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"
The Ion Tail of Comet Hyakutake
19 March 1996
This picture of Comet Hyakutake was taken on March 14, 1996. Structure in the ion tale of Comet Hyakutake is now clearly visible. An ion tale forms as a comet nears the Sun. Sunlight causes gas and dust to boil off the comet's solid nucleus. Charged gas - called ions - are then accelerated away from the Sun by the solar wind - fast moving particles streaming out from the Sun's corona. The ion tale will appear blue and glows by fluorescence. As Comet Hyakutake gets closer to the Sun during the next month, a dust tail is expected to be visible as well. Dust tails shine by light reflected from the Sun. Comet tails point away from the Sun, even as a comet moves away from the Sun. For observers in the Northern Hemisphere, Comet Hyakutake should appear tonight in the eastern part of the constellation of Virgo and should be about magnitude 2.5. The comet will look the most impressive in the darkest skies - in a city you are likely to see only a fuzzy blob! Information: The Scale of the Universe Debate in April 1996