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"
Stardust Capsule Returns to Earth
16 January 2006
A flying saucer from outer space crash-landed in the Utah desert yesterday after being tracked by radar and chased by helicopters and airplanes. Like last time, no space aliens were involved. The saucer, the Stardust return capsule, is carrying bits of Comet Wild 2 captured two years ago during a rendezvous between Stardust and the ancient comet. The capsule is pictured above entering a temporary clean room in Utah before part of it is shipped to NASA's Johnson Space Center in Houston, Texas, USA. In the inset on the lower right, an artist depicts the capsule as it likely looked after it parachuted to a landing in the Utah desert. The upper right insert shows an image of the streaking capsule taken yesterday by a DC-8 chase plane. The bits of Comet Wild 2 stored in the Stardust return capsule are likely older than the Sun and will be inspected over the next few years for clues about the early years of our Solar System. You, too, can help look for dust grains in the Stardust aerogel!