Comets are icy planetesimals usually from 1 to 50 km across and containing bits of
fragile dust resembling
carbonaceous
chondrite material. They probably formed among the outer planets and were ejected into the
Oort cloud, from which they occasionally reenter the inner
solar system.
When a reentering comet reaches a distance of about 2 to 4 AU from the
sun, material begins to
sublime off its surface, producing a
gaseous
coma and a gas-and-dust tail that is blown outward by the
solar wind. The solid debris become meteoroids strewn along the comet's
orbit, since they leave the comet at too low a speed to change orbit very much.
These
particles are affected by
Poynting-Robertson and radition
forces and occasionally hit
Earth, producing meteors or their larger cousins,
fireballs.
Comets age slowly as more and more gas and dust are lost. Some of them may burn out, leaving inert residues of stony material-possibly cataloged in some cases as
Apollo asteroids of spectral class C, P or D. Comets in their various states would provide interesting targets for close investigation by spacecraft.
Hollywood tried to personify this with their asteroidal adventure -
Armageddon, more recently
NASA theorized the possibilities using remote robot landings.