Comets and asteroids are not only a resource. They are also a clear and present danger to all life on Earth, a fact brought home to many for the first time when they saw kilometer sized fragments of Shoemaker-Levy-9 slamming into Jupiter. It doesn't just happen to gas giants. In the not so distant past of our own planet, a mere 65 million years ago, half of all species were wiped out by a single event. Within this century a 50 mile radius of forest at Tunguska in Siberia was flattened by the airburst of a small comet or a carbonaceous asteroid. In the mid 70's a chunk of rock with the kinetic energy of an Hiroshima bomb skipped through the upper atmosphere over the north central USA. The photo appeared in National Geographic. Recently declassified american early warning satellite data has shown that there are many kiloton sized airbursts every year from small objects self destructing in the upper atmosphere. Not a hazard in and of themselves - but a definite warning. The impact threat is not science fiction.
These articles discuss the recent discovery that the KT extinction was partly due to the stratigraphy of the Chixculub region. The high Sulfur content of the rocks was responsible for the severity of the environmental impact. Unstated in this particular document is that this explains the source of the acid rain that killed trees and other plants and set the stage for planetary scale conflagrations.
There are a number of impact sites on Earth:
Only a few asteroids have been imaged:
The Kuiper Belt is the second major source of small bodies that will of of use for economic exploitation by a space based civilization. Until recent Hubble results the Kuiper Belt was primarily theoretical. Now it is know to be real.