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"
Here Comes Comet Hyakutake
13 March 1996
The reaction of ancient peoples to the appearance of bright comets has toppled empires, de-throned kings, and been taken as a sign of great things to come. Probably some of these comets did not get as bright as Comet Hyakutake ("hyah-koo-tah-kay") will in the next two weeks. It is likely that every major news organization will soon cover Comet Hyakutake extensively. Comet Hyakutake is shown above already showing a tail of dust. This image, taken directly from a photographic negative, shows stars as black spots and the bright comet coma and tail as dark clouds against the white background of space. During its closest approach to the Earth on March 25th, Comet Hyakutake will appear in the Northern hemisphere as a diffuse ball of light brighter than most stars. Comet Hyakutake will be visible then most of the night even without binoculars, passing above the stars in the handle of the Big Dipper. Comet Hyakutake will be best seen in dark skies far from city lights, where its tail - possibly extending 20 degrees or more - will be most easily visible. As Comet Hyakutake recedes from the Earth it will fade, but brighten again as it nears the sun later in April. At that time it will be best seen in the southern hemisphere. There is no chance Comet Hyakutake will hit the Earth. Information: The Scale of the Universe Debate in April 1996