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 New Comet SWAN
29 April 2020
Newly discovered Comet SWAN has already developed an impressive tail. The comet came in from the outer Solar System and has just passed inside the orbit of the Earth. Officially designated C/2020 F8 (SWAN), this outgassing interplanetary iceberg will pass its closest to the Earth on May 13, and closest to the Sun on May 27. The comet was first noticed in late March by an astronomy enthusiast looking through images taken by ESA's and NASA's Sun-orbiting SOHO spacecraft, and is named for this spacecraft's Solar Wind Anisotropies (SWAN) camera. The featured image, taken from the dark skies in Namibia in mid-April, captured Comet SWAN's green-glowing coma and unexpectedly long, detailed, and blue ion-tail. Although the brightness of comets are notoriously hard to predict, some models have Comet SWAN becoming bright enough to see with the unaided eye during June. Experts Debate: How will humanity first discover extraterrestrial life?