http://www.nineplanets.org/comets.html

As of 1995, 878 comets have been cataloged and their orbits at least roughly calculated. Of these 184 are periodic comets (orbital periods less than 200 years); some of the remainder are no doubt periodic as well, but their orbits have not been determined with sufficient accuracy to tell for sure. 





http://en.wikipedia.org/wiki/Comet

While the solid body of comets (called the nucleus) is generally less than 50 km across, the coma may be larger than the Sun, and ion tails have been observed to extend 1 astronomical unit (150 million km) or more."[4]
Surprisingly, cometary nuclei are among the darkest objects known to exist in the solar system. Solar heating drives off volatile compounds leaving behind heavy long-chain organics that tend to be very dark, like tar or crude oil. The very darkness of cometary surfaces allows them to absorb the heat necessary to drive their outgassing.
In 1996, comets were found to emit X-rays.


SL9 was in pieces ranging in size up to 2 kilometres in diameter, and is believed to have been pulled apart by Jupiter's tidal forces during a close encounter in July 1992. These fragments collided with Jupiter's southern hemisphere between July 16 and July 22, 1994, at a speed of approximately 60 kilometres per second (37 miles per second). 
Tracing back the comet's orbital motion revealed that it had been orbiting Jupiter for some time. It seems most likely that it was captured from a solar orbit in the early 1970s, although the capture may have occurred as early as the mid-1960s. Observers soon saw a huge dark spot after the first impact. The spot was visible even in very small telescopes, and was about 6,000 km (one Earth radius) across. Over the next 6 days, 21 discrete impacts were observed, with the largest coming on July 18 at 07:34 UTC when fragment G struck Jupiter. This impact created a giant dark spot over 12,000 km across, and was estimated to have released an energy equivalent to 6,000,000 megatons of TNT (750 times the world's nuclear arsenal).  15 kilotons of TNT (~6.3×1013 joules). 1 joule in everyday life is approximately:
the energy required to lift a small apple (102 g) one meter against Earth's gravity. 
the amount of energy, as heat, that a quiet person produces every hundredth of a second. 
the energy required to heat one gram of dry, cool air by 1 degree Celsius. 
one hundredth of the energy a person can get by drinking a single 5 mm diameter droplet of beer. 

things to compare: car crash,