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"
ISON, Mars, Regulus
17 October 2013
In order top to bottom this celestial snapshot features Comet ISON, planet Mars, and Regulus, alpha star of the constellation Leo, in the same frame. The scene spans about 2 degrees near the eastern horizon in early morning skies of October 15. Closest of the three, the much heralded Comet ISON (C/2012 S1) is by far the faintest at 14 light-minutes (1.7 AU) away. Mars is only slightly farther from our fair planet. About 16.5 light minutes (2 AU) away its normal ruddy color is washed out in the exposure. Regulus outshines both comet and planet from a distance of 75 light-years. Just above Regulus, the very faint smudge of light is actually the Leo I dwarf galaxy, 800,000 light-years away and almost lost in the glare of the bluish hued bright star. Comet ISON is expected to grow brighter, though. How bright is still not clear, but not as bright as a Full Moon in night skies. Estimated to be 1 to 4 kilometers in diameter, ISON's nucleus might substantially survive its very close encounter with the Sun on November 28. If so, the comet will climb back above the eastern horizon in planet Earth's northern hemisphere before dawn in early December.