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"
Near the Nucleus of Hyakutake
28 March 1996
NASA's Hubble Space Telescope captured this image of the near-nuclear region of Comet Hyakutake on March 25 as the comet approached within 9.3 million miles of the Earth. It covers a relatively "small" 2,000 mile wide area with the sunward direction toward the lower right (tailward is upper left). The image shows large amounts of dust jetting from the sunward side of the nucleus as the sun heats the surface of this dirty "orbiting iceberg". Pressure from sunlight eventually pushes the dust tailward and as the dust production increases, the already visually impressive tail will grow even brighter! The actual size of the nucleus is uncertain but is estimated to be 5-10 miles, similar to Comet Halley. As seen here, the brightest point is probably the tip of the strongest dust jet rather than the nucleus itself. A dramatic dust jet feature also appears to arise from the nightside but its true angle to our line of sight is difficult to judge. Some large fragments which have broken away from the comet nucleus are visible in the upper left of the image, producing dust tails of their own. Comet dust may represent primordial material from the formation of the solar system and NASA has plans for a comet dust sample return mission. Latest Comet Hyakutake images: JPL, Crni Vrh Observatory, Slovenia, Fayetteville Observer-Times, NASA's Night of the Comet