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"
Blue Comet in the Hyades
18 January 2018
Stars of the Hyades cluster are scattered through this mosaic spanning over 5 degrees on the sky toward the constellation Taurus. Presently cruising through the Solar System, the remarkably blue comet C/2016 R2 PanSTARRS is placed in the wide field of view using image data from January 12. With the apex of the V-shape in the Hyades cluster positioned near the top center, bright Aldebaran, alpha star of Taurus, anchors the frame at the lower right. A cool red giant, Aldebaran is seen in orange hues in the colorful starfield. While the stars of the Hyades are gathered 151 light-years away, Aldebaran lies only 65 light-years distant and so is separate from the cluster stars. On January 12, C/2016 R2 was over 17 light-minutes from planet Earth and nearly 24 light-minutes from the Sun. Its blue tinted tail largely due to CO+ gas fluorescing in sunlight, the head or coma of the comet appears with a slightly greenish hue, likely emission from diatomic carbon.