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"
Cloud, Clusters and Comet Siding Spring
4 September 2014
On October 19th, a good place to watch Comet Siding Spring will be from Mars. Then, this inbound visitor (C/2013 A1) to the inner solar system, discovered in January 2013 by Robert McNaught at Australia's Siding Spring Observatory, will pass within 132,000 kilometers of the Red Planet. That's a near miss, equivalent to just over 1/3 the Earth-Moon distance. Great views of the comet for denizens of planet Earth's southern hemisphere are possible now, though. This telescopic snapshot from August 29 captured the comet's whitish coma and arcing dust tail sweeping through southern skies. The fabulous field of view includes, the Small Magellanic Cloud and globular star clusters 47 Tucanae (right) and NGC 362 (upper left). Worried about all those spacecraft in Martian orbit? Streaking dust particles from the comet could pose a danger and controllers plan to position Mars orbiters on the opposite side of the planet during the comet's close flyby.