The idea of dropping raw materials from the Moon into the deep gravity well of the Earth is not new. The first description of a primitive implementation of this idea (called impact reaction engine) was mentioned in:

Roger Arnold and Donald Kingsbury, "The Spaceport, Part II," Analog Science Fiction/Science Fact, Vol. 99, No. 12, December 1979, pp. 69-74.

Tethers in Space Handbook (W. A. Bacarat & C. L. Butner, Bantam Books 1986) describes various methods including towers and slings. These methods are not economical because they require at least one year to transport cargo equal their own mass.


This is my favorite system of Earth-to-orbit transportation in the long term. It can bootstrap itself, which means that a small, one ton system can lift into space a much more massive system. The lunavator bolo relay is trivial when compared to rocket launchers, and it may reduce the cost of space access to a few dollars per kilogram! The system is based on orbital slings and precise maneuvers guided by GPS (Global Positioning System). There are two versions of the lunavator bolo relay: lunavator bolo rocket relay, and lunavator bolo sling relay.

The lunavator bolo rocket relay is very simple and easy to implement. It consists of a large reusable sounding rocket, terrestrial bolo with built-in electrodynamic tethers, lunar rotovator spacecraft, and lots of disposable cargo sacks.

Single tether lunavator spacecraft

Single tether lunavator spacecraft

Multiple tether lunavator spacecraft

Multiple tether lunavator spacecraft
(proposed by Henry Spencer and improved by Andrew Nowicki)

The lunar rotovator is called lunavator. It picks up sacks filled with regolith (Moon dust) from the surface of the Moon and hurls the sacks toward the Earth. The lunavator is mounted on a rotating arm which is attached to a large, rotating, toroidal greenhouse, or any large, rigid, rotating object. The arm rotates independently of the greenhouse, so it can easily change the angular velocity of the lunavator. The maximum length of the lunavator is about 200 km. When a winch reels its cargo sack in, the sack moves faster to conserve its angular momentum. This fact makes it possible to increase the orbital energy of the lunavator and the greenhouse without the need for any external thrust. It is as simple as picking the sack from the Moon, reeling it in, and tossing it backward. This maneuver increases angular momentum of the lunavator. The excess angular momentum is lost either by dragging a chain on the lunar surface, or by using gravity. The latter method consists of synchronizing periodic reeling of the lunavator with its rotation. The orbital velocity of the lunavator is only 1.6 km/s, much less than the Moon's escape velocity (2.4 km/s). When the sack is released from the lunavator, its velocity relative to the Moon is 3.2 km/s. It is gradually slowed down by the lunar gravity to 0.8 km/s (3.2 km/s - 2.4 km/s = 0.8 km/s). Gravitational pull of the Earth accelerates the sack by 11.2 km/s, which is the Earth's escape velocity. A Hall thruster guides the sack toward the bolo. When the sack is captured by the bolo, its velocity is 4.3 km/s relative to the bolo and 12 km/s relative to the Earth. (The cargo gains 3.5 km/s, which is the difference between the Earth's escape velocity, and the orbital velocity of the bolo, which is 7.7 km/s). The bolo hurls the sack backward and a moment later it picks up a payload from the sounding rocket. This maneuver is not possible unless the rocket lifts the payload to the altitude of 100 km and accelerates it to a horizontal velocity of 3.4 km/s, which matches the tip velocity of the bolo. The best design for the sounding rocket is the engine cluster. When the bolo hurls the payload forward, payload velocity relative to the Earth is 12 km/s, which is more than the Earth escape velocity. A Hall thruster guides the payload to its final destination.

If the payload is going to be used in the bolo's orbit, a manipulator riding on the bolo hauls it to the center of the bolo's mass. This maneuver has a side effect of increasing both the orbital and angular energy of the bolo. The energy is lost by converting it to electric energy, which is generated in aluminum wires secured to the bolo. This means that the wires are the electrodynamic tethers. The same wires are used as electric power supply for the manipulator which rides on the bolo. Most of the electric energy is dissipated as thermal radiation, but some may be used for other purposes. The electrodynamic tethers are very efficient, so their mass is very small in comparison with the bolo's mass. The wires must be hidden inside the bolo so that they are not short circuited by the ionosphere.

The lunavator and the bolo do not have to be made of unobtanium or buckytubes. Perhaps the best material for the lunavator and the bolo is a rope made of strong (6.5GPa) carbon fibers coated with a thin layer of aluminum and fused together in a hot press.

All the winches and manipulators are powered by electric motors. Commercial high performance direct current motors use samarium cobalt magnets and have power-to-weight ratio of about 7 watts per gram not counting the gearbox. Cooling the motors in the vacuum of outer space is a challenge, so careful design is needed to minimize the amount of heat produced by the motors. Whenever possible, the heat is absorbed by the cargo sacks. If this is not possible, the heat must be radiated to the outer space.

It may be desirable to launch cargo sacks into lunar and terrestrial orbits and use them in emergency as the momentum exchange mass.

The lunavator bolo sling relay is complex but has the advantage of consuming very little rocket propellants. It consists of one small reusable sounding rocket, two terrestrial bolo spacecrafts, one lunavator spacecraft, a small Zylon sling, and lots of disposable cargo sacks.

The lunavator is the same as the lunavator of the lunavator bolo rocket relay, but the simple bolo is replaced with two identical bolo spacecrafts which are similar to the lunavator spacecraft. One bolo spacecraft is ahead of the other spacecraft and is called the first bolo spacecraft. When the first bolo spacecraft captures the cargo sacks, a remotely controlled manipulator secures three cargo sacks to the bolo and two cargo sacks to both ends of a spinning sling which is placed at the outer tip of the bolo. Both ends of the sling rotate about its center with a velocity of about 2 km/s. When the bolo is accelerated to its maximum angular velocity, the sling and the sacks are released almost simultaneously. The first bolo spacecraft reverses velocity of the sacks and thus gains lots of orbital energy. The sling is released first. Its velocity relative to the Earth is 3.4 km/s. Before it is released, a sounding rocket lifts a payload to the altitude of 100 km and a ball is launched from a loaded balloon gun at a velocity of about 1 km/s. The balloon gun is tethered to a ship. To improve safety, the balloon gun is filled with helium rather than hydrogen. The ball is a massive, ball shaped hydrogen peroxide monopropellant rocket. This is the best propellant for the ball and the sounding rocket because it is safe and clean. The ball has enough propellant to simultaneously reach the altitude of 100 km and horizontal velocity of about 0.5 km/s. The payload, the sling, the empty ball, and the cargo sack filled with regolith have the same mass. The sling is made of Zylon and is strong enough to reverse relative velocity of the cargo sacks and the payload. Small rocket engines permanently attached to both ends of the sling control its angular velocity and guide it toward the cargo sacks. The sling releases one sack, captures the ball with its empty end, releases the second sack, a moment later captures the payload, releases the ball, and then captures and releases the three remaining sacks. After the momentum exchanges with all the sacks the payload has the same velocity (3.4 km/s relative to the Earth) as both bolo tips, and is captured by the second terrestrial bolo. At the end of this celestial ballet the first bolo spacecraft gains lots of orbital energy, while the second bolo spacecraft looses a little orbital energy. As a result, the second bolo spacecraft overtakes the other bolo spacecraft and becomes the first bolo spacecraft.


The orbital loop relay provides an economical method of transporting large quantities of raw materials from the Earth and the Moon to low Earth orbit. It is inferior to the other relays because it has greater minimum mass and greater complexity. The raw materials are processed into orbital greenhouses. Water and ammonia are transported from the Earth by a two stage light gas gun. Moon dust (regolith) is transported by a plastic gunsling and solar sails. Loops orbiting the Moon and the Earth transfer reusable cargo containers among the gun, gunsling, and sails. The loops do not contribute net orbital energy to the cargo, so powerful winches are not needed. For every 3 kg of Moon dust, 1 kg of water and ammonia is brought to the Earth loop. The momentum of raw materials brought from the Moon is balanced by the momentum of raw materials delivered from the Earth. No bibliography. The minimum mass (for 1-ton cargo) is about 1000 tons. The orbital loop relay can transport its own mass in about two weeks.

Hydrogen and nitrogen are essential to life, and yet they are rare on the Moon. Both elements have to be transported from the Earth in a convenient form, e.g., frozen ammonia. The ammonia is dissolved in water, poured into the reusable container and frozen into a block of ice (clathrate hydrate). The loaded container is fired from the gun and enters the Earth loop tube which accelerates it to orbital velocity. Next, the container is emptied, picked up by the solar sail, slowed down by the Moon loop and dropped on the Moon surface near the mining site. When the solar sail approaches the Moon loop, its orbit has an opposite direction to the loop. The sail must then return to the Earth before it can reverse its direction and pick up a container loaded with Moon dust.

A mining settlement is erected on the Moon's equator. The settlement is inactive during the lunar night which lasts approximately two weeks. When it is active, its main task is loading the Moon dust into containers and launching them via a plastic gunsling. The gunsling is shielded by rocks from radiation and temperature extremes. A loop orbiting the Moon picks up the containers and releases them when the solar sails pass by. The solar sails then guide the containers to the Earth loop.

Orbital loop relay

Orbital loop relay

Most solar sails look like kites or balloons. Kites are difficult to erect in space, while balloons are being punctured by space junk and meteoroids. A more practicable sail design is aluminum foil transported to space on a spool and bent into a C-shaped beam by rollers. The foil is 1 micrometer thick except for ribs which are about 100 micrometer thick. The foil is made by vaporizing aluminum on a water surface. The same method can be used to bend thick sheet metal into rigid booms.

Aluminum ribbon shaped in space into solar sail

Aluminum ribbon shaped in space into solar sail
(The rollers are red, and the ribs are dark.)
(large image 53k)


Moon dust propulsion is very lightweight but it is so unconventional that its feasibility remains a mystery. Its minimum mass (for 1-ton cargo) is only 1 ton!

Large number of small robots are launched into elliptic orbits linking the Earth and the Moon. Their orbits are corrected by on-board ion thrusters. The robots transport Moon dust (lunar regolith) from a Moon orbit to the Earth's ionosphere.

The spacecraft is lifted above the atmosphere by a sounding rocket. As soon as it is above the atmosphere, it unfurls a large loop which generates magnetic field of about 1 mT. At the same time the robots release the dust in the ionosphere above the magnetic equator of the Earth. The initial trajectory of the dust is similar to a polar orbit. When the dust turns into ions, the ions follow the lines of force of the Earth magnetic field. Ion trajectories are helixes parallel to the lines of force of the magnetic field. Diameter of the helixes is on the order of one millimeter, so the plasma cloud is very streamlined.

Shortly before the moving plasma cloud hits the loop of the spacecraft, the electric current in the loop is moderate. The moving plasma intensifies the electric current in the loop and it exerts magnetic force on the loop. The moving plasma is constrained by the magnetic force of the loop, but it leaks through the loop.

Experimental data produced by stellarators and tokamaks would be very helpful in designing this contraption. Studying the solar wind trapped by the Earth's magnetic field would also help -- the ionized wind follows the lines of force of the Earth's magnetic field and it bounces off the magnetic poles. We can see it as the auroras. The northern aurora is the mirror reflection of the southern aurora.