COMETS AND ASTEROIDS

In Solar System astrophysics, isolationism is unfashionable and compar ative planetology is all the rage. Astronomers compare Venus with Eart h, Mars with Moon and Jupiter with Saturn; so why not compare comets w ith asteroids? This is the task that I set out to fulfil, but it must be stressed at the outset that, just like the previous examples, comet s and asteroids are completely different objects. They have different origins, evolutionary histories, orbits, size ranges, size distributio ns, compositions, surface features, internal structures, apparent magn itudes, observational histories and end products. Comets and asteroids are often grouped together under the epithet 'minor body'. They are m inor because they are smaller than the majority of planetary satellite s, and their eccentric orbits often put them on collision courses with the planets. Both comets and asteroids are capable of cratering plane tary surfaces and in the present epoch, a minor body > 1 km will hit E arth about every 2 X 10(6) years. This impactor is 20 times more likel y to be an asteroid than a comet. Corners decay and asteroids break up . Both these processes produce dust that starts off as a co-orbital me teoroid stream and eventually feeds the general zodiacal dust cloud th at surrounds the Sun.