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"
Going Wild
19 March 2004
Dynamic jets of gas and dust surround one of the most active planetary surfaces in the solar system in this wild-looking picture of a comet nucleus. The comet's designation is 81P/Wild 2 of course (sounds like "vilt 2"), and the picture is a composite of two images recorded by the Stardust spacecraft's navigation camera during its January 2nd flyby. The composited images consist of a short exposure recording startling surface details of Wild 2's nucleus and a longer exposure, taken 10 seconds later, revealing material streaming from the surface. The left edge of the nucleus appears extremely jagged due to a strong shadow. Pitted and eroded after billions of years of outgassing and meteorite impacts, the nucleus pictured is only about 5 kilometers in diameter, while the jets of dust and gas ultimately leave trails millions of kilometers long. Stardust is scheduled to return samples of Wild 2's cometary dust, picked up during the flyby, to Earth in January 2006.