine and again to make a copy of the instructions for the offspring machine. Notice that although the instructions available to the system yield a duplicate of the original system, this need not be the case. Thus, in the biological example, even though some DNA made available to a cell does not code the instructions for a duplicate cell, the cellmachine still may proceed to obey the instructions. This means that a cell can generate offspring not only different from itself and its normal constituents and products, but even inimical to it. This is precisely what happens when a virus possessing no metabolic machinery and no enzymatic protein machinery to read DNA or to manufacture anything parasitically insinuates itself into a host cell. The virus co-opts the host cell's interpreting and manufacturing capacity, causing it to make virus particles until the cell fills with them, bursts open, and is destroyed. The greatly multiplied viral agents are then free to parasitize other cells.

In artificial systems as well, machines may read and interpret instructions without knowing what they are being called upon to do. The instructions might call for some computational, constructional, or program-copying activities. The machine can make machines unlike itself, and can give these "unnatural" offspring copies of the instructions which were employed in their manufacture. If the offspring are also equipped to read and follow instructions, and if they have a constructional capability, their offspring in turn would be replicas of themselves—which might not resemble their "grandparent" machine at all. Thus, an original construction machine can follow instructions to make an indefinitely large number of diverse machines, that are like or unlike themselves, capable or not capable of constructing, reproducing, etc. And though a universal constructing machine might make large numbers of "sterile" machines, if it should once make a duplicate of itself which is also equipped with the instructional program for making duplicates of itself, the process can become "explosive." Such machines would tend to drive out all other "species" not possessing this reproductive "autocatalytic" property. Thatcher's variant: inferring structure. Thatcher (1970) showed that a machine need not have an explicit construction program made available to it initially in order to create a duplicate of itself. First, it is sufficient that a machine can secure a description of itself (in place of instructions) if the machine is equipped with the capacity to read the description and convert this into the necessary constructive actions. Second, using a result obtained by Lee (1963) and himself (Thatcher, 1963), Thatcher showed that such a machine need not have its description loaded beforehand into its accessible memory organ. Instead, the machine has a partial self-description hard-wired into itself in the form of circuits which, when stimulated, make the description available to the machine in its accessible memory organ. These data describe all of the machine except the hardwired part which was stimulated to emit the description in the first place. The problem then, for the machine, is to obtain the description of this hidden part of itself. Lee and Thatcher showed that this section of the device can be constructed in such a simple fashion that the system can infer how this part must have been constructed merely by examining the consequences of its actions (e.g., the partial description it produced). After inferring the nature of this hidden part of itself, the machine possesses a complete selfdescription and can then follow von Neumann's paradigm for reproduction.

The principal practical significance of this form of automata replication is that it reminds the designer that the information required for machine construction (whether reproduction or not) need not be in the form of instructions for constructions but can be in the form of a description. Moreover, the description need not even reside in an accessible organ such as memory registers but may be embedded in "inaccessible" hardware. The hypothetical infinite regress likewise is shown to be baseless—it is possible for a machine to have within itself only a part of its own description, and from this to infer the rest.

Reproduction by component analysis. In von Neumann's cellular system, an embedded machine cannot send out an inspection arm to an encountered machine to identify all of its states. However, the cell-space system could be redesigned to permit this. In such a system an analyzing machine could examine an encountered passive machine and identify the type and location of all its cell-automata. (The analyzer might of course have to penetrate the machine, thus altering its automaton states, so the inspecting arm would have to send out appropriate restoration construction signals.)

In von Neumann's kinematic model a machine ostensibly could identify all parts of the system and thus determine the type and location of all components. This opens the possibility that a machine system might, for example, reproduce essentially two machines — one active, the other passive or able to assume passivity under a signal from the active machine. This possibility and others have been explored by Laing (1975, 1976, 1977, 1978, 1979) in a series of papers presenting alternative reproductive strategies which include the following:

• Beginning with two identical machines, one active and one passive, the active machine "reads" the passive machine twice, producing one active and one passive machine, thus completing reproduction.

• Beginning with two machines (not necessarily identical) one machine reads the second, and makes a duplicate of it. Then the second reads the first, and makes a duplicate of it, active and passive status being exchanged.

• By combining the capacity of machines to read machines with the Thatcher result, one can hardwire a machine to construct a second machine which is a duplicate of the original except for the hardwired part which produced the second machine. The original machine then "reads" the newly constructed partial duplicate, and infers what the missing hardwired part must be. The original machine then constructs the missing part, completing the reproductive process. This result explicitly confronts and overcomes the "necessary machine degeneracy" criticism of automata self-replication.

Machine reproduction without description. In the machine reproduction schemes explained thus far, some arbitrary part of the machine which cannot be inferred is always made explicitly available in memory initially, or is implicitly made available iwn memory or for inspection by means of an internal wired-in memory, also not directly accessible. Laing (1976) showed that even this wired-in description is not necessary. In effect, a machine can carry out a self-inspection which can yield a description which in turn can be made available to the machine in constructing a duplicate of itself.

The process begins with a wired-in construction routine which produces a semiautonomous analyzer machine. This analyzer moves over the original machine and identifies the type and location of its componentry. This is reported back to the original machine, which uses this information to make a duplicate of itself. Thus, though it may be that a part of a machine "may not comprehend the whole" in a single cognitive act, a part of a machine can examine in serial fashion the whole machine, and in time can make this information available to the machine for purposes of replication.

Exploitation of basic machine capabilities. The "simplified von Neumann" automata reproductive strategy — whereby a machine employs a stored program of instructions to make other machines (including duplicates of itself) and then also provides the program or parts of programs of instructions to newly constructed machines— should probably be the central strategy for any actual physical machine reproducing systems. -The other strategies are, from most points of view, more complex than this and thus perhaps are less preferable. The virtue of the alternative strategies is not as practical ways of implementing machine reproduction but rather in suggesting many basic capabilities, which, in a complex automated replicating LMF, may be usefully employed. The following are some of the behaviors of which, under suitable conditions and design, machines are actually and potentially capable:

(1) A machine can be "hard-wired" to carry out a computation.

(2) A machine can be programmed to carry out a computation.

(3) A machine can be a general-purpose computer, in that it can be given a set of instructions which will enable it to carry out the computation of any other computer. Alternatively, a general-purpose computing machine can be given the description of any other computing machine, and can carry out the computational actions of the machine described .

(4) A machine can be hard-wired to carry out a construction activity.

(5) A machine can be programmed to carry out a constructional activity.

(6) A sufficiently complex machine can be a generalpurpose constructor, vis-a-vis a set of machines, in that it can be given a set of instructions which enables it to carry out the construction of any of the set of machines. Alternatively, a machine can be given the description of any machine of the set, and can, from this description, construct the machine described.

(7) A machine can construct a duplicate of itself, including the instructions or description used to guide the construction process.

(8) A machine, given a coded set of instructions for machine actions, or a coded description of a machine, can make a copy of the instructions or coded description.

(9) A machine, given a coded set of instructions for machine actions, can infer the structure of a machine which can carry out the actions described, and can construct such a machine.

(10) A machine, given a coded set of instructions for a machine, or a description of a machine, can carry out the actions of the machine whose instructions are given or whose description is supplied.

(11) A machine, given the instructions for or the description of an unknown machine, can examine the instructions or description and can (a) infer some of the properties of the machine, (b) simulate the actions of the machine, (c) construct the machine, and (d) observe the actions of the constructed machine.

(12) A machine can determine the component types of encountered machines.

(13) A machine can determine the structure (the component type and arrangement of components) of encountered machines.

(14) A machine can thus obtain a structural description of an encountered machine and simulate its actions, construct a duplicate, and then observe the duplicate in action.

(15) A machine can possess a copy of its own description, perhaps stored in a memory organ.

(16) A machine can obtain a copy of its own present structure. Note that the present structure of a machine may deviate from the original design, and also from its present stored description of itself (which may be out of date).

(17) A machine can compare the stored description of itself with the description obtained by inspection, and note the discrepancies.

(18) A machine can make a duplicate of itself on the basis of its stored "genetic" description or on the basis of its present (possibly altered) structure. This latter is an example of transmission of acquired characteristics.

(19) A machine can examine duplicates of itself constructed on the basis of an examination of itself, and note the discrepancies.

(20) The duplicates made from either of these two bases (genetic and observed) can be set in action and observed.

(21) For diagnostic purposes, the two kinds of descriptions can be compared, the two passive structures compared, the two kinds of structures in action observed and compared. The basis for machine self-diagnosis is thus available.

(22) A machine noting the discrepancies between two machine descriptions, or machine structures, or two machine behaviors, can in some cases act so as to resolve the discrepancies. That is, a machine in some cases can repair or reject or reconstruct deviant machines (including itself) .

(23) A machine encountering an "unknown" machine can observe the behavior of that machine and compare this to the behavior of other machines, both directly and by simulating the behavior of those machines for which it already has or can obtain descriptions.

(24) A machine encountering an unknown machine can examine the structure of the machine and obtain a structural description which can be compared with other structural descriptions.

(25) Encountering an unknown device, a machine can use the structural description of the unknown to simulate its actions. These simulated actions can be compared to those of other machines whose descriptions are stored or which can be made available.

(26) Having the description of an encountered device, a machine.can construct a duplicate of it. This duplicate can be set in action and observed, and its behavior compared with the behavior (actual or simulated) of other machines.

(27) The structure and behavior of encountered machines can be compared with those of known useful or benign machines, including that of the inspecting machine itself. This comparison, and the degrees of similarity discerned, can serve as the basis for a subsequent policy of "friendship," "tolerance," "avoidance," "enmity," etc.

(28) The descriptions of encountered machines can be incorporated into the reproductive construction cycle so that these new machines or their features become part of the continuing and evolving machine system. This is an analogue to biological symbiosis.

Machine multiplication strategies. In describing the logical process of machine reproduction we have concentrated on the means by which the parent system could come to possess the information needed to carry out a replication and the associated question of how offspring would if necessary acquire the programs needed to continue the machine reproduction process. Although these questions, logically, are at the heart of machine replication, they leave open many issues concerning creation and siting of new machine systems as well as the ultimate fate of such systems.

This matter can be approached by considering certain biological analogues to the machine situation. In the known biological realm, all living organisms use the same underlying reproductive logic of protein synthesis and nucleotide sequence copying but employ vastly different broad strategies in producing more of their own kind.

One strategy is seen in the case of seed-bearing plants (as well as most fish and insects), in which vast numbers of "minimal" genetic packets are produced by the parent system and dispersed in the hope that a sufficient number will, largely by chance, find an appropriate site at which to survive and complete growth and development to maturity. At the other end of the scale is human behavior, whereby "construction" and nurture of the offspring may continue under the control and protection of the parent system until near maturity.

The particular multiplication strategy for artificial reproducing systems must of course be adjusted to intentions. The swift utilization of large rich environments might justify a "seed" dispersal strategy, with early maturity of new systems so as to retain a high reproductive rate. On the other hand, an environment consisting of scattered pockets of valuable resources, or situations with less pressure for immediate "explosive" utilization might suggest fewer offspring, possibly more fully developed in regard to their capacity for seeking out and efficiently utilizing the scarce resources available. In this case, the offspring might also be expected to receive longer tutelage from the parent system or from outside controllers (such as humans).

Similarly, the presence of a large contiguous valuable ore body might dictate the extensive ramification of a single machine factory system consisting of many laboring submachines. The model of a colonial organism such as coral, or of a social insect such as ants or termites, might make more sense. Zoological and sociobiological studies of animal and plant multiplication strategies may prove valuable in suggesting optimal machine system growth and reproduction strategies. One important difference must be borne in mind: biological organisms often have adapted their strategies to compete with other organisms, as well as to survive in a world where resources are renewed at certain rates over varying seasons. Some of these factors may be nonexistent or present in very different form in a nonterrestrial machine-inhabited environment.

A few questions that should be considered in determin ing optimal replicating machine behavior include:

• How large should a system be allowed to grow?

• How large should a system grow before it reproduces.

• What sorts of offspring (e.g., minimal vs mature should be produced? A mixture?

• How many offspring should be produced? How manz offspring should be produced from a single parent machine?

• When should offspring be produced?

• Where and how should offspring be sited? Specific sites? Near? Far? Randomly dispersed?

• What offspring transport mechanisms should b/ employed? Should new systems be mobile? Under own control? Parent? Human operator?

• When should sited machine systems be turned off? Abandoned? Should lifespan of a machine system be a function of time alone? Reproductive life? Exhaustion of local resources? Work experience and use? Detection of malfunction? When should subsystems be turned off? What growth and death patterns of individual machine systems should be adopted?

• What should be done with unsited offspring systems? Allowed to wander indefinitely?

• What should be done with outmoded machine systems? Dismantle them? Abandon them?

Intergeneration information transmission among replicating machines. Throughout most of the present discussion it has been assumed that the goal was to have the parent machine transmit to its offspring machine the same genetic information it received from its parent, regardless of the logical strategy of reproduction employed. This genetic fidelity is not necessary or even desirable in all cases. Nor mally the parent should transmit all information necessary for offspring to do their jobs and to construct further offspring in turn, but beyond this simple requirement there are many alternatives. For example, a parent machine might augment its program during its lifetime with some valuable information, and this augmented part of the program could then be transmitted to its offspring.

A few possible variations of interest include:

(1) The parent machine program is not altered in the course of its lifetime and is transmitted unaltered to offspring.

(2) The parent machine program is altered (e.g., by intervention, or by some machine adaptive process of a more or less complex sort) during the course of its lifetime, but again only the program originally received from the parent is transmitted to the offspring.

(3) The parent machine program is altered during the course of its lifetime, and the altered program is transmitted to the offspring machine. The parent machine (being a constructing machine) may make changes in its structure beyond those called for in its received genetic program.

(4) Changes in parent structure are not made part of the offspring structure.

(S) Changes in parent structure are made part of the offspring structure.

(6) Changes in parental structure are not made part of the offspring structure, but are made part of the offspring genetic program. Thus, the offspring can, under its own control, modify its structure to conform to that of its parent machine.

5.2.3 Information and Complexity in Self-Replicating Systems

The design and implementation of a self-replicating lunar factory represents an extremely sophisticated undertaking of the highest order. It is useful to consider the complexity of this enterprise in comparison with the information requirements of other large systems, natural or artificial, replicating or not (Stakem, 1979).

It is not irnmediately clear what the proper measure should be. One way to look at the problem of machines reproducing themselves is to consider the flow of information that occurs during reproduction. A fully generalized self-replicating system could possess a reproductive behavior of such complexity that the information necessary to describe that behavior is complete to atomic level specifications of machine structure. Such a machine has behavior so complex and complete that it might produce a copy of itself almost from complete chaos—say, a plasma containing equal concentrations of all isotopes. In this case the machine reproduction is essentially complete—given sufficient energy, the system can make copies of itself in any arbitrary environment even if that environment contains virtually no information relevant to replication.

At the other extreme, consider a long row of Unimate PUMA-like industrial robots side by side, each requiring merely the insertion of a single fuse to render it functional. The first working robot, its fuse already in place, seeks to "reproduce" itself from a "substrate" of dormant machines. It accomplishes this by reaching onto a nearby conveyor belt, picking up a passing fuse part, and plugging it into the neighboring robot. The adjacent machine now begins to function normally as the first (indeed, as an exact duplicate), so it can be said that in some sense the first machine has reproduced itself. Before the reproductive act there was no second working robot; afterwards, one exists. However, this is almost the most trivial case of replication imaginable, since the substrate for reproductive activity in this case completed machines lacking only fuses—is extremely highly organized. Hence, the operative complexity resides in the substrate, and the action of the machine in "making a new machine" is trivial.

This latter example may be compared to the case of a bacteriophage. The phage particle infects a healthy bacterium, using the captive cellular machinery to manufacture new virus particles. Only the DNA of the virus enters the bacterium, instructing the cellular machinery to make new viral DNA and to interpret the DNA to create protein and polysaccharide components which form the coat or carrier of the viral DNA. Thus the foreign DNA, like the PUMA robot which inserts fuses to "self-replicate," must situate itself in a very rich complex environment, one already containing a great deal of machinery and information. In this case, the complexity of the virus-making enterprise probably can be gauged by the length of the viral DNA inserted into the host cell, just as the true complexity of the fuseinsertion behavior can be gauged by the length of the program needed to permit location of the supply of fuses and the fuse holder on an adjacent machine in physical space, and to insert the part properly. It is suggested, therefore, that the length of the shortest program which can carry out the process of replication may be an appropriate measure of the complexity of the task.

For instance, in the case of the von Neumann cellular reproducing system each part is already located in its proper place in space, but signals must be injected into that space to cause it to take on the properties desired in the offspring machine. It has been estimated that such a reproducing machine might consist of a minimum of 105 cells, with offspring cell type and location the principal parameters which must be specified for each. The length of the shortest program would represent perhaps 106 bits of information (Kemeny, 1955).

If the construction of a replicating growing lunar factory was purely a matter of machine parts assembly, then the length of the replication program could be determined by the necessity to locate various required parts in the environment and then to specify and execute the proper placement of each part to construct the desired system (Heiserman, 1976). However, it is likely the reproductive process will be vastly more complicated than this, since it is not likely that all parts can be supplied "free" from Earth. If the lunar factory must begin, not with completed machines or parts, but rather with a raw lunar soil substrate, the task quickly becomes many orders more difficult—though not impossible. Based on the estimates outlined in section 5.3 and the appendixes, the lunar factory replication program length should not exceed roughly 1012 bits of information. This compares to about 101° bits coded in the human genome and about 1014 bits stored in the human brain. Terabit (1012 bits) memories are considered state-of-the-art today.

Complexity of a selt-replication program may also be viewed as an index of versatility or system survivability. The more complex the program, the more likely it is that the machine system can bring about its own replication from increasingly disordered substrates. This is an interesting observation because it suggests that reproduction is an activity defined along a broad continuum of complexity rather than as a single well-defined event. Both the chaosreplicator and the fuse-insertion robots described above perform acts of self-reproduction. Fundamentally, these systems differ only in the degree to which they are capable of bringing order to the substrate in which they are embedded.

It is interesting to note that human beings fall somewhere in the middle of this broad reproductive spectrum. A 100 kg body mass, if composed of purely random assortments of the 92 natural elements, would contain roughly 1027 atoms and hence require about 1028 bits to describe. Yet a 100 kg human body is described by a chromosome set containing just 101 ° bits. The difference must be made up by the "substrate" in which people are embedded - a highly ordered rich environment, namely, the Earth. Human beings thus are conceptually remarkably similar to von Neumann's kinematic self-reproducing automata, moving around in a "stockroom" searching for "parts."

5.2.4 Conclusions

The Replicating Systems Concepts Team reached the following conclusions concerning the theory of machine reproduction:

(1) John von Neumann and a large number of other researchers in theoretical computer science following him have shown that there are numerous alternative strategies by which a machine system can duplicate itself.

(2) There is a large repertoire of theoretical computer science results showing how machine systems may simulate machine systems (including themselves), construct machine systems (including machine systems similar to or identical with themselves), inspect machine systems (including themselves), and repair machine systems (including, to some extent, themselves). This repertoire of possible capabilities may be useful in the design and construction of replicating machines or factories in space.

5.3 Feasibility

The design and construction of a fully self-replicating factory system will be a tremendously complicated and difficult task. It may also be fairly expensive in the near-term. Before embarking upon such an ambitious undertaking it must first be shown that machine self-replication and growth is a fundamentally feasible goal.

5.3.1 Concept Credibility

The plausibility of the theoretical notion of selfreplicating machines already has been reviewed at length (see sec. 5.2). It remains only to demonstrate concept credibility in an engineering sense (Bradley, 1980, unpublished memorandum, and see appendix SA; Cliff, 1981; Freitas, 1980a; von Tiesenhausen and Darbro, 1980) - that is, is it credible to consider building real physical machines able to replicate themselves?

The credibility of any design proposed for such a machine or machine system depends first and foremost upon whether that design is consistent with reasonably foreseeable automation and materials processing technologies. These technologies need not necessarily be well established or even state-of-the-art, but should at least be conceivable in the context of a dedicated R&D effort spanning the next two decades. It is interesting to note that computer programs capable of self-replication have been written in many different programming languages (Burger et al., 1980; Hay, 1980), and that simple physical machines able to replicate themselves in highly specialized environments have already been designed and constructed (Jacobson, 1958;Morowitz, 1959;Penrose,1959).

Another major requirement for concept credibility is a plausible system configuration. Proposed designs for selfreplicating systems (SRS) must be sufficiently detailed to permit the generation of work breakdown structures, subsystem operational flowcharts, mass and energy throughput calculations, and at least preliminary closure (see sec.5.3.6) analyses.

A related requirement is plausible mission scenarios. Research and development costs for the proposed design should be many orders of magnitude less than the Gross National Product. The mission must not require launch and support facilities which cannot or will not be available in the next two or three decades. The mission must entail reasonable flight times, system lifetimes, growth rates, production rates, and so forth. The problems of reliability and repair should be addressed.

The final requirement for concept credibility is positive societal impact. A given SRS design must be economically, politically, and socially feasible, or else it may never be translated into reality even if the technology to do so exists. A general discussion of the implications of replicating systems appears in section 5.5, but the team has arrived at no firm conclusions regarding concept feasibility in this area. More research is clearly required.

5.3.2 Concept Definition

In order to demonstrate SRS concept credibility, specific system designs and mission scenarios must be subjected to a detailed feasibility analysis. The first step in this process is to conceptualize the notion of replicating systems in as broad an engineering context as possible. Many kinds of replicating machine systems have been proposed and considered during the course of the study. Some of these place emphasis on different types of behavior than others.

Consider a "unit machine" which is the automata equivalent of the atom in chemistry or the cell in biology—the smallest working system able to execute a desired function and which cannot be further subdivided without causing loss of that function. The unit machine may be comprised of a number of subunits, say, A, B, C, and D. These subunits may be visualized in terms of structural descriptions (girders, gearboxes, generators), functional descriptions (materials processing, parts fabrication, mining, parts assembly), or any other complete subset-level descriptions of the entire system.

SRS may be capable of at least five broad classes of machine behavior:

Production—Generation of useful output from useful input. The unit machine remains unchanged in the process. This is a "primitive" behavior exhibited by all working machines including replicating systems.

Replication - Complete manufacture of a physical copy of the original unit machine, by the unit machine.

Growth—Increase in mass of the original unit machine by its own actions, retaining the physical integrity of the original design.


Figure 5.5.- Five basic classes of SRS behavior.

Evolution—Increase in complexity of structure or function of the unit machine, by adding to, subtracting from, or changing the character of existing system subunits.

Repair—Any operation performed by a unit machine upon itself, which does not alter unit population, designed unit mass, or unit complexity. Includes reconstruction, reconfiguration, or replacement of existing subunits.

These five basic classes of SRS behavior are illustrated in figure 5.5.

Replicating systems, in principle, may be designed which can exhibit any or all of these machine behaviors. In actual practice, however, it is likely that a given SRS format will emphasize one or more kinds of behaviors even if capable of displaying all of them. The team has considered two specific replicating systems designs in some detail. The first (cf. von Tiesenhausen and Darbro, 1980), which may be characterized as a unit replication system, is described in section 5.3.3. The second (cf. Freitas, 1980a; Freitas and Zachary, 1981), which can be characterized as a unit growth system, is outlined in section 5.3.4. The team decided to concentrate on the possibility of fully autonomous or "unmanned" SRS, both because these are more challenging from a technical standpoint than either manned or teleoperated systems and also because the latter has already been lredted to some degree elsewhere in this report (see chap. 4).


5.3.3 Unit Replication: A Self-Replicating System Design

The SRS design for unit replication is intended to be a fully autonomous, general-purpose self-replicating factory to be deployed on the surface of planetary bodies or moons. The anatomy of an SRS is defined by two end conditions: (I) the type and quantity of products required within a certain time, and (2) the available material needed to manufacture these products as well as the SRS itself.

There are four major subsystems which comprise each SRS unit, as shown in figure 5.6. First, a materials processing subsystem acquires raw materials from the environment and prepares industrial feedstock from these substances. Second, a parts production subsystem uses the feedstock to make machines or other parts. At this point SRS output may take two forms. Parts may flow to the universal constructor subsystem, where they are used to construct a new SRS (replication). Or, parts may flow to a production facility subsystem to be made into commercially useful products. The SRS also has a number of other important but subsidiary subsystems, including a materials depot, parts depots, product depot, control and command, and an energy system.

The work breakdown structure given in figure 5.7 lists all SRS elements studied, and each is briefly described below.






Materials processing and feedstock production. In this system, raw materials are gathered by strip or deep milling. They are then analyzed, separated, and processed into industrial feedstock components such as sheets, bars, ingots, castings, and so forth, which are laid out and stored in the materials depot. The processing subsystem has a high degree of autonomy including self-maintenance and repair. It is linked to a central supervisory control system (see below).

The materials processing subsystem is shown schematically in figure 5.8.

Materials depot. The materials depot collects and deposits in proper storage locations the various feedstock categories according to a predetermined plan. This plan ensures that the subsequent fabrication of parts proceeds in the most efficient and expeditious manner possible. The depot also serves as a buffer during interruptions in normal operations caused by failures in either the materials processing subsystem (depot input) or in the parts production subsystem (at depot output).

Parts production plant. The parts production plant selects and transports industrial feedstock from the materials depot into the plaet, thee (6<i>~tAs %11 p1tt I~ qBilG } for SRS production or replication activities. Finished parts are stored in the production parts and the replication parts depots, respectively. The parts production plant is highly automated in materials transport and in distribution, production, control, and subassembly operations.

Figure 5. 6. - Functional schematic of unit replication SRS.

Figure 5. 7.- Work breakdown structure for SRS.

The parts production plant subsystem is shown schematically in figure 5.9.

Parts depots. There are two parts depots in the present design. These are called the production parts depot and the replication parts depot.

Parts are stored in the production parts depot exclusively for use in the manufacture of useful products in the production facility. If certain raw materials other than parts and subassemblies are required for production, these materials are simply passed from the materials depot through the parts production plant unchanged. The parts production depot also acts as a buffer during interruptions in normal operations caused by temporary failures in either the parts production plant or the production facility.

Parts and subassemblies are stored in the replication parts depot exclusively for use in the replication of complete SRS units. Storage is in lots earmarked for specific facility construction sites. The replication parts depot also serves as buffer during interruptions in parts production plant or universal constructor operations.

Figure 5. 7.- Concluded.

Production facility. The production facility manufactures the desired products. Parts and subassemblies are picked up at the production parts depot and are transported to the production facility to be assembled into specific useful products. Finished products are then stored in the products depot. Ultimately these are collected by the product retrieval system for outshipment.

Universal constructor. The universal constructor manufactures complete SRS units which are exact duplicates of the original system. Each replica can then, in turn, construct more replicas of itself, and so on. The universal constructor retains overall control and command responsibility for its own SRS as well as its replicas, until the control and command functions have also been replicated and transferred to the replicas. These functions can be overridden at any time by external means.

The universal constructor subsystem consists of two major, separate elements — the stationary universal constructor (fig. 5.10) and the mobile universal constructors (fig. 5.11). This composite subsystem must successfully perform a number of fundamental tasks, including receiving, sorting, loading, and transporting parts and subassemblies; assembling, constructing, installing, integrating, and testing SRS systems; starting and controlling SRS operations; and copying and transferring instructions between system components.

Figure 5.8. - SRS materials processing subsystem.

Products depot. The outputs of the production facility are stored in the products depot, ready for retrieval. Major hardware components are neatly stacked for ready access by the product retrieval system. Consumables such as elemental oxygen are stored in reusable containers that are returned empty to the production facility. The products depot also serves as a buffer against variable output and retrieval rates.

Product retrieval system. The product retrieval system collects the outputs of all SRS units in an "SRS field" and carries them to an outside distribution point for immediate use or for subsequent outshipment. The dashed lines in figure 5.11 indicate one possible solution to this problem in a typical SRS field. Other solutions are possible—careful consideration must be given to SRS field configuration to arrive at an optimum product retrieval system design.

Command and control systems. The master control and command system, located within the stationary universal constructor, is programmed to supervise the total SRS operation and to communicate both with the peripheral controls of the mobile universal constructors during the selfreplication phase and with the replicated stationary universal constructor during the transfer of command and control for the operation of the new SRS unit.

(fig. 5.11). This composite subsystem must successfully perform a number of fundamental tasks, including receiving, sorting, loading, and transporting parts and subassemblies; assembling, constructing, installing, integrating, and testing SRS systems; starting and controlling SRS operations; and copying and transferring instructions between system components.


Figure 5.8. - SRS materials processing subsystem.

Products depot. The outputs of the production facility are stored in the products depot, ready for retrieval. Major hardware components are neatly stacked for ready access by the product retrieval system. Consumables such as elemental oxygen are stored in reusable containers that are returned empty to the production facility. The products depot also serves as a buffer against variable output and retrieval rates.

Product retrieval system. The product retrieval system collects the outputs of all SRS units in an "SRS field" and carries them to an outside distribution point for immediate use or for subsequent outshipment. The dashed lines in figure 5.11 indicate one possible solution to this problem in a typical SRS field. Other solutions are possible - careful consideration must be given to SRS field configuration to arrive at an optimum product retrieval system design.

Command and control systems. The master control and command system, located within the stationary universal constructor, is programmed to supervise the total SRS operation and to communicate both with the peripheral controls of the mobile universal constructors during the selfreplication phase and with the replicated stationary universal constructor during the transfer of command and control for the operation of the new SRS unit.

Figure 5.9.- SRS parts production plant subsystem.


Figure 5.10. - SRS stationary universal constructor.

The master control and command system operates its own SRS unit through individual communication links which address the local control and command systems of individual SRS elements. In this way the master control and command system supervises the condition and operations of its own system elements, from materials acquisition through end product retrieval.

Energy system. The power requirements for the present design may be in gigawatt range. Hence, a single energy source (such as a nuclear power plant) would be excessively massive, and would be difficult to replicate in any case. This leaves solar energy as the lone viable alternative. Daylight options include: (1) central photovoltaic with a ground cable network, (2) distributed photovoltaic with local distribution system, (3) individual photovoltaic, and (4) satellite power system, with microwave or laser power transmission to central, local, or individual receivers. Nighttime power options include MHD, thermionics, or turbogenerators using fuel generated with excess capacity during daytime. Oxygen plus aluminum, magnesium, or calcium could be used for fuel. A 155to efficient central silicon photovoltaic power station has been assumed in the reference design, with an output of tens of gigawatts and a size on the order of tens of square kilometers.

Figure 5.11.- SRS mobile universal constructors.

Each SRS produces, in addition to its scheduled line of regular products, a part of the photovoltaic energy system equal to the energy needs of its replicas. These are retrieved along with the regular products by the product retrieval system and are assembled on-site to increase energy system capacity according to demand during the self-replication phase.

SRS deployment and expansion. A complete SRS factory unit, erected on the surface of the Moon, might appear as illustrated in figure 5.12.

As a unit replication scheme, the multiplication of SRS units proceeds from a single primary system to many hundreds of replica systems. This expansion must be carefully planned to reach the desired factory output capacity without running out of space and materials. Figure 5.13 shows one possible detailed growth plan for the geometry of an SRS field. In this plan, each SRS constructs just three replicas, simultaneously, then abandons replication and goes into full production of useful output. After the three generations depicted, an SRS field factory network 40 units strong is busy manufacturing products for outshipment.

The routes taken by mobile universal constructors are shown as solid lines, the product retrieval routes as dashed lines.

Figure 5.14 shows another possible expansion geometry. Again, each SRS constructs just three replicas, but sequentially rather than simultaneously. The end result is a field of 326 individual units after nine cycles of replication. Output is collected by the product retrieval system and taken to an end product assembly/collection system where end products undergo final assembly and other operations preparatory to outshipment. A more detailed discussion of expansion scenarios for SRS fields may be found in von Tiesenhausen and Darbro (1980).

Proposed development and demonstration scenario. It is proposed that the practical difficulties of machine replication should be confronted directly and promptly by a dedicated development and demonstration program having four distinct phases.

In Phase A of the development scenario, a robot manipulator will be programmed to construct a duplicate of itself from supplied parts and subassemblies. The original robot then makes a copy of its own operating program and inserts this into the replica, then turns it on, thus completing the duplication process (see appendix 5J). To complete Phase A; the replica must construct a replica of itself, repeating in every way the actions of the original robot. The rationale for the second construction, called the Fertility Test, is to demonstrate that the capacity for self-replication has in fact been transmitted from parent machine to offspring.

Figure 5.12.- Self-replicating lunar factory.

In Phase B of the development and demonstration scenario, the robot manipulator will be supplied with numerous additional parts so it can assemble objects of interest other than replicas of itself. This is intended to show that the system is able to construct useful products in addition to the line of robot duplicates.

In Phase C the manipulator system is still required to construct replicas and useful products. However, the robot now will be supplied only with industrial feedstock such as metal ingots, bars, and sheets, and must fabricate all necessary parts and subassemblies on its own. Successful completion of Phase C is expected to be much more difficult than the two earlier phases. The reason is that the parts fabrication machines must themselves be constructed by the robot manipulator and, in addition, all parts and subassemblies comprising the newly introduced fabrication machines must also be made available to the manipulator. Fabricator machines thus must be programmed to make not only the parts required for robot manipulators and useful products, but also their own parts and subassemblies as well. This raises the issue of parts closure, a matter which is discussed in section 5.3.6.

In Phase D, the system developed in the previous phase is retained with the exception that only minerals, ores, and soils of the kind naturally occurring on terrestrial or lunar surfaces are provided. In addition to all Phase C capabilities, the Phase D system must be able to prepare industrial feedstock for input to the fabrication machines. Successful completion of Phase D is expected to be the most difficult of all because, in addition to the parts closure problem represented by the addition of materials processing machines, all chemical elements, process chemicals, and alloys necessary for system construction and operation must be extracted and prepared by the materials processing machines. This raises the issue of materials closure (see also sec. 5.3.6). The completion of Phase D will yield an automatic manufacturing facility which, beginning with "natural" substrate, can replicate itself.

Figure 5.13. - Possible growth plan with simultaneous replica construction, suitable for geometry of an SRS field.

Figure 5.14.- SRS growth plan with sequential replication.

This progressive development of a replicating factory will serve to verify concept feasibility, clarify the functional requirements of such a system, and identify specific technological problem areas where additional research in automation and robotics is needed. A minimum demonstration program should be designed to gain engineering under standing, confidence, and hands-on experience in the design A and operation of replicating systems. (See sec. 5.6.) The question of when the results of an Earth-based development and demonstration project should be translated to lunar requirements, designs, and construction remains open. On the one hand, it may be deemed most practical to complete Phase D before attempting a translation to a design better suited to a lunar or orbital environment. On the other hand, major system components for a lunar facility undoubtedly could be undertaken profitably earlier in concert with Phase C and D development. The proposed development and demonstration scenario is described in greater detail in von Tiesenhausen and Darbro (1980).

5.3.4 Unit Growth: A Growing Lunar Manufacturing Facility

The Lunar Manufacturing Facility (LMF) demonstrating SRS unit growth is intended as a fully automatic general purpose factory which expands to some predetermined adult size starting from a relatively tiny "seed" initially deposited on the lunar surface. This seed, once deployed on the Moon, is circular in shape, thus providing the smallest possible perimeter/surface area ratio and minimizing interior transport distances. Expansion is radially outward with an accelerating radius during the growth phase. Original seed mass is 100 tons.

The replicating LMF design encompasses eight fundamental subsystems. Three subsystems are external to the main factory (transponder network, paving, and mining robots). The LMF platform is divided into two identical halves, each comprised of three major production subsystems: (I) the chemical processing sector accepts raw lunar materials, extracts needed elements, and prepares process chemicals and refractories for factory use; (2) the fabrication sector converts these substances into manufactured parts, tools, and electronics components; and (3) the assembly sector, which assembles fabricated parts into complex working machines or useful products of any conceivable design. (Each sector must grow at the same relative rate for uniform and efficient perimeter expansion.) Computer facilities and the energy plant are the two remaining major subsystems. (See fig. 5.15 .)

Transponder network. A transponder network operating in the gigahertz range assists mobile LMF robots in accurately fixing their position relative to the main factory complex while they are away from it. The network, described briefly in appendix SB, is comprised of a number of navigation and communication relay stations set up in a well defined regular grid pattern around the initial seed and the growing LMF complex.

Figure 5.15.- Functional schematic of unit growth SRS.

Paving robots. In order to secure a firm foundation upon which to erect seed (and later LMF) machinery, a platform of adjoining flat cast basalt slabs is required in the baseline design. A team of five paving robots lays down this foundation in a regular checkerboard pattern, using focused solar energy to melt pregraded lunar soil in situ. (See app. 5C.)

Mining robots. As described in appendix SD, LMF mining robots perform six distinct functions in normal operation: (1) strip mining, (2) hauling, (3) landfilling, (4) grading, (5) cellar-digging, and (6) towing. Lunar soil is stripmined in a circular pit surrounding the growing LMF. This material is hauled back to the factory for processing, after which the unused slag is returned to the inside edge of the annular pit and used for landfill which may later be paved over to permit additional LMF radial expansion. Paving operations require a well graded surface, and cellar digging is necessary so that the LMF computer may be partially buried a short distance beneath the surface to afford better protection from potentially disabling radiation and particle impacts. Towing is needed for general surface transport and rescue operations to be performed by the mining robots. The robot design selected is a modified front loader with combination roll-back bucket/dozer blade and a capacity for aft attachments including a grading blade, towing platform, and a tow bar.

Chemical processing sectors. Mining robots deliver raw lunar soil strip-mined at the pit into large input hoppers arranged along the edge of entry corridors leading into the chemical processing sectors in either half of the LMF. This material is electrophoretically separated (Dunning and Snyder, 1981; see sec. 4.2.2) into pure minerals or workable mixtures of minerals, then processed using the HF acid-leach method (Arnold et al., 1981; Waldron et al., 1979) and other specialized techniques to recover volatiles, refractories, metals, and nonmetallic elements. Useless residue and wastes are collected in large output hoppers for landfill. Buffer storage of materials output is on site. Chemical processing operations are shown schematically in figure 5.16, and are detailed in appendix SE.

Fabrication sectors. The LMF fabrication sector outlined in appendix SF is an integrated system for the production of finished aluminum or magnesium parts, wire stock, cast basalt parts, iron or steel parts, refractories, and electronics parts. Excepting electronics (Zachary, 1981) there are two major subsystems: (1) the casting subsystem, consisting of a casting robot to make molds, mixing and alloying furnaces for basalt and metals, and automatic molding machines to manufacture parts to low tolerance using the molds and alloys prepared; and (2) the laser machining and finishing subsystem, which performs final cutting and machining of various complex or very-close-tolerance parts. The basic operational flowohart for parts fabrication is shown in figure 5.17.

Assembly sectors. Finished parts flow into the automated assembly system warehouse, where they are stored and retrieved by warehouse robots as required. This subsystem provides a buffer against system slowdowns or temporary interruptions in service during unforeseen circumstances. The automated assembly subsystem requisitions necessary parts from the warehouse and fits them together to make subassemblies which are inspected for structural and functional integrity. Subassemblies may be returned to the warehouse for storage, or passed to the mobile assembly and repair robots for transport to the LMF perimeter, either for internal repairs or to be incorporated into working machines and automated subsystems which themselves may contribute to further growth. The basic operational flowchart for SRS parts assembly is shown in figure 5.18, and a more detailed presentation may be found in appendix SG.

Computer control and communications. The seed computers must be capable of deploying and operating a highly complex, completely autonomous factory system. The original computer must erect an automated production facility, and must be expandable in- order to retain control as the LMF grows to its full "adult" size. The computer control subsystem coordinates all aspects of production, scheduling, operations, repairs, inspections, maintenance, and reporting, and must stand ready to respond instantly to emergencies and other unexpected events. Computer control is nominally located at the hub of the expanding LMF disk, and commands in hierarchical fashion a distributed information processing system with sector computers at each node and sector subsystems at the next hierarchical level of control. Communications channels include the transponder network, direct data bus links, and E2ROM messenger chips (firmware) for large data block transfers.

Using ideas borrowed from current industrial practice, top-down structured programming, and biology, Cliff (1981) has devised a system architecture which could perform automated design, fabrication, and repair of complex systems. This architecture, presented in appendix SH, is amenable to straightforward mathematical analysis and should be a highly useful component of the proposed lunar SRS. Further work in this area should probably include a survey of industrial systems management techniques (Carson, 1959) and the theory of control and analysis of large-scale systems (Sandell et al., 1978).

Figure 5.16.- LMF chemical processing sector. Operations.

In a practical sense, it is quite possible to imagine the lunar SRS operating nonautonomously (Johnsen, 1972). For instance, the in situ computer could be used simply as a teleoperation-management system for operations controlled directly by Earth-based workers. Material factory replication would proceed, but information necessary to accomplish this would be supplied from outside. An intermediate alternative would permit the on-site computer to handle mundane tasks and normal functions with humans retaining a higher-level supervisory role. Yet another possibility is that people might actually inhabit the machine factory and help it reproduce—manned machine economies can also self-replicate.

Solar canopy. The solar canopy is a "roof" of photovoltaic solar cells, suspended on a relatively flimsy support web of wires, crossbeams and columns perhaps 3-4 m above ground level. The canopy covers the entire LMF platform area and expands outward as the rest of the facility grows. The solar canopy and power grid provide all electrical power for LMF systems. Canopy components may be stationary or may track solar motions using heliostats if greater eff1ciency is required. A further discussion of canopy design and rationale may be found in appendix 5I.

Mass, power, and information requirements. Seed subsystem masses and power requirements scale according to the total system mass assumed. SRS can be reduced indefinitely in size until its components begin to scale nonlinearly. Once this physical or technological limit is reached for any subsystem component, comprehensive redesign of the entire factory may become necessary.

Figure 5.18.- LMF assembly sector: Operations.

A seed mass of 100 tons was selected in the present study for a number of reasons. First, 100 tons is a credible system mass in terms of foreseeable NASA launch capabilities to the lunar surface, representing very roughly the lunar payload capacity of four Apollo missions to the Moon. Second, after performing the exercise of specifying seed components in some detail it is found that many subsystems are already approaching a nonlinear scaling regime for a 100-ton LMF. For instance, according to Criswell (1980, private communication) the minimum feasible size for a linear-scaling benchtop HF acid-leach plant for materials processing is about 1000 kg; in the present design, two such plants are required with a mass of 1250 kg each. Third, the results of a previous study (Freitas, 1980a) which argued the feasibility of 433-ton seed in the context of an interstellar mission (inherently far more challenging than a lunar factory mission) were compared with preliminary estimates of 15-107 tons for partially self-replicating lunar factories of several different types (O'Neill et al., 1980), and an intermediate trial value of 100 tons selected. The 100-ton figure has appeared in numerous public statements by former NASA Administrator Dr. Robert A. Frosch (lecture delivered at Commonwealth Club, San Francisco, Calif., 1979, and personal communication,1980) and by others in prior studies (Bekey and Naugle, 1980; Giacconi et al., working paper of the Telefactors Working Group, Woods Hole New Directions Workshop, 1979). Finally, it was decided to use a specific system mass rather than unscaled relative component mass fractions to help develop intuitive understanding of a novel concept which has not been extensively studied before.

For reasons similar to the above, an SRS strawman replication time of 1 year was taken as appropriate. The ranges given in table 5.1, drawn from the analysis presented in appendixes 5B-51, are estimates of the mass and power requirements of an initial seed system able to manufacture 100 tons of all of its own components per working year, hence, to self-replicate. These figures are consistent with the original estimate of a 100 ton circular LMF seed with an initial deployed diameter of 120 m, so feasibility has been at least tentatively demonstrated. However, it must be emphasized that the LMF seed design outlined above is intended primarily as a proof of principle. Numerical values for system components are only crude estimates of what ultimately must become a very complex and exacting design.

Information processing and storage requirements also have been collected and summarized in table 5.1, and lie within the state-of-the-art or foreseeable computer technologies. These calculations, though only rough approximations, quite likely overestimate real needs significantly because of the conservative nature of the assumptions employed. (See also sec.5.2.3.)

SRS mission overview. In the most general case of fully autonomous operation, a typical LMF deployment scenario might involve the following initial sequence:

(1) The predetermined lunar landing site is mapped from orbit to l-m resolution across the entire target ellipse.

(2) Seed lands on the Moon, as close to dead center of the mapped target area as possible navigationally.

(3) Mobile assembly and repair robots, assisted by mining robots, emerge from the landing pod and erect a small provisional solar array to provide interim power until the solar canopy is completed.

(4) LMF robots, with the computer, select the precise site where erection of the original seed will commence. This decision will already largely have been made based on orbital mapping data, but ground truth will help refine the estimate of the situation and adjust for unexpected variations.

(5) Mobile robots emplace the first three stations of the transponder network (the minimum necessary for triangulation), calibrate them carefully, and verify that the system is in good working order.

(6) Mining robots equipped with grading tools proceed to the construction site and level the local surface.

(7) Five paving robots disembark and begin laying down the seed platform in square grids. This requires one working year for completion.

(8) When a sufficiently large platform section has been completed, seed mobile robots transfer the main computer to a place prepared for it at the center of the expanding platforrn disk.

(9) Erection of the solar canopy begins, followed by each of the seed sectors in turn, starting with the chemical processing. Total time to unpack the landing pod after moonfall is one working year, conducted in parallel with paving and other activities. The completed seed factory unit, unfurled to a 120 m diam on the surface of the Moon 1 vear after landing, might appear as shown in figure 5.19.


Seed subsystem

Transponder network

Paving robots

Mining robots

Chemical processing sector (S)

Fabrication sector (S) Electronics Floor map Totals

Assembly sector (S) Assembly robots Warehouse subsystem Floor map

Automated transport vehicles

Mobile assembly and repair robots

Computer central orbital site map

Solar canopy



Estimated mass of

100 ton/yr seed,







Estimated power of 100 ton/yr seed, W

Up to 104

Up to 104


137-20,400 270-345,000

83-1,150 83-19,600

1,000 10,000


4,000 2,200




Computer Computer processor, memory, bits to operate hitc tr, {1>coriho

105 ?

l-lOX 106

4-7X 108

9.4X 107




6,000 107

40,000 4X 109

37.000 (1.6X 101 °)

Nomirial annual seed output 100,000 1.7 MW


The LMF has two primary operational phases—growth and production. The optimal program would probably be to "bootstrap" (grow) up to a production capacity matching current demand, then reconfigure for production until demand increases, thus necessitating yet further growth (O'Neill et al., 1980). Growth and production of useful output may proceed sequentially, cyclically, or simultaneously, though the former is preferred if large subsystems of the lunar factory must be reconfigured to accommodate the change.

The LMF also may exhibit replicative behavior if and when necessary. Replicas of the original seed could be constructed much like regular products and dispatched to remote areas, either to increase the total area easily subject to utilization or to avoid mortality due to depletion of local resources or physical catastrophes. The scheduling of factory operational phases is very flexible,-as shown schematically in figure 5.20, and should be optimized for each mission and each intended use.

5.3.5 Lunar SRS Growth and Productivity

As the study progressed, the team noted a developing convergence between the two designs for SRS described in sections 5.3.3 and 5.3.4. Both require three major subsystems—materials processing, fabrication, and assembly plus a variety of support systems, and each is capable of replication and useful production. Both display exponential expansion patterns.

Figure 5.19.- Self-growing lunar factory.

Figure 5.20. - Flexible scheduling of LMF operational phases.

Of course, in a finite environment exponential growth cannot continue indefinitely. Geometrical arguments by - Taneja and Walsh (1980, Summer Study document) suggest that planar packing of triangular, cubic, or hexagonal units can expand exponentially only for as many generations as each unit has sides, assuming that once all sides are used up no further doubling can occur by the enclosed unit.

Growth is quadratic from that time on. However, in real physical systems such as the developing LMF, enclosure need not preclude material communication with exterior units. Selected ramification of communication, control, and materials transportation channels or internal component rearrangement, reconfiguration, or specialization can prevent "starvation" in the inner regions of the expanding system. Hence, SRS exponential growth may continue until limited either by purposeful design or by the - specific configuration of the external environment. Assuming that a lO0-ton seed produces 100 tons/year of the same materials of which it is composed, then if T is elapsed time and N is number of seed units or seed mass-equivalents generated during this time, T = I + log2 N for simple exponential "doubling" growth. (There is no replication in the first year, the time required for initial setup.) If P is productivity in tons/year, then P = 100 log2 N.; However, the above is valid only if each unit works only on its own replica. If two or more units cooperate in the construction of a single replica, still more rapid "fast exponential" growth is possible. This is because new complete replicas or LMF subsystems are brought on line sooner, and thence may begin contributing to the exponentiation earlier than before. Using the above notation, the "fast exponential" growth rate is given by T= 1 + 1/2 + + 1/N in the optimum case where all available machines contribute directly to the production of the next unit.

Growth rates and productivities are tabulated for exponential and "fast-exponential" expansion in table 5.2. Note that in just 10 years the output of such a facility could grow to approximately one million tons per year. If allowed to expand for 18 years without diversion to production, the factory output could exponentiate to more than 4 X 109 tons per year, roughly the entire annual industrial output of all human civilization.

Useful SRS products may include lunar soil thrown into orbit by mass drivers for orbital processing, construction projects, reaction mass for deep space missions, or as radiation shielding; processed chemicals and elements, such as oxygen to be used in space habitats, as fuel for interorbital vehicles, and as reaction mass for ion thrusters and mass drivers; metals and other feedstock ready-made for space construction or large orbital facilities for human occupation (scientific, commercial, recreational, and medical); components for large deep-space research vessels, radio telescopes, and large high-power satellites; complex devices such as machine shop equipment, integrated circuits, sophisticated electronics gear, or even autonomous robots, teleoperators, or any of their subassemblies; and solar cells, rocket fuels, solar sails, and mass driver subassemblies. Also, a 100-ton seed which has undergone thousand-fold growth or replication represents a 2 GW power generating capacity, plus a computer facility with a 16,000 Gbit processing capability and a total memory capacity of 272,000 Gbits. These should have many useful applications in both terrestrial and space industry.

5.3.6 Closure in Self-ReplicatingSystems

Fundamental to the problem of designing self-replicating systems is the issue of closure.


"Fast-exponential" growth, T = 1 yr

(About 3 billion seed units would completely cover the entire lunar surface)

In its broadest sense, this issue reduces to the following question: Does system function (e.g., factory output) equal or exceed system structure (e.g., factory components or input needs)? If the answer is negative, the system cannot independently fully replicate itself; if positive, such replication may be possible.

Consider, for example, the problem of parts closure. Imagine that the entire factory and all of its machines are broken down into their component parts. If the original factory cannot fabricate every one of these items, then parts closure does not exist and the system is not fully self-replicating .

In an arbitrary system there are three basic requirements to achieve closure:

(1) Matter closure - can the system manipulate matter in all ways necessary for complete self-construction?

(2) Energy closure—can the system generate sufficient energy and in the proper format to power the processes of self-construction?

(3) Information closure can the system successfully command and control all processes required for complete self-construction?

System productivity, tons/yr

100 400 1 ,100 3,100 8,300

22,700 61,600 167,400 455,000 1,236,700 3,361,700 9,138,000 24,839,800 67,521,500

183,542,600 498,920,500 1,356,206,600 3,686,551,700 10,021,086,500 27,240,137,200

(~2 km-wide asteroid/yr)

Partial closure results in a system which is only partially self-replicating. Some vital matter, energy, or information must be provided from the outside or the machine system will fail to reproduce. For instance, various preliminary studies of the matter closure problem in connection with the possibility of "bootstrapping" in space manufacturing have concluded that 90-96% closure is attainable in specific nonreplicating production applications (Bock, 1979; Miller and Smith, 1979; O'Neill et al., 1980). The 4-10% that still must be supplied sometimes are called "vitamin parts." These might include hard-to-manufacture but lightweight items such as microelectronics components, ball bearings, precision instruments and others which may not be costeffective to produce via automation off-Earth except in the longer term. To take another example, partial information closure would imply that factory-directive control or supervision is provided from the outside, perhaps (in the case of a lunar facility) from Earth-based computers programmed with human-supervised expert systems or from manned remote teleoperation control stations on Earth or in low Earth orbit.

The fraction of total necessary resources that must be supplied by some external agency has been dubbed the "Tukey Ratio" (Heer, 1980). Originally intended simply as an informal measure of basic materials closure, the most logical form of the Tukey Ratio is computed by dividing the mass of the external supplies per unit time interval by the total mass of all inputs necessary to achieve self-replication. (This is actually the inverse of the original version of the ratio.) In a fully self-replicating system with no external inputs, the Tukey Ratio thus would be zero (0%).

It has been pointed out that if a system is "truly isolated in the thermodynamic sense and also perhaps in a more absolute sense (no exchange of information with the environment) then it cannot be self-replicating without violating the laws of thermodynamics" (Heer,1980). While this is true, it should be noted that a system which achieves complete "closure" is not "closed" or "isolated" in the classical sense. Materials, energy, and information still flow into the system which is thermodynamically "open"; these flows are of indigenous origin and may be managed autonomously by the SRS itself without need for direct human intervention.

Closure theory. For replicating machine systems, complete closure is theoretically quite plausible; no fundamental or logical impossibilities have yet been identified. Indeed, in many areas automata theory already provides relatively unambiguous conclusions. For example, the theoretical capability of machines to perform "universal computation" and "universal construction" can be demonstrated with mathematical rigor (Turing, 1936; von Neumann, 1966; see also sec. 5.2), so parts assembly closure is certainly theoretically possible.

An approach to the problem of closure in real engineering-systems is to begin with the issue of parts closure by asking the question: can a set of machines produce all of its elements? If the manufacture of each part requires, on average, the addition of >1 new parts to product it, then an infinite number of parts are required in the initial system and complete closure cannot be achieved. On the other hand, if the mean number of new parts per original part is <1, then the design sequence converges to some finite ensemble of elements and bounded replication becomes possible.

The central theoretical issue is: can a real machine system itself produce and assemble all the kinds of parts of which it is comprised? In our generalized terrestrial indus-trial economy manned by humans the answer clearly is yes, since "the set of machines which make all other machines is a subset of the set of all machines" (Freitas et al.,1981). In space a few percent of total system mass could feasibly be supplied from Earth-based manufacturers as "vitamin parts." Alternatively, the system could be designed with components of very limited complexity (Heer, 1980). The minimum size of a self-sufficient "machine economy" remains unknown.

Von Tiesenhausen and Darbro (1980) similarly argue that a finite set of machines can produce any machine element . Their reasoning, outlined in figure 5.21, is as follows:

(1) If all existing machines were disassembled into their individual parts there would obviously be a finite number of parts, many of them identical, and a large number would be of common categories like shafts, motors, wiring, etc. The only differences between the machines would be a different selection, different arrangement, and different dimensions of this finite number of parts.

(2) A finite number of parts involves a finite number of machine operations, this number being less than the number of parts because some machines can make more than one kind of parts.

(3) Therefore, the number of machines is finite and less than the number of operations.

This reasoning can then be generalized to say: "Every existing machine can be reduced to a finite set of machine elements, and there exists a finite set of machine operations." (Still, of course, a limited number of standard elements should be developed and machine operations limited as much as practical by substitution, in order to minimize the number of parts and machine operations.)

Figure 5.21.- Closure of SRS parts production.

Similar arguments may be applied to materials processing and feedstock production. There exists a finite number of different materials anywhere. There is a finite number of materials processes which is less than the number of materials because single processes result in various materials (e.g., silicon and oxygen). Hence, there is a finite number of materials processing robot systems needed for an SRS. Also, there is a flnite and rather limited number of feedstock requirements such as bars, rods, ingots, plates, etc. The number of materials is much less than the number of parts; therefore, a finite number of parts fabrication robots is required for an SRS.

Closure engineering In actual practice, the achievement of full closure will be a highly complicated, iterative engineering design process. Every factory system, subsystem, component structure, and input requirement (Miller and Smith, 1979) must be carefully matched against known factory output capabilities. Any gaps in the manufacturing flow must be filled by the introduction of additional machines, whose own construction and operation may create new gaps requiring the introduction of still more machines.

The team developed a simple iterative procedure for generating designs for engineering systems which display complete closure. The procedure must be cumulatively iterated, first to achieve closure starting from some initial design, then again to eliminate overclosure to obtain an optimally efficient design. Each cycle is broken down into a succession of subiterations which ensure three additional dimensions of closure:

(1) Qualitative closure—can, say, all parts be made?

(2) Quantitative closure - can, say, enough parts be made?

(3) Throughput closure — can parts be made fast enough?

In addition, each subiteration sequence is further decomposed into design cycles for each factory subsystem or component, as shown in figure 5.22.

The procedure as outlined, though workable in theory, appears cumbersome. Further work should be done in an attempt to devise a more streamlined, elegant approach.

Quantitative materials closure - numerical results. In the context of materials processing, "closure" is a relationship between a given machine design and a given particular substrate from which the machine's elemental chemical constituents are to be drawn. Hence the numerical demonstration of closure requires a knowledge of the precise composition both of the intended base substrate to be utilized and of the products which the SRS must manufacture from that substrate. Following a method suggested by the work of Freitas (1980a), a modified "extraction ratio" Ry is defined as the mass of raw substrate material which must be processed (input stream) to obtain a unit mass of useful system output having the desired mass fraction of element n (output stream).

Consider the significance of the extraction ratio to the problem of materials closure. Assume that the final product is to be composed of elements x, y, and z. An Rx = 1 means that 1 kg of lunar soil contains exactly the mass of element x needed in the manufacture of 1 kg of the desired output product. On the other hand, Ry = 10 means that 10 kg of lunar regolith must be processed to extract all of element y required in 1 kg of final product. The difference between Rx and Ry may signify that y is more rare in lunar soil than x, or that the two elements are equally abundant but ten times more y than x is required (by weight) in the final product. When the output stream is identical to the machine processing system itself, then the system is manufacturing more of itself -- self-replicating—and the extraction ratio becomes an index of system materials closure on an element-by-element basis.

The total net extraction ratio R is some function of the individual extraction ratios Rns and depends on the methods of materials processing employed. At worst, if only one element is recovered from a given mass of input stream ("parallel processing"), then R is the sum of all Rn. At best, if the input stream is processed sequentially to extract all desired elements in the necessary amounts ("serial processing"), then R is driven solely by the Rn of the element most difficult to extract, say, element z. That is, R = (Rn)maX = Rz, which is always equal to or smaller than the sum of all Rn. As serial processing should dominate in the lunar factory the latter formula is assumed for purposes of the present calculations. Note that Rn can be less than 1 for individual elements, but for an entire machine systemR must always be greater than or equal tol .

As a general rule, a low value for R implies that the system is designed for low mass throughput rates and is built from relatively few different chemical elements. A high value of R implies that many more elements are necessary and that a higher mass throughput rate will be accommodated to obtain them.

The "closure" of a given output stream (product) relative to a specified input stream (substrate) is computed by treating R as an independent variable. If In is the concentration of element n in mineral form in the input stream of lunar soil (kg/kg), En is the efficiency of chemical extraction of pure element n from its mineral form which is present in lunar soil (kg/kg), and °n is the concentration of element n in the desired factory output stream (kg/kg), then Rn = On/EnIn. Closure Cn for each element is defined as the mass of pure element n available in a system with a total net extraction ratio R per unit mass of output stream. For any given element, if R > Rn then all pure element n needed is already available within the system. In this case, Cn = °n. On the other hand, if R < Rn then the choice of R is too low; all the pure element n needed cannot be recovered, and more lunar soil must be processed to make up the difference if 100% closure is to be achieved. In this case, Cn = On(R/Rn), since the closure deficit is measured by the ratio of the chosen R to the actual Rn of the given element (i.e., how much the factory has, divided by how much the factory actually needs). Total net system closure C is simply the sum of all Cn for all elements n required in the output stream of the SRS factory (Freitas and Zachary, 1981)


















Cla,b,c C2a,b,c C3a,b,c C4a,b,c C5a,b,c C6a,b,c C7a,b,c C&, b, c C9a,b,c ClOa,b,c C 11s, b, c














Figure 5.22.- Generalized closure engineering design cycles.





A#a —Add nrnv machines to make

A#b —Add nevs machines to make

A#c —increase replication time available or change machine design

To estimate the quantitative materials closure for the lunar SRS baseline designs proposed in sections 5.3.3 and 5.3.4, three different approaches were taken in an attempt to converge on a useful estimate of the composition of the output stream necessary for LMF selfreplication. First, the "seed" element distribution given by Freitas (1980a) in the context of a self-reproducing exploratory spaceprobe was adopted. These figures are derived from published data on the material consumption of the United States (the world's largest factory) during the years 1972-1976 (U.S. Bureau of Mines, 1978; U.S. Bureau of the Census, 1977, 1978). A second but less comprehensive measure called "demandite" is based on 1968 U.S. consumption data (Goeller and Weinberg, 1976). A molecule of "nonfuel demandite" is the average nonrenewable resource used by humans, less fuel resources (Waldron et al., 1979). Third, the direct estimate of LMF elemental composition presented in appendix 5E was used to obtain additional trial values for °n. (Appendix 5E also represents a first attempt to deal with qualitative materials closure for SRS.) In all cases the input stream was assumed to consist of lunar maria regolith, with values for In averaged from published data (Phinney et al., 1977) and listed in table 5.3. Following earlier work, for simplicity all efficiencies En were taken to be 093 (Rao et al., 1979; Williams et al.,1979).

The closures calculated from these data are plotted against extraction ratio in figure 5.23. (Data for the human body are included for purposes of comparison.) Note that 100% closure (C = 1) is achieved for the "U.S. Industrial" estimate (84 elements of the spaceprobe "seed") at R = 2984; for "Demandite" (28 elements) at R = 1631; and for the appendix 5E "LMF" (18 elements) at R = 45. This suggests that the fewer the number of different elements, and the more common and more efficiently extractable are Mthe elements the factory system needs for replication to occur, the lower will be the total mass of raw materials which must be processed by the LMF.

Note also that in all three cases, virtually complete (>90%) closure is achieved for extraction ratios of 2 to 14. The incremental gains in closure after 90% are purchased only at great price—from 1 to 3 orders of magnitude more raw materials mass must be processed to achieve the last bit of full materials autonomy. Two conclusions may be drawn from this observation. First, for any given SRS design it may well be more economical to settle for 90-95% system closure and then import the remaining 5-10% as "vitamins" from Earth. Second, in those applications where 100% closure (full materials autonomy) is desirable or required, great care must be taken to engineer the self-replicating system to match the expected input substrate as closely as possible. This demands, in the case of quantitative materials closure, a design which minimizes the value of R, thus optimizing the use of abundantly available, easily extractable elements.






Al Ca Cr

Fe K Mg Mn Na O P S si Ti

Ag Ar As Au B

Ba Be Bi Br C

Cd Ce Cl Co Cs Cu Dy Er Eu F

Ca Cd Ce H

He Hf Hg


















45.2 ppb

0.800 ppm

0.206 ppm

2.66 ppb

4.78 ppm

195 ppm

2.63 ppm

3.19 ppb

0.178 ppm

104 ppm

0.197 ppm

48.8 ppm

25.6 ppm

40.3 ppm

0.392 ppm

14.4 ppm

15.3 ppm

9.24 ppm

1.77 ppm

174 ppm

4.99 ppm

14.3 ppm

0.626 ppm

54.8 ppm

28.5 ppm

7.77 ppm

0.019 ppm




In Ir La Li Lu Mo N

Nb Nd Ne Ni Os Pb Pd Pr Rb Re Rh Ru Sb Sc Se Sm Sn



la Tb Te Th Tl Tm U V W Y Yb Zn Zr



3.73 ppm

2.00 ppb

32.9 ppb

6.32 ppb

17.2 ppm

129 ppm

1.22 ppm

0.520 ppm

95.4 ppm

19.6 ppm

38.2 ppm

2.75 ppm

169 ppm

12.9 ppb

3.11 ppm

12.3 ppb

7.20 ppm

3.21 ppm

1.36 ppb

0.192 ppm

0.231 ppm

22.1 ppb

48.8 ppm

0.306 ppm

109 ppm

0900 ppm

167 ppm

1.26 ppm

2.58 ppm

0.0545 ppm

2.50 ppm

1.61 ppb

1.42 ppm

0.805 ppm

114 ppm

0.358 ppm

84.2 ppm

8.40 ppm

23.4 ppm

311 ppm



1 t0 102 103 104 105

EXTRACTION RATIO(R), kg lunar raw materials/kg useful output

Figure 5.23.- Quantitative materials closure data for various self-replicating systems.



5.3.7 Conclusions

The team reached the following major conclusions regarding the feasibility of self-replicating machine systems:

• The basic concept of physical machine systems capable of self-replication appears credible both from a theoretical and a practical engineering standpoint.

• It is reasonable to begin designing replicating systems based on current knowledge and state-of-the-art technology, but final design definition will require significant further research.

• Complete systems closure is achievable in principle, though partial closure may be more feasible from an economic and pragmatic engineering standpoint in the near term.

• It is feasible to begin immediate work on the development of a simple demonstration SRS on a laboratory scale, with phased steps to more sophisticated levels as the technology is proven and matures.

5.4 Applications

Having shown that machine SRS is, in principle, both theoretically possible and feasible in terms of engineering systems design, their usefulness in some economic or commercial sense remains to be demonstrated. That is, what might such systems permit humankind to do that could not be done before? The main advantage in using SRS over other methods of space exploration and industrialization is that a very large capability for performing any desired task can be rapidly achieved at arbitrary remote locations, starting with a relatively small investment of time, money, energy, and mass in the original "seed" mechanism.

The team has identified four general criteria for determining the most probable and profitable application of replicating systems technology:

(1) A large number of identical or similar products is required;

(2) Excessively long production periods for alternate approaches are required;

(3) Raw materials or parts are available onsite; and

(4) Sufficient physical space is available for replication.

Each of these criteria should be applicable, or largely so, in a specific case before the use of SRS technologies is considered.

Replicating systems will find many applications on Earth, in near-Earth and lunar space, throughout the Solar System, and in the interstellar realm, for both exploration and utilization. SRS also provides a number of fascinating applications in basic and applied research in automata theory, theoretical biology, experimental evolution, and machine intelligence and robotics architecture.

5.4.1 Terrestrial Applications

The early development of replicating systems technology on Earth will be the history of modern industfial automation. The United States at one time enjoyed the highest productivity in the world, and still partakes of the prosper ity that that has brought. Recently, however, competition Q from other nations who are more rapidly automating their I industries is seriously eroding the U.S. position of leader ship. The resulting economic forces are impelling domestic industry to accelerate the automation of its factories.

The space program is viewed by many as a high technology venture which predominantly makes use of computers, robot spacecraft, and other trappings of automation. In reality, NASA's activities are strongly people intensive. For example, large teams of trained technicians and scientists are required to operate a robot space probe by remote control. The same economic forces at work in the marketplace are forcing NASA to rethink its traditional way of doing business. Not only will there be more automation in the space rogram for this reason, but also there will be missions that are difficult or impossible to conduct without using advanced machine intelligence and robotics technologies. The harsh environment of space, the significant costs of life support systems for human beings and of "man-rating" space systems for safety, and the communications problems caused by the immense distances involved in interplanetary travel have given NASA additional incentives to develop systems of total automation beyond those commonly employed in industry. The sheer magnitude of many potentially interesting missions requires massive automation.

Accordingly, NASA should strongly participate in automation research and development in anticipation of spinoffs to industry of great potential value. The agency also should closely monitor industrial R&D efforts, remaining alert for new developments on the commercial front which might prove beneficial to the space program. The infusion of NASA funds at critical points could allow the agency to exert subtle influence on industrial development so as to provide for NASA's special needs at less cost than an independent program to achieve the same ends.

Similarly, the Department of Defense (DOD) is embarking upon an ambitious program of industrial automation. The aim is to produce war materiel in the most economical and flexible manner possible, and to shorten the time between concept and field deployment of weapons systems.

Much of the DOD effort will produce results useful for the space program. To take maximum advantage of this, NASA should maintain close liaison with DOD and should join in various cooperative efforts in areas of overlapping interests.

Computer-aided design {CAD), manufacturing (CAM), and testing (CAT), and robotics. Automation for replication will require extensive application of computer science and robotics. At the initial stage of development, and during periods when repair or reconstruction operations must be performed, computers can be used in many ways to aid the design process (CAD). They are excellent for generating and maintaining documentation. Computer-executed graphics are invaluable in assisting human operators to visualize complex objects in the absence of a real, physical construction. Simulation using computer models is used in place of, or as a cost-saving adjunct to, physical models or prototypes. Recent developments in machine intelligence research has made far easier the complete automation of the entire design process. Ultimately, the capability will exist for a human to carry on a dialog with a computer system in which the person merely defines the functional specifications of the desired product and the computer determines the remaining design details autonomously.

Computers have been used in manufacturing (CAM) for more than two decades. The most common modern application is business data processing. Computerized inventory control and scheduling are two promising uses rapidly gaining prominence today. Process control using analog computers began many years ago in chemical plants, steel mills, and paper mills. Newer facilities rely instead upon digital computing. An important subset of process control is numerical control (N/C) of machine tools, with instructions traditionally recorded on punched paper tape. Today it is feasible to connect N/C machine tools directly to a computer able to generate and store instructions in electronic memory, and increasingly this is being done, especially in the aerospace industry.

Computers can also be used to great advantage in the testing of products (CAT). (This is distinguished from measurements of process variables, which is considered a process control function.) Highly complex products such as microprocessor integrated circuits cannot realistically be tested without the aid of computer technology. A standard interface protocol (the IEEE488 bus) has been defined for the interfacing of test instrumentation to a host computer.

In the context of a factory, robotics generally is understood to refer to materials handling and assembly functions. Typical operations include loading/unloading machine tools and spot-welding automobile bodies. Hard automation (special-purpose robots of very limited versatility) commonly are used in applications requiring high volume output. But computer-controlled general-purpose robot manipulators are becoming increasingly popular, as exemplified by the rather anthropomorphic PUMA device (a robot arm system manufactured by Unimation).

Replicative automation. CAD, CAM, CAT, and robotics technologies could be combined to produce an almost totally automated factory. The Department of Defense has instituted an ongoing program designed to promote this very concept, called Integrated Computer-Aided Manufacturing or ICAM. The technology now exists to design integrated circuits in one location (CAD), then fabricate the masks for microelectronic manufacture in another (CAM) under the direction of several intercommunicating computers. Further developments and advances in ICAM techniques are imminent.

In a very real sense, an industrialized nation is a symbiotic self-replicating, growing "organism" consisting of humans and machines working together. At the beginning of the industrial revolution the "organism" consisted chiefly of human beings, who, aided by a few machines, performed logical and physical functions. In later years more and more of the heavy and most dangerous work was delegated to machines. As ICAM increasingly enters the mainstream of industrial automation, the logical processes of man-machine manufacturing "organisms" will begin to be taken over by sophisticated computer systems and the physical functions will be dominated by commercial robot devices.

When ICAM techniques are directed toward the production of components of their own systems (CAD, CAM, CAT, and robot machines), a regenerative effect occurs in which each generation of automated factories is cheaper to construct than the preceding one. By the time this regeneration, which has been termed "superautomation" (Albus, 1976), is achieved on Earth, there may be very little human intervention in the replication process except for supervisory and top-level guidance functions. The final step in achieving totally autonomous machine replication requires the replacement of the human top-level managers with computers and turning over any remaining physical tasks to robot devices.

The near-term removal of all human intervention from the industrial "organisms" on Earth is highly unlikely. Certainly people may want to continue to perform various logical and physical functions for social or psychological reasons, and man may always remain the decisionmaker in control of which products are produced. Certain tasks are likely to prove more difficult to automate than expected, and human beings will continue to perform these jobs for economic reasons for a long time to come. Superautomation on Earth will proceed only as far and as fast as is economically advantageous.

The long-termfuture almost certainly will see the development of full replicative automation capability on Earth. Whether it is economical remains an open question at present. The main advantage of pure machine replicating systems over man-machine symbiotic systems is that autonomous factories can be sent to locations where there is not, or cannot be without great expense, a population of human workers adequate to operate and maintain the factory complex.

Prime candidates for terrestrial replicating systems applications will most likely be mass-produced products for use in inaccessible or hostile places requiring large spaces to perform the specified tasks. Possibilities include large photovoltaic arrays for centralized power plants in the southwestern regions of the United States (Leonard, in-house document, Bechtel Natl. Inc., San Francisco, Calif., 1980), desert irrigation and soil conditioning equipment covering vast areas, agricultural or military robots, ocean-bottom roving mineral retrievers and seawater extractors patrolling the vast continental shelves, or solarpower satellite ground receiver (rectennae) devices. Each of these machine systems could probably be made to selfreplicate from a basic feedstock substrate, possibly even from a raw material substrate ultimately.

A few somewhat more speculative terrestrial applications have been proposed by imaginative writers. For instance, Moore (1956) suggested the idea of an artificial living plant able to extract its own nutrients from the sea. These machines could obtain energy from sunlight to refine and purify materials, manufacture them into parts, and then assemble the parts to make duplicates of themselves. Such plants could be harvested for a material they extracted or synthesized. Thus, an artificial plant which used magnesium as its chief structural material could be cannibalized for its metal content. Like lemmings, schools of artificial living machines could be programmed to swim to a harvesting factory when they reached adulthood.

Clearly there would be need for international controls and allocation of areas for production and harvesting. This would involve not only the political rights of nations but also questions of natural conservation. Social problems could arise in connection with the selection of products to be manufactured. An artificial plant might be designed to make a product useless to the plant itself. It might extract gold from seawater, refine it, and cast it into an ingot, which would be harvested as the crop from the plant. But this would be a shortsighted choice. Multiplying at an exponential rate, the gold-making plant would soon produce so much that gold would lose its scarcity value and probably end up being worth very little. An excellent candidate for production by an artificial plant is fresh water, which is needed in great quantities in various parts of the world.

Dyson (1979) suggests a small self-reproducing automaton well adapted to function in terrestrial deserts. It builds itself mainly out of silicon and aluminum which it extracts from ordinary rocks wherever it happens to be. Its source of energy is sunlight, its output electricity and high-tension transmission lines. There is bitter debate in Congress over licensing this machine to proliferate over our Western states. The progeny of one robot can easily produce ten times the present total power output of the United States. Legislation is finally passed authorizing the automaton to multiply, with the proviso that each machine shall retain a memory of the original landscape at its site, and if for any reason the site is abandoned the device is programmed to restore it to its original appearance.

After its success with the rock-eating automaton in the United States, the company places on the market an industrial development kit, designed for the needs of developing countries. For a small down payment, a country can buy an egg machine which will mature within a few years into a complete system of basic industries together with the associated transportation and communication networks, custom made to suit the specifications of the purchaser. The vendor's guarantee is conditional only on the purchaser's excluding human population from the construction area during the period of growth. After the system is complete, the purchaser is free to interfere with its operation or to modify it as he sees fit. (A technological spinoff is the Urban Renewal Kit—a city's architects and planners work out a design for urban rebuilding, then the kit is programmed to do the job for a fixed fee.)

Theodore Taylor calls all such devices "Santa Claus Machines" because of their almost "magical" behavior (Calder, 1978). In his version of SRS, a fully automatic mining, refining, and manufacturing facility gathers scoopfuls of raw lunar materials and then processes them by means of a giant mass spectrograph with huge superconducting magnets. This device converts mined material into an ionized atomic beam which is deflected by the magnetic field. Lighter elements curve more than heavier atomic species, so the material is sorted into stockpiles of constituent elements atom by atom. To manufacture any item, the Santa Claus Machine selects the necessary metals and plastics, then vaporizes and sprays them onto a mold. Instructions for manufacturing, including directions for adapting to new processes and replication, are stored on magnetic tapes in the machine, perhaps activated by radio command from Earth. Conceivably, costs eventually could fall to zero; and if the workload grows too large, the machine simply reproduces itself.

5.4.2 Near-Earth and Lunar Space Applications

While terrestrial self-replicating systems may be limited for some time to coevolution with Earth-based industry constrained by normal economic factors, the prospect for extraterrestrial applications is quite different. The difficulty of surmounting the Earth's gravitational potential makes it more efficient to consider sending information in preference to matter into space whenever possible. Once a small number of self-replicating facilities has been established in space, each able to feed upon nonterrestrial materials, further exports of mass from Earth will dwindle and eventually cease. The replicative feature is unique in its ability to grow, in situ, a vastly larger production facility than could reasonably be transported from Earth. Thus, the time required to organize extraordinarily large amounts of mass in space and to set up and perform various ambitious future missions can be greatly shortened by using a self-replicating factory that expands to the desired manufacturing capacity.

In the not-too-distant future such facilities could be sited either in Earth or lunar orbit, or on the surface of the Moon. The chief advantages of orbital factories are nearzero gravity, absence of lunar dust or atmosphere, convenience in choice of orbit, proximity to Earth (relative ease of transport of finished products), and unobstructed view of virtually the entire celestial sphere. For some applications, however, the lunar surface may be the preferred location. Many manufacturing processes require at least small amounts of gravity, and the availability of solid ground for physical support may be important too. The main advantage to factories on the lunar surface is that the raw materials to be processed into finished products are right at hand—only relatively low-mass final products need be lifted from the lunar surface, rather than bulky raw materials as in the case of an orbital factory. The Moon can also be used as a shield to block sunlight or electromagnetic interference from Earth during highly sensitive observations.

The useful applications of replicating factories with facilities for manufacturing products other than their own components are virtually limitless.

Manufacturing. Huge solar power satellites with dimensions 1-10 km on a side could be constructed in Earth orbit by a fleet of free-flying assembly robots or teleoperators manufactured by a replicating factory complex using material from the Moon. Components for very large structures, including communications, storage, recreational, penal, or even military platforms could be fabricated, and later assembled, by an SRS. Another exciting mass-production possibility is the notion of orbital habitats, or "space colonies" (O'Neill, 1974, 1976), by which increasingly large populations of human beings could be safely and comfortably maintained in a support capacity for the space program. Additionally, a replicating factory could build more copies of itself, or new variants of itself capable of manifesting different behaviors and producing different outputs, in almost any desired location. Possible useful output of such facilities already has been summarized in section 5.3.4.

Observation. Exceedingly large sensor arrays for Earth or astronomical observations could be rapidly constructed from nonterrestrial materials by a self-replicating manufacturing facility. This technology could be used to make feasible such advanced missions as optical extrasolar planet imaging (using millions of stationkeeping mirror assemblies arranged in an array with an aperture diameter on the order of kilometers); complex multisensor arrays; very large, high-resolution x-ray telescopy; and other self-organizing optical or radio telescopic arrays of grand proportions to permit such ambitious undertakings as galactic core mapping, continuous observation of large numbers of passive fiducial markers for Earth crustal plate motion monitoring, and various SETI (Search for Extraterrestrial Intelligence) observations including beacon acquisition, radio "eavesdropping," or, ultimately, active communication. Automated mass production will make possible arrays with heretofore unattainable sensitivity and spatial resolution.

Experimentation. Replicative automation technology will permit a tremendous expansion of the concept of a "laboratory" to include the Earth-Moon system and ultimately all of the bodies and fields in the Solar System. A number of grand experiments could be undertaken which would prove too costly if attempted by any other means. For example, an Earth orbital cyclotron could be constructed as a series of thousands of robot-controlled focusing coils and stationkeeping target assemblies within the terrestrial magnetosphere, with operating energies possibly as high as TeV for electrons and GeV for protons. Additional experiments on magnetospheric propulsion and energy generation could be conducted by free-flying robot drones manufactured on and launched en masse from the lunar surface. Gravity field probes, including mascon mappers and drag-free satellites, could be coordinated to perform complex experiments in kinematics, special and general relativity, and celestial mechanics. Investigations of artificial in situ lunar crater formation dynamics, solar wind composition and utilization, unmanned ecological simulation modules, and isolation or "hot lab" module manufacturing for conducting dangerous experiments with explosive, radioactive, or biologically engineered materials are still further possibilities.

Exploration. The Moon is largely unexplored. A growing, self-replicating factory could be reprogrammed to massproduce modified mining or other mobile robots, including orbiters and rovers, for detailed investigation of the lunar surface. This would augment orbital sensing and intelligent image processing systems (see chap. 2) around the Moon, and could be linked to lunar subsurface explorers and other automated surface prospecting equipment to assist in new resource location, colony siting, and the further acquisition of scientific knowledge. Subselene or subterrene (see discussion of the "Coal Mgle" in Heer, unpublished draft notes, Pajaro Dunes Workshop, 1980) mining robots could burrow deep into the lunar or terrestrial crust in search of pockets or veins of useful substances, and then dig them out. A self-replicating manufacturing facility could produce thousands of meter-long robot rovers equipped with cameras, core samplers, and other instrumentation which could survey the entire Moon —or any other planet, for that matter—in just a few years. Such exploration would take a century by more conventional methods. Similarly, due to the low gravity, lack of atmosphere, and relative abundance of energy and raw materials, the Moon is an excellent location for the construction and launching of future generations of interplanetary exploratory spacecraft.

Human resources. The augmentation of human services and the extension and safety of the human habitat is yet another near-term application of self-replicating systems. In principle, it is possible to construct a completely autonomous lunar-based facility, but it may turn out to be inefficient or uneconomical in the future unless a few human beings are present onsite to handle unforeseen problems with the machinery. (Humans are the most compact and efficient general-purpose self-replicating systems of which we have certain knowledge.) Initial crew quarters and supplies can be transported from Earth, but much larger and more pleasant living accommodations could be manufactured in situ by lunar or orbital replicating systems. The inexpensive mass-production of habitation and agricultural modules (or their components) could help open the door to more extensive lunar and space colonization by people, including recreational, industrial, medical, and educational uses, especially because of the abundant solar energy and the expected ability of replicating factories to manufacture and implement a low-cost lunar-surface-to-orbit launch capability. A comprehensive, highly sophisticated automated astronaut search and rescue system may also become necessary as the human population in space begins to grow, with system components mass-produced by SRS.

Presently, there are about 6000 known and tracked pieces of debris orbiting the Earth at various altitudes and inclinations, and countless additional shards which lie below observational thresholds in near-Earth space. These represent an ever-increasing danger of collision with spacecrah. Debris-catchers or "scavengers" mass-produced by SRS technology could be automatically launched into various Earth orbits, seek out and recognize space debris, report ephemerides in the case of satellite-like objects to avoid destruction of operational equipment and, upon go-ahead, collect the debris. Scavengers would be programmed either to enter the Earth's atmosphere after a specified time in orbit and self-destruct, or to return their collections to orbital manufacturing facilities for recycling of high-level components and materials to help build new robots. A more advanced network could offer protection from possible ecological disasters caused by terrestrial meteorite impacts (Alvarez et al., 1980).

Another possibility, however controversial, is meteorological and climatological intervention on both a local and global scale. A number of interesting alternatives were discussed by the participants of the recent Pajaro Dunes Workshop (Heer, unpublished draft notes, 1980), including:

• Manufacture of 107 copies of a 1-km2 sunshade to achieve global cooling, if required, which could be deployed most effectively for the polar regions at Earth-Sun L1 (losses due to image diffusion) or in

LEO (serious orbital problems).

• Deployment of 1 to 10 million copies of 1-km2 mir rors in LEO, to cause localized heating effects by

concentrating incident solar radiation.

• A system of several 1 to 10 GW microwave frequency solar power satellites to add 100 to 200 W/m2 toselected terrestrial ground spots 10 km diam, to be deployed in geosynchronous Earth orbit (GEO). The replicative manufacturing facility needed to economi cally produce such large numbers of similar system elements would make possible at least a rudimentary global homeostatic environmental control by humanity.

Given the exotic conditions prevailing on the lunar sur face and in space, and the novel materials and processes that may become available, it is highly probable that a self replicating growing lunar facility will be able to economi cally produce many goods directly for use in space and for export to Earth. What these goods might be is not now certain. However, the economic importance of the tele phone, steamboat, airplane, television, office copying machine, etc., during their early stages of development like wise were not at all obvious to most people.

5.4.3 Solar System Applications

The technology of replicating systems will become increasingly important as humanity expands its theater of operations from near-Earth space out to encompass the entire Solar System. Mankind has fallen heir to an incredible treasure trove of nonterrestrial energy and material resources (see sec. 4.2.1). It is likely that replicating machines will provide the only "lever" large enough to explore, and ultimately manipulate and utilize in a responsible fashion, such tremendous quantities of organizable matter. Lacking this advanced automation capability, most of the more ambitious Solar System applications appear uneconomical at best, fanciful at worst.

Observation. Exceedingly far-reaching observational possibilities may become feasible with the advent of SRS technology. Very large baseline interferometry (VLBI) may be attempted with components distributed across the entire Solar System, perhaps located at the stable Trojan points of the Jovian planets or their moons, providing multiplanar baselines of from 1 to 100 AU and complete spherical coverage with the use of out-of-ecliptic robot sensor devices that are mass-manufactured by replicating factories. The solar wind could also be mapped in three dimensions, and by using the entire Sun as a gravitational lens focal lengths on the order of the size of the Solar System can in theory be obtained (Ingel, 1974). This may permit simultaneous observation of the entire celestial sphere across the full spectrum of gravitational radiation using fleets of gravity-wave detectors manufactured by SRS and stationed along the focal plane. A Solar System surveillance network could be constructed to track and warn of objects approaching human habitats, facilities, or the Earth on collision courses, allowing mankind to avoid potentially severe catastrophes.

Exploration. The technologies developed for a generalized lunar autonomous replicative manufacturing facility should be directly applicable in the exploration of all planetary and satellite surfaces. One early possibility is a mission to land a single replicative "seed" on Mars which would then use local materials to produce large numbers of rovers (including, perhaps, fliers, crawlers, walkers, or rollers) and orbiters. A population of 1000 to 10,000 surface rovers each perhaps 100 kg in mass, coupled with a chain of orbital monitors, might continuously monitor and explore the planetary surface and leave stationary probes (active or passive) behind in permanent emplacements. The probes need only have lifetimes on the order of a year or so, since they could constantly be repaired and replenished by the rovers (each of which could last 10 years or more). This system would provide complete surface exploration and continuous status monitoring of all areas on the planet, including temperatures, pressures, wind velocities, seismic events and crustal creeps, meteorite impacts, surface and subsurface compositions, illumination, precipitation, and numerous other phenomena of interest. Automated balloon explorers could be mass-produced and released in Jovian atmospheres, and "trains" of deep solar probes (Heer, unpublished draft notes, 1980) could be hurled into the Sun to obtain direct information on internal conditions there.

Materials retrieval. Replicating systems would make possible very large-scale interplanetary mining and resource retrieval ventures. Nonterrestrial materials could be discovered, mapped, and mined using teams of surface and subsurface prospector robots manufactured en masse in an SRS factory complex. Raw materials could be dug up and sent back to wherever they were needed in the Solar System, or could be refined along the way and the waste slag used as reaction mass, or could be utilized in situ for manufacturing useful products which would then be exported. Atmospheric mining stations could be established on many different planets—Jupiter and Saturn for hydrogen, helium (and rare isotopes potentially useful for fusion power generation, Martin, 1978), and hydrocarbons, using "aerostats" (Parkinson, 1978); Venus for carbon extraction; Europa for water; Titan for hydrocarbons; etc. Comets could be intercepted to obtain large quantities of useful volatiles, and Saturn's rings could be mined for water-ice by large fieets of mass-produced robot craft. Heavy metals may be retrieved in great quantities from asteroids. Replicating systems might manufacture huge mining, processing, even ground-to-orbit and interplanetary transportation capabilities using local materials in surprisingly short periods of time.

The general product factory. The team has proposed the design and construction of an automatic multiproduct replicating lunar factory. The reason for the factory having multiproduct capability is to permit it to be able to respond to any changing requirements in kind or amount of product output. This leads to a still broader concept—the notion of a general product factory.

A general product factory is one which can be instructed to manufacture anything which is physically possible to make. Such a system is the physical realization of von Neumann's "universal constructor" automaton, which can construct anything constructable, given an adequate substrate and the rules of operation of his artificial cell-space universe. In the context of drawing upon planetary resources, we should think of each celestial body in terms of its menu of possible materials and the repertoire of processes theoretically available there (see sec. 4.5.4). The following questions should then be considered:

• What is the total range of things which can be made using these processes acting upon these material resources? (See sec. 5.3.6.) This query should be viewed in the broadest possible fashion, including biological as well as mechanical entities.

• Does there exist, for this planetary environment, a factory design which is capable of making all of these entities?

• Can an initial system be designed which, when introduced into the target environment, will yield such a general product factory? A few important developmental milestones are suggested in table 5.4.

The notion of a general product factory using asteroidal material was briefly considered at the Pajaro Dunes Workshop. The "Hive," as it was called, would consist of "an autonomous space island 'beehive' of independently intelligent machines . . . specialized in mining and production, experts in planning, navigation and repair." The product of the Hive would be solar power satellites, "asteroids turned into space colonies, vacuum-filled balloons of nickel floated down to a resource-hungry Earth, spaceships, telescopes, or even another Hive." The Hive was envisioned as an independent economy, using raw materials gathered from the Asteroid Belt, refined and processed with solar or fusion energy, then fashioned into useful output by robot hands. Workshop participants suggested a timetable in which the first fully autonomous replicating system could be in operation in the Asteroid Belt by 2040, commencing exponential growth with a replication time of 5 years, resulting in a total of 1000 new Hives available for production by the year 2080.



1. Design and construct a system which, when supplied only with parts and subassemblies, can duplicate itself.

2. Design and construct a system which can duplicate itself, and in addition produce some useful product.

3. Design and construct a system which, when supplied only with feedstock, can duplicate itself.

4. Design and construct a system which, when supplied with raw materials only, can duplicate itself.

5. Design and construct an automated, reprogrammable, multiproduct system which can, from raw materials, duplicate itself.

6. Design and construct an automated, reprogrammable, multiproduct system which, using only lunar materials and employing only those processes possible in the lunar environment, can duplicate itself.

7. Design and construct an initial automatic "seed" system which, if placed on the lunar surface, could unpack itself and develop into an automated, reprogrammable, multiproduct replicating system, using lunar resources and lunar processing modes only.

8. Design and construct an initial seed which can, in the lunar environment, develop and augment itself so as to become a general-product factory, relative to the lunar environment.

9. Design and construct a seed which can, in an arbitrary planetary environment, develop into a general product factory.

Buman resources. From the human standpoint, perhaps the most exciting consequence of self-replicating systems is that they provide a means for organizing potentially infinite quantities of matter. This mass could be so organized as to produce an ever-widening habitat for man throughout the Solar System. Self-replicating homes, O'Neill-style space colonies, or great domed cities on the surfaces of other worlds would allow a niche diversification of such grand proportions as never before experienced by the human species.


SRS provides such a large amplification of mattermanipulating capability that it is possible even to consider the "terraforming" of the Moon, Mars, Venus, and other worlds. Terraforming is a theoretical concept in which a planetary environment with otherwise inhospitable conditions for life is purposefully and artificially altered so that humans may live there with little or no life support equipment. The "traditional" approach is to suggest biological means, such as the proposal to seed the atmosphere of Venus with genetically tailored algae to convert excess carbon dioxide into combined carbon and free oxygen. This would have the incidental salutary effect of lowering the planetary surface temperature so that people could live unaided on the surface. However, it is not known whether biological organisms can be found or developed which are able to withstand present conditions in the Venusian atmosphere.

An alternative approach is to use nonbiological replicating systems which may be far more durable under extreme conditions. A few simple calculations reveal the approximate magnitude and duration of such an enterprise. Consider the terraforming of Mars. For simplicity it is assumed that the planetary crust is largely silicon dioxide and that a general-purpose 100-ton SRS factory "seed" which lands there can replicate itself in 1 year. In just 36 years such a system could theoretically manufacture an SiO2 reduction capability able to release 220,000 tons/sec of pure oxygen into the Martian atmosphere, which in only 60 years is sufficient to produce 4X 101 7 kg °2 . Assuming negligible leakage through the Martian exosphere, this is enough oxygen to establish a 0.1 bar breathable oxygen atmosphere planet-wide—approximately equivalent to normal air on Earth at an altitude of 3000 m (16,000 ft). This plan requires a solar power satellite system in near-Mars orbit with a total generating capacity of about 101 7 W, a network which would take less than a year for the finished replicating factory system to produce. The total material thus excavated to terraform Mars is of the order of 1018 kg SiO2, enough to fill a surface depression 1 km deep and 600 km diam. This is roughly the size of the crater Edom near the Martian equator, or Mare Crisium on the Moon.

Of course, far more efficient methods for terraforming planets may eventually be found, such as Dyson's proposal to mine the Saturnian moon Enceladus for its water-ice and return the material to Mars (Dyson, 1979). But the utility of self-replicating systems is clear, and it appears that terraforming times on the order of one century are conceivable using the SRS approach.

Technology requirements. Additional technology over and above "superautomation" (sec. 5.4.1) will be required for the highly ambitious ventures described in this section using advanced space-based self-replicating systems. The most important new technology in this regard is "closure engineering," discussed in section 5.3.6. Some of the enterprises proposed above are of such large scale that it is difficult to envision a feasible mode of operation with anything less than 100% materials and energy closure and virtually 100% information closure as well. No doubt there exist manufacturing operations which are not economically

viable candidates for total automation in terrestrial industry—in these instances the functions either must be redesigned for full automation or else people must be permanently incorporated as symbionts of a locally teleoperated or remotely human-supervised system. Manufacturing processes developed for terrestrial environments must be re-engineered to accommodate the input and production environments found in space or on the surfaces of other planets, and output streams must be sufficiently flexible to make feasible the notion of a general products factory.

5.4.4 Interstellar and Galactic Applications

Replicating systems technology is the key to exploration and human habitat expansion beyond the confines of the Solar System. Although these kinds of missions necessarily are highly speculative, and admittedly exceed the limits of current or projected technology in many areas, a consideration of possible interstellar and galactic applications is nonetheless a useful exercise because it serves to illustrate the fantastic power and virtually limitless potential of the SRS concept.

Extrasolar exploration. Before humankind can move out into interstellar space, automated probes will scout the way ahead. The distances are so large and the volumes so vast that self-replicating probes are highly desirable, even essential, to adequately and efficiently perform a reconnaissance of extrasolar star systems in a search for human habitable worlds and extraterrestrial life. A preliminary design for a self-reproducing interstellar probe has been presented in the scientific literature (Freitas, 1980a), and another study of the comparative benefits of reproducing and nonreproducing galactic exploration strategies by unmanned probes suggests that search patterns using semi-intelligent automata involving more than about the nearest 100 stars would probably be optimized (in terms of economy and productivity) if self-replicating systems are employed (Valdes and Freitas, 1980). Reproductive probes could permit the direct investigation of the nearest million stars in about 10,000 years and the entire Milky Way galaxy in less than 10^6 years, starting with a total investment by humanity of a single self-replicating exploratory spacecraft.

The problems in keeping track of, controlling, and assimilating data returned by an exponentially growing number of self-reproducing space probes are staggering. Part of the solution may lie in the use of an extremely high level of autonomy in operations management and reasoning such as discussed in chapter 3 of this report; part may lie in the utilization of high levels of abstraction in the information returned to Earth after the fashion of the World Model sensing and data-processing philosophy articulated in chapter 2. Another major piece of the solution is the development of a hierarchical command, control, and information-gathering architecture in which any given probe communicates directly only with its own parent and offspring. Control messages and exploration reports would pass up and down the chain of ancestral repeater stations erected by earlier generations (Valdes and Freitas, 1980). Certain highly critical but low probabaity- signals might perhaps be broadcast in an omnidirectional alarm mode to all members of the expanding network (and to Earth) by individual probes which encountered specific phenomena or events — such as the discovery of an extrasolar planet suitable for human habitation or a confrontation with intelligent alien lifeforms or their artifacts.

Extrasolar utlization. Before mankind can venture out among the stars, his artifacts and replicating machines must blaze the trail. Ultimately, however, one can envision freeflying space colonies journeying through interstellar space (Matloff, 1976). Upon reaching some new solar system or other convenient source of raw materials, these mobile habitats would reproduce themselves with the human passengers redistributed among the offspring colonies. The original space habitats would serve as extraterrestrial refuges for humanity and for other terrestrial lifeforms that man might choose to bring along. This dispersal of humankind to many spatially separated ecosystems would ensure that no planetary-scale disaster, and, as people travel to other stars, no stellar-scale or (ultimately) galactic-scale event, could threaten the destruction of all mankind and his accomplishments. Replicating systems may be the only rational means to attempt large-scale astroengineering projects usually relegated to the domain of science fiction, such as the construction of "Dyson Spheres" which enclose and utilize the energy output of entire suns (Dyson, 1959).

The limits of expansion. The expansion of a population of replicating systems in any environment is restricted largely by two factors: (1) replication time, and (2) maximum velocity of the outer "envelope" which defines the physical extent or dispersion of the population. No population can accrue at a faster rate than its components can reproduce themselves. Similarly, no population can disperse faster than its medium will permit, no matter how fast components are manufactured — assuming number density remains essentially constant, corresponding to continuous maximum utilization of the environment. Neither factor may be ignored during any phase of population growth.

If envelope expansion velocity does not constrain a population because components are produced only relatively very slowly, then that population will experience exponential multiplication according to:

N(T) = exp(T/t) (I)

where N(T) is the number of replicating units comprising the population at time T (replication starts at T = 0) and t is the replication time per unit, assumed constant. On the other hand, if unit reproduction is so swift that multiplication is not constrained by replication time, then the population can grow only as fast as it can physically disperse that is, as fast as the expansion velocity of the surface of its spherical outer envelope—according to:

N(T) = 4/3 77 d ( VT)3 (2)

where V is peak dispersion velocity for individual replicating units at the periphery and d is the number density of useful sites for reproduction. Expansion cannot exceed the values for N(T) given either by equations (1) or (2) at any time T, provided all replication sites receive maximum utilization as stipulated (e.g., constant number density of units).

Populations of machines expanding across the surfaces of worlds with replication times on the order of 1 year will not achieve mean envelope growth speeds in excess of a few meters per hour, even in later phases of extreme enlargement when the population of SRS covers a large fraction of the available planetary surface. This figure is well within anticipated nominal ground transport capabilities, so exponential extension should remain largely velocityunconstrained on such bodies if replication time remains constant at greater population sizes.

Similarly, three-dimensional populations of replicating systems in interplanetary space using Solar System materials and solar energy ultimately are restricted to spherical circumstellar shells where SRS units can collect virtually all energy radiated by the Sun. If a "Dyson Sphere" of 100-ton replicating "seed" units is assembled near the orbit of Earth, approximately one terrestrial mass is required to manufacture the more than 101 9 individual units needed to completely enclose the star. But maximum expansion velocity even in this case never exceeds about 100 m/sec, hence interplanetary replicating systems as well in theory may spread at purely exponential rates.

In the interstellar realm, however, the situation is far more complex. Depending on the maximum dispersal velocity and interstellar probe replication time, either equation (1) or (2) may control. Figure 5.24 compares pure exponentiation and dispersal speed effects for t = 1 year (see sec. 5.3.4) and t = 500 years (Freitas, 1980a), and for V = c (since the theoretical maximum envelope expansion rate is the speed of light) and V= 10%c (Martin, 1978) for an assumed homogeneous stellar distribution of "habitable" star systems (taken as 10% of the total) in the galactic disk. In most cases, exponential multiplication soon is halted by the speed-of-light barrier to dispersion, after which the SRS population expansion proceeds only polynomially.

Technology requirements. In order to sustain the expansion of a potentially infinite replicating system, new dispersal mechanisms must be developed. Initially, self-replicating machines or their "seeds" must be capable of motion across a planetary surface or through its atmosphere or seas. Later, interplanetary, interstellar, and, ultimately, intergalactic dispersal mechanisms must be devised. Supplies of energy, stored and generated, must be established if extrasolar spacecraft are to survive in the depths of interstellar space far from convenient sources of power (such as stars) for a major portion of their lives. The technologies of command, control, and communication over stellar and galactic distances ultimately also must be developed.



Figure 5.24.- Limits to exponential and polynomial expansion of self-replicating interstellar probe populations dispersing throughout the galactic disk.

5.4.5 Applications to Basic Research

In addition to specific applications of replicating systems technology to future missions in space, a number of applications to basic research in biology, computer science, and automata theory have been identified by the team. These are discussed below.

Automaton theory. Automaton theory is the abstract and precise study of all mechanistic devices and processes. At times this has been restricted to the theory of discrete and deterministic machines with a fixed finite number of states. In this narrow sense it is the abstract mathematical counterpart of physical devices such as existing digital computers. In the broadest sense, though, automaton theory can include the study of all mechanisms, discrete or continuous, deterministic or probabilistic or even indeterministic, with a fixed, variable, or indefinitely large number of possible states. Included in this wider definition is the notion of devices which can alter the number of their states by growth or by contraction in respect to certain of their organs, much like the way a Turing machine or a pushdown automaton (or a linear-bounded automaton) can increase or decrease the number of its states by increasing or decreasing the length of its memory tape—but also can grow by increasing or decreasing the numbers of its more active computing components. This is representative of machines which can construct or dismantle other machines.

These machines can not only increase their memory capacity but can augment their computing power by the construction of additional active computing organs (registers, control units, etc.) and by constructing machines separate from themselves, including duplicates of themselves. Von Neumann had begun to develop a general and logical theory of automata which would have embraced all these machine types. Automaton theory has, however, never achieved the generality he sought, at least not in the sense he seems to have intended.

The very general theory of automata has become increasingly abstract, moving from describing mechanistic processes in terms of algebraic concepts such as groups and semigroups to employing category theory, the most abstract and general of algebraic theories. Although a certain level of understanding of what mechanisms might exist has thereby been developed, the applicability of such approaches to the design of complex systems of automata is very slight or nonexistent. In this regard, von Neumann once lamented that "... at a great distance from its empirical source, or after much abstract inbreeding, a mathematical subject is in danger of degeneration.... Whenever this stage is reached, the only remedy seems to me to be the rejuvenating return to the source—the reinjection of more or less directly empirical ideas." (von Neumann, 1966).

It may be that an effort to actually design and implement a system of machines which can construct more machines like themselves would encourage theorists again to attempt to develop a very general automaton theory including as a part of its subject matter the spatial and communicatory interactions of vast and increasing numbers of submachines. (Perhaps the automatic telephone system provides us with the closest physical analogy to such systems, aside from the analogy of human societies themselves.)

Such a theory would enable one to ask what is the best organization of a system of (potentially) arbitrarily increasing numbers of active components, arranged in various spatial geometries. How might the interacting activites of vast numbers of submachines be optimized? What rules of interaction and of interconnection can be imposed on such a system in order to attain efficient and stable behaviors? What are the safest physical and behavioral interactions, and which lead to instabilities and pathologies?

A general theory would also take as part of its subject matter the flow of parts and materials. It might, like the von Neumann cellular system, treat the creation and flow of materials and the movement of machinery as a form of information flow. It might distinguish information, materials (raw materials, feedstock, and parts) and the movement and siting of machines, but treat them in an identical format so that tradeoffs and exchanges in these categories could be computed (while retaining the essential differences among these types of flow important to the working of the system). The theory would answer such questions as: When will more information be the best substitute for more parts or more feedstock? Under what conditions in the vast assemblage of machines should parts be made anew, from raw materials and feedstock, and when should information or already finished parts be employed to the same purpose? When should machines which are likely to fail be abandoned? When should machines in the assemblage which are still in good condition nevertheless be shut down, moved, sacrificed for parts or dismantled, or sealed off? Under what local and global conditions should submachines be retired, repaired, or replaced?

Theoretical biology. Machines which can construct machines, and machines which can construct replicas of themselves, display behavior which in many ways is analogous to that of natural organisms. Furthermore, as machines are designed to examine their own structure and the structures of other machines, to repair themselves and other machines, and generally to become more autonomous and more reliable, the analogies become even more apparent.

The ways in which machines carry out these processes of growing, repairing, regenerating, and reproducing may or may not be similar to those carried out by natural organisms—which, in many cases, are not yet even known.

One goal of theoretical biology is to develop an under standing of the mechanisms of living systems, to the point where these systems can be characterized in a precise mathematical fashion (Miller, 1978). To attain such a char acterization one needs a good intuitive feeling for the full possible range of lifelike forms. For example, a theory of biology that takes as its subject matter only Earth-evolved forms would be as unlikely to be capable of providing adequate explanation for non-Earth forms as were attempts to account for the forms of extant organisms quite apart from their extinct progenitors. It seems, therefore, likely that an adequate explanatory theory of biology of any elegance and simplicity must embrace not only all biological forms which presently exist, but all those which have ever existed, or will exist, or could exist. Indeed, the proper subject matter for a true theoreti cal biology in its broadest sense would be the study of life like behavior wherever it occurs whether now, or in the past, or the future; whether on Earth or elsewhere in the universe and whether it is exemplified in artificial or natural forms (Freitas, 1980b), a field of study termed "xenobiol ogy" by one author (Freitas, 1981). This suggests that research on complex automata able to reconstruct, repro duce, and repair themselves might serve as a fertile source of hypotheses as to the logical control and organizational aspects of how living organisms in fact carry out these processes. Such explanatory hypotheses can apply to life like systems generally and have the advantage that they are likely to be simpler and more elegant than the necessarily ad hoc explanations of behavior for the particular organ isms of particular worlds, at particular times. Thus, research in self-growing and self-replicating machine systems can be viewed as a contribution to, even as F a central part of, a true theoretical biology which takes as its subject matter not merely the evolved, naturally occur ring living organisms of Earth, but lifelike mechanisms, natural or artificial, having existed or possible, wherever in L the universe they might arise.

Design of biological and hybrid organisms. The forms and processes of artificial organism-like systems are not bound to follow the particular structure and logical organizations of known naturally evolved organisms. As the design of increasingly complex artificial systems capable of drawing materials and energy from natural surroundings and possessing more and more organism-like properties proceeds, it may become apparent that there are artificial organism functions which, if embodied in biological organisms, would be of value. With advances in "genetic engineering" it may become possible to create new biological forms, possessing the desired features.

Just as the design of artificial mechanisms can be inspired by contemplation of evolution's apparent solutions to various design problems, so might new biological systems also be created, drawing upon designs originally conceived for artificial systems—a kind of inverse bionics. Taking this a step further, one can envisage as a research goal the gradual elimination of the perhaps arbitrary line now drawn between artificial and natural organisms, and the consideration of a more deliberate systematic investigation of the creation of hybrid biological-mechanical systems.

Experimental evolution. Studies of form and function in biological and artificial systems may contribute to an understanding of the design and construction of both biological and mechanical organisms. This interdisciplinary exchange should not be limited to studies of the relationship between individual classes of lifelike entities, but should also extend to studies of the consequences of large numbers of such entities interacting and competing for resources. Replications of programs and creation of new machines (including replicas), and compounds and combinations of initially existing machines, can be a feature of the proposed machine replicating systems. It seems clear that development of a science of evolving systems is needed (Miller, 1978). (This would again be a part of a very general "true" theoretical biology, which takes all possible lifelike systems as its subject matter.)

For example, one putative value of sexual over asexual reproduction is the enormously increased mobility of genetic variation in the species population. This widely available variation tends to ensure that environmental changes can be accommodated or exploited with great swiftness by at least some members of the population (Smith, 1978). In a "designed" universe, one is free to consider the advantages (if any) of three or more sexes (Freitas, 1980c; Smith, 1978) or of the consequences of other, even more radical redesignings of existing natural systems. In particular, the actual behavior of largely autonomous growing replicating machine "species" with differing capabilities and reproducing strategies certainly should be an object of study by evolutionary biologists who might be able to predict the forms which would persist and come to dominate in systems left unperturbed by external pressures and commands.

The existence of large interacting populations of entities whose "genetics" are precisely known, bu. whose global behavior over time cannot readily be predicted, may be of great experimental value to evolutionary biologists. At present, computer simulation is the usual tool of choice for such problems. However, if the physical creation of machine populations becomes sufficiently inexpensive,


Figure 5.30.- Schematic of simple robot replication exponentiation.

5.6.5 Initiation of the Three Approaches

Section 5.6.1 proposed a Generalized Lunar Autonomous Replicating Manufacturing Facility as a strawman mission to bring NASA up to speed in advanced automation technology, in particular the technologies relevant to SRS. Succeeding discussions dealt with the top-down, bottom-up, and middle-out approaches to achieving such a facility. Figure 5.31 shows how the three approaches relate in achieving the overall program goals.

The various approaches, stages, and phases fit together in coordinating industry, university, and NASA expertise in the fields of self-replicating systems and extraterrestrial utilization of materials. Once the benefit of this expertise has been obtained, the mission design and realization of a GLARMF can begin. A proposed timeline for the development and demonstration of replicating systems may be found in figure 5.32.

It would be most advantageous for NASA to begin activities in advanced automation research and development at the present time. By beginning now, NASA will be in a strong position to seriously demonstrate and deploy advanced autonomous systems after the Shuttle becomes operational. At that time the Shuttle will not be making such large demands on the NASA budget and a means of transporting the systems will exist. This will also be an opportune moment to begin attracting a cadre of bright, enthusiastic robotics practitioners by offering them both the chance to enter robotics as it begins to take off as a well defined field and the opportunity to contribute to the development of what may be one of mankind's most far-reaching achievements.

The 1980s may be the "Decade of the Robot." Many lay magazines such as Newsweek and Business Week have run major articles on robotics. Professional journals such as Science and IEEE Computer have also published prominent articles on robotics. In 1980 a new professional organization, the American Association for Artificial Intelligence, was created. Its first "Annual National Conference on Artificial Intelligence" was held at Stanford University during the Summer Study which produced this report. Momentum is gathering as the robotics and automation wave begins to take form. The team suggests that NASA ride the crest of this wave rather than stand back and be engulfed when it breaks.

NASA will be able to use the results of programs in automation sponsored by other government organizations; however, the space agency has some unique requirements which are unlikely to be met unless NASA takes an active role in automation research and development. Failing this, the infusion of relevant new technology into NASA programs can be expected to be slow. A recent report concluded that "NASA is 5 to 15 years behind the leading edge in computer science and technology" (Sagan, 1980). It was recommended that NASA take a more active role in computer science research and development to remedy the problem. The same phenomenon can be expected to occur with automated manufacturing. Unless NASA performs in-house R&D and sponsors university and industry R&D, significant infusion of the automation technology NASA needs in its future programs is unlikely. Many of NASA's unique needs cannot be satisfied unless the agency takes an active role in the development of automated manufacturing.

Figure 5.31.- Relationship of three R&D approaches to SRS development and demonstration.












Figure 5.32.- Suggested timeline for development and demonstration of replicating systems technologies.

In particular, NASA is more concerned with total automation — the use of either teleoperated or completely autonomous systems than are most government agencies or industry. NASA should, therefore, perform or sponsor significant amounts of research and development in total automation. Special emphasis should be placed on the nonterrestrial environment, where such factors as vacuum or unusual atmospheres, nonterrestrial raw materials, and various gravity fields down to zero-g might be used to advantage (and must be dealt with in any case).

Replicative automation - the automation of automation — wherein robots are used to produce robots will happen in the terrestrial environment for economic reasons. There is, however, a synergism between replicative automation and total automation which has special relevance for NASA. For operations such as lunar manufacturing or planetary terraforming exceedingly large amounts of mass must be manipulated in the extraterrestrial realm. Because of high transportation costs due to the Earth's gravitational influence, the most desirable method of achieving these missions is to send a "seed"—a replicative manufacturing facility with the minimum necessary closure for remote autonomous replication and repair—to distant operational sites. The seed can then produce, from in situ materials, and perhaps through several generations, the required machines to perform desired tasks. If the seed can manufacture propulsion systems and other seeds, then significant interstellar exploration becomes a very real possibility (Freitas, 1980a).

5.7 Conclusions and Recommendations

The Replicating Systems Concepts Team reached the following technical conclusions:

• The theoretical concept of machine duplication is well developed. There are several alternative strategies by which machine self-replication can be carried out in a practical engineering setting.

• There is also available a body of theoretical automation concepts in the realm of machine construction by machine, in machine inspection of machines, and machine repair of machines, which can be drawn upon to engineer practical machine systems capable of replication.

An engineering demonstration project can be initiated immediately, to begin with simple replication of robot assembler by robot assembler from supplied parts, and proceeding in phased steps to full reproduction of a complete machine processing or factory system by another machine processing system, supplied, ultimately, only with raw materials.

The raw materials of the lunar surface, and the materials processing techniques available in a lunar environment, are probably sufficient to support an automated lunar manufacturing facility capable of self-replication and growth.

Tentative design of a lunar manufacturing facility capable of self-replication can begin, when current knowledge and state-of-the-art technologies are employed, but final design awaits the initial results of the demonstration-development program. Significant further research in lunar materials processing and in the design and operation of automated factories, should be conducted at once.

In addition, the team considers that the replicating sysns concept, if implemented, can have the following important consequences:

• It will accelerate the design and development of sophisticated automated assembly techniques useful in carrying out future NASA missions.

• It will accelerate the design and development of improved automated assembly and processing techniques applicable to the problems of achieving increased Earth-based manufacturing productivity.

• By establishing an automated, growing, selfreplicating, multipurpose, multiproduct lunar manufacturing facility, NASA capacity for space exploration and research can be enormously expanded and permanently enhanced with only modest continuing expenditures.

• The virtually cost-free expansion of mining, processing, and manufacturing capacity, once an initial investment is made in an autonomous SRS, makes possible the commercial utilization of the abundant energy and mineral resources of the Moon for the benefit of all mankind.

• The establishment of a replicating lunar manufacturing facility can be a stepping stone to the design and construction of replicating manufacturing complexes on the surfaces of other planets. These new complexes themselves may be products of autornated, self-replicating manufacturing facilities located elsewhere.

Finally, the team offers the following general recommendations to NASA in furtherance of the basic objective of achieving practical self-replicating, growing machine systems in the shortest reasonable time:

(1) NASA should begin immediately the development of a simple demonstration replicating system on a laboratory scale, with teleoperated to fully automated phased steps to higher levels of sophistication as the technology is proven and matures.

(2) The space agency should support significant further research in lunar materials processing, lunar resource exploration, and the design and operation of automated manufacturing facilities.

(3) NASA should implement the design, development, and construction of an automated, multiproduct, remotely reprogrammable lunar factory system to begin operation on the lunar surface early in the next century.

(4) Studies should be conducted of scenarios in which a succession of replicating, multipurpose, multiproduct, automated, remotely reprogrammable factories could be placed in orbit or on other planets, these systems perhaps themselves products of earlier established nonterrestrial replicating facilities.

(5) NASA should initiate additional studies of the social, political, military, and economic consequences of the proposed work, and of various other as yet unresolved issues and concepts (see app. SK).

5.8 References

Albus, James A.: People's Capitalism: The Economics of the Robot Revolution. New World Books, College Park, Maryland, 1976.

Alvarez, L. W.; Alvarez, W.; Asaro, F.; and Michel, H. V.: Extraterrestrial Cause for the Cretaceous-Tertiary Extinction. Science, vol. 208, 6 June 1980, pp. l095-1108.

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