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.
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APODs including "Comet"
Gas and Snow Jets from Comet Hartley 2
23 November 2010
Unusual jets have been discovered emanating from Comet Hartley 2. The EPOXI spacecraft imaged the jets in unprecedented detail during its flyby of the comet earlier this month. Pictured above, sun-illuminated jets shoot away from the two-kilometer long decaying iceberg that orbits the Sun between Earth and Jupiter. Comet Hartley 2 became active recently as it neared the Sun and sunlight warmed the comet. Preliminary analyses of images like that shown above indicate that the smooth regions around the middle are porous and leak frozen water vapor directly out into space. Unexpectedly, however, the rough regions at either end appear to shoot carbon dioxide jets that expel fluffy snowballs, some as large as basketballs, from the nucleus. Many of the dots in the above image are thought to be snowballs. The unusual jets will continue to be studied, and may yield further clues as to how comets and asteroids formed and evolved during the early years of our Solar System. Comet Hartley 2 is slowly evaporating and may completely break up over the next 1,000 years.