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.
Sources
APODs including "Comet"
The Hunter, the Bull, and Lovejoy
14 January 2015
Heading north, Comet Lovejoy (C/2014 Q2) is putting on its best show for comet watchers now, with moonlight absent from mid-January's early evening skies. An easy binocular target and just visible to the unaided eye from dark sites, the comet sweeps across the constellation Taurus the Bull in this deep night skyscape. The starry scene was captured just two days ago on January 12, from Jackson Hole, Wyoming, planet Earth. In fact, the head of Taurus formed by the V-shaped Hyades star cluster points toward Lovejoy at the right. The comet's greenish coma and tail streaming in the anti-sunward direction also seem to have been shot from Orion's bow. You can spot the familiar stars of the nebula rich constellation of the Hunter on the left, and follow this link to highlight Comet Lovejoy in the wide field of view.