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 Galaxy is not a Comet
12 April 2002
This gorgeous galaxy and comet portrait was recorded on April 5th in the skies over the Oriental Pyrenees near Figueres, Spain. From a site above 1,100 meters, astrophotographer Juan Carlos Casado used a guided time exposure, fast film, and a telephoto lens to capture the predicted conjunction of the bright Comet Ikeya-Zhang (right) and the Andromeda Galaxy (left). This stunning celestial scene would also have been a rewarding one for the influential 18th century comet hunter Charles Messier. While Messier scanned French skies for comets, he carefully cataloged positions of things which were fuzzy and comet-like in appearance but did not move against the background stars and so were definitely not comets. The Andromeda Galaxy, also known as M31, is the 31st object in his famous not-a-comet catalog. Not-a-comet object number 110, a late addition to Messier's catalog, is one of Andromeda's small satellite galaxies, and can be seen here just below M31. Our modern understanding holds that the Andromeda galaxy is a large spiral galaxy some 2 million light-years distant. The photogenic Comet Ikeya-Zhang, now a lovely sight in early morning skies, is about 80 million kilometers (4 light-minutes) from planet Earth.