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"
Perseid from Torralba del Burgo
13 August 2016
Perseid meteors rained on planet Earth last night. This year the stream of dust from periodic comet Swift-Tuttle has produced a stunningly active shower of bright cosmic streaks. In this 25 second long exposure, one luminous Perseid trail, fast and colorful with a small explosion at the end, is witnessed by night skygazers from Torralba del Burgo, Soria, Spain. A second fainter meteor trail appears well below the first. The two can be extended to intersect at the meteor shower's radiant just above the brighter stars of the heroic constellation Perseus. Though the meteor shower's activity is waning, in the coming days Perseids will still flash through the night. But you won't see any if you don't go outside and look up.