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"
Comet Lovejoy Before Galaxy M63
2 December 2013
Comet Lovejoy was captured last week passing well in front of spiral galaxy M63. Discovered only three months ago and currently near its maximum brightness, Comet Lovejoy can be seen near the Big Dipper from dark northerly locations before dawn with the unaided eye. An unexpected rival to Comet ISON, C/2013 R1 (Lovejoy), pictured above, is currently sporting a large green coma and a beautifully textured ion tail. Comet Lovejoy is now headed back to the outer Solar System but should remain a good sight in binoculars for another few weeks. Conversely, spiral galaxy M63, lies far in the distance and is expected to remain stationary on the sky and hold its relative brightness for at least the next few million years. Late ISON News: The continuing saga of Comet ISON | In memoriam