A Comet is an object orbiting the Sun that displays a visible Atmosphere (Coma) and possibly a tail, on the side away from the sun. They are icy, i.e., made of substances solid but that can be turned to gas at Temperatures reached in the inner part of their orbit, creating the coma and possible tail. Classically, they have long-period orbits (tens to thousands of years) that are highly elliptical so a brief portion of the orbit is visible and displays the coma and tail effects. It is thought that many comets reside at the far end of their orbits, where they move relatively slowly, which, together, are known as the Oort Cloud.
Two well-known comets are Comet Halley (recognized in 1705 by Edmund Halley as orbiting approximately every 76 years) and Comet P/Shoemaker-Levy 9 which was observed colliding with Jupiter in 1994.
Catalina Sky Survey (CSS)
Deep Lens Survey (DLS)
ESO/Uppsala Survey Of The ESO (B) Atlas
Origins Space Telescope (OST)