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"
Comet Hale-Bopp Outbound
10 September 1997
Hale-Bopp, the Comet of the Century, is leaving the inner Solar System. Outbound at about 12 miles per second it is presently nearing the main asteroid belt between Mars and Jupiter. This false-color image represents a recent view from low Earth orbit showing the comet surrounded by its shrinking coma against a background of stars. It was constructed from a fraction of the data taken aboard the Space Shuttle Discovery in August by a small innovative telescopic camera known as the Southwest Ultraviolet Imaging System or SWUIS. SWUIS (sounds like "swiss") images will be particularly interesting to astronomers who wish to continue to follow the Great Comet's interaction with the Solar Wind. The once bright Hale-Bopp has faded below 4th magnitude but is still visible to Earthbound observers south of 35 degrees north latitude.