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"
The Comet Hartley 2 Cruise
7 October 2011
rly last November, small but active Comet Hartley 2 (103/P Hartley) became the fifth comet imaged close-up by a spacecraft from planet Earth. Still cruising through the solar system with a 6 year orbital period, Hartley 2 is making astronomical headlines again. New Herschel Space Observatory measurements indicate that the water found in this comet's thin atmosphere or coma has the same ratio of the hydrogen isotope deuterium (in heavy water) as the oceans of our fair planet. Hartley 2 originated in the distant Kuiper Belt, a region beyond the orbit of Neptune that is a reservoir of icy cometary bodies and dwarf planets. Since the ratio of deuterium is related to the solar system environment where the comet formed, the Herschel results indicate that Kuiper Belt comets could have contributed substantial amounts of water to Earth's oceans. Comet Hartley 2 appears in this starry skyscape from last November sporting a tantalizing greenish coma appropriately sailing through the nautical constellation Puppis. Below the comet are open star clusters M47 (right) and M46 (left).