What is a Comet?

In ancient history, comets were traditionally considered to be bad omens. They are small Solar System bodies (SSSB) that will display a visible coma (a thin, fuzzy, temporary atmosphere) when close to the Sun. The main difference between an asteroid and a comet is that a comet shows a coma. Asteroids are also thought to have a different origin from comets, having formed inside the orbit of Jupiter rather than in the outer Solar System. This gives the orbital history of comets more importance.

The coma of a comet is formed when it passes by the Sun. The coma is generally made of ice and dust, and can grow to be incredibly large. In October 2007, comet 17P/Holmes briefly had a tenuous dust atmosphere larger than the Sun. It has been estimated that roughly one comet is discovered each year that is visible to the human eye. In some rare cases, a Great Comet can form which is brighter than any star in the sky. It has been estimated that one Great Comet will appear every decade.
The requirements for a Great Comet include a large and active nucleus, a close approach to the Sun, and a close approach to the Earth. In 1996, Comet Hyakutake, which had a similar sized nucleus as Comet Elenin, made an extremely close approach to Earth. The Ulysses spacecraft crossed the Hyakutake’s tail at a distance of more than 500 million kilometers (3.3 AU or 3×108 mi) from the nucleus, showing that Hyakutake had the longest tail known for a comet.