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"
A Sudden Jet on Comet 67P
18 November 2015
There she blows! A dramatic demonstration of how short-lived some comet jets can be was documented in late July by the robotic Rosetta spacecraft orbiting the nucleus of Comet 67P/Churyumov-Gerasimenko. The featured animation depicts changes in the rotating comet with three illuminating stills. Although the first frame shows nothing unusual, the second frame shows a sudden strong jet shooting off the 67P's surface only 20 minutes later, while the third frame -- taken 20 minutes after that -- shows but a slight remnant of the once-active jet. As comets near the Sun, they can produce long and beautiful tails that stream across the inner Solar System. How comet jets produce these tails is a topic of research -- helped by images like this. Another recent Rosetta measurement indicates that the water on Earth could not have come from comets like 67P because of significant differences in impurities. Comet 67P spans about four kilometers, orbits the Sun between Earth and Jupiter, and has been the home for ESA's Rosetta spaceship since 2014 August. Rosetta is currently scheduled to make a slow crash onto Comet 67P's surface in late 2016.