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"
Hyakutake: The Great Comet of 1996?
8 February 1996
Get ready for one of the most impressive but least anticipated light shows in modern astronomical history. Next month, newly discovered Comet Hyakutake will pass closer to the Earth than any recent comet. Unknown before its discovery by Yuji Hyakutake on 30 January 1996, the fuzzy spot in the above photograph is a comet now predicted to become bright enough to see without a telescope. Although comets act in such diverse ways that predictions are frequently inaccurate, even conservative estimates indicate that this comet is likely to impress. For example, even if Comet Hyakutake remains physically unchanged, its close pass near the Earth in late March 1996 should cause it to appear to brighten to about 3rd magnitude - still bright enough to see with the unaided eye. In the next two months, though, the comet will continue to approach the Sun and hence should become brighter still. Optimistic predictions include that Comet Hyakutake will change physically, develop a larger coma and tail, brighten dramatically, move noticeably in the sky during a single night, and may ultimately become known as the "The Great Comet of 1996." Move over Hale-Bopp!