There are two main types of comets, those on short period orbits, like 19P/Borrelly (6.8 yr) and 81P/Wild (6.4 yr) and those on much longer period orbits like Hale-Bopp and Hyakutaki. Hale-Bopp previously passed the Sun about 4200 years ago and will come back again in 2380 years The reason for the difference is that it passed within 0.77 astronomical units of Jupiter in April 1966 and the jovian gravitational field altered its path. Hyakutaki was previously seen about 17500 years ago and will come back in about 29500 years. Short period comets have been captured into the inner solar system by Jupiter’s gravity. If they pass Jupiter in a different way they can be placed back onto much longer orbits. Short-period comets travel around the Sun in the same direction as the planets and their orbits have low inclinations with respect to the Earth’s orbit. Long-period comets have orbits that are randomly inclined. The orbit of Hale-Bopp was at right-angles to the planetary plane.
The cometary nucleus is a deep-freeze mixture of ‘snow’ and ‘dirt’, the mass ratio between these being about two to one. About 19 out of 20 of the snow molecules are water, the contaminants being carbon dioxide, ammonia, methane, carbon monoxide, and more exotic coal-tar-like combinations of hydrogen, oxygen, carbon and nitrogen. The dirt is phylosilicate material - the material which makes up the bulk of the terrestrial planets.
Once you have got below the few-centimetres-thick, ice-depleted surface layer, you enter the low-density interior. We have very little idea what a comet is like inside. Unlike large planets, cometary interior temperatures have not been increased by radioactive decay. There is no differentiation and we expect no variation with depth. But what is the scale of the uniformity? Maybe the interior has a constant low density throughout. Maybe some regions have slightly higher densities and others are completely void. Hopefully the large crater that the 2005 Deep Impact mission is aiming to create will provide some clues. We know that the cometary nucleus is very weak. This is demonstrated by the ease with which the nucleus of comet Shoemaker-Levy-9 was pulled apart by tidal forces when it passed too close to Jupiter in July 1992.
Comet, Asteroid, Meteor, Meteorite
Let me end by dispelling certain myths. Comets and asteroids are completely different types of object. Comets are the left-over building blocks of the interiors of the gas-giant planets. Comets are unaltered remnants of the dawn of our solar system. Asteroids on the other hand were formed between the orbits of Mars and Jupiter. They are fragments of large, sometimes differentiated, planetesimals (small bodies formed from the solar nebulae) that were trying to come together to form a large terrestrial planet but were prevented from doing so by the gravitational influence of Jupiter.
When a sand-grain sized piece of cometary dust hits the Earth’s upper atmosphere it quickly burns up forming a meteor (or more-prosaically, a shooting star). When a small asteroid hits the Earth’s atmosphere, friction slows it down and the remnant rock free-falls to the Earth’s surface. This meteorite can be retrieved, placed in our geological museums and analysed in laboratories.