Comet
A comet is a small Solar System body composed mainly of ice, dust, and organic compounds. As they approach the Sun, cometary ices sublimate, releasing gas and dust that form the glowing coma and distinctive tails—often extending millions of kilometers.
Comets originate from two primary reservoirs: the Kuiper Belt—home to short-period comets with orbits under ~200 years—and the distant Oort Cloud, which sends long-period comets with orbits spanning thousands of years. Occasionally, interstellar objects like ‘Oumuamua pass through our system, appearing comet-like.
They are “dirty snowballs” and among the most primitive remnants of the early Solar System, preserving original ices and organic material from 4.6 billion years ago.
Landmark missions include ESA’s Rosetta—with its lander Philae—which orbited and sampled Comet 67P/Churyumov–Gerasimenko (2014–2016). NASA’s Deep Impact struck Comet Tempel 1 in 2005 to analyze interior composition.
These missions discovered complex organic molecules, including amino-acid precursors, and supported the theory that comets may have delivered water and prebiotic compounds to early Earth.
Looking ahead, ESA–JAXA’s Comet Interceptor (launch planned for 2029) intends to perform a rapid flyby of a dynamically new or interstellar comet, using multiple spacecraft to sample both nucleus and coma environments.
Comets are studied across the electromagnetic spectrum—from radio to gamma rays—using telescopes and spacecraft to understand composition, internal structure, activity, and their roles in planetary formation, the evolution of the solar system, and the origin of life.
Sources
APODs including "Comet"
A Cosmic Snowball
30 May 1997
Like cosmic snowballs, fluffy comet-like objects the size of houses and composed mostly of water-ice, may be pummeling planet Earth 5 to 30 times a minute. This controversial theory was originally proposed in 1986 by Dr. Louis Frank (U. Iowa) based on data from NASA's Dynamics Explorer 1. It is further supported by recently reported findings from the one year old POLAR spacecraft. Representing a previously unknown class of Solar System objects, these proposed small, icy comets disintegrate in the upper atmosphere at altitudes of 600 to 15,000 miles and so do not pose an impact threat to the Earth's surface or even to spacecraft in low Earth orbit. On breaking up, however, they produce a fleeting trail of clouds of water vapor. Traces of these transient, extremely high altitude clouds can be detected by down looking spacecraft designed to monitor the near-Earth environment. The suspected trail of one such cosmic snowball vaporizing over the Atlantic Ocean and Western Europe at an altitude of 5,000 to 15,000 miles is seen above. It was recorded in a 54 second exposure by POLAR's Visible Imaging System in September of 1996. A map has been added as a background for location reference. If continuous over the history of the Earth's formation, this relatively gentle cosmic snow shower would have been a major source of water for Earth's present life-nurturing oceans and possibly even a source of simple organic compounds.