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"
This Comet Lovejoy
25 December 2014
Comet Lovejoy, C/2014 Q2, is framed like a cosmic Christmas tree with starry decorations in this colorful telescopic portrait, snapped on December 16th. Its lovely coma is tinted green by diatomic C2 gas fluorescing in sunlight. Discovered in August of this year, this Comet Lovejoy is currently sweeping north through the constellation Columba, heading for Lepus south of Orion and bright enough to offer good binocular views. Not its first time through the inner Solar System, this Comet Lovejoy will pass closest to planet Earth on January 7, while its perihelion (closest point to the Sun) will be on January 30. Of course, planet Earth's own 2015 perihelion passage is scheduled for January 4. A long period comet, this Comet Lovejoy should return again ... in about 8,000 years.