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"
Catching Falling Stardust Credit:
1 February 1997
This carrot shaped track is actually little more than 5 hundredths of an inch long. It is the trail of a meteroid through aerogel exposed to space by the shuttle launched EURECA (European Recoverable Carrier) spacecraft. The meteoroid itself, about a thousandth of an inch in diameter, is visible where it came to rest, just beyond the tip of the carrot (far right). Chemical analyses of interplanetary dust particles similar to this one suggest that some of them may be bits of comets and represent samples of material from the early stages of the formation of the Solar System. NASA's Stardust mission, planned for launch in 1999, will attempt to directly collect dust from the tail of a comet and return it to Earth -- the first non-lunar sample return mission ever! In addition to peering into the chemistry and history of the solar system, the composition of cometary dust has important implications for the possibility of past life on Mars.