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 Bradfield Rising
27 April 2004
Comet Bradfield has become quite a sight just before sunrise -- for those with binoculars or cameras. Although fading noticeably each day, a sky chart, a northern location, and some persistence will allow curious sky gazers to locate the cosmic snowball and its spectacular tail. One might call Bradfield a "camera" comet as its extended tail is too long for most telescopes but caught nicely by normal cameras capable of long exposures and set to rotate with the sky. Pictured above just yesterday, Comet C/2004 F4 (Bradfield) was caught as it rose on successive three-minute exposures above the Rocky Mountains near Yampa, Colorado, USA. Visible on the upper left as a bright fuzzy smudge is the Andromeda Galaxy (M31), far in the distance. Comet Bradfield was discovered only last month and was briefly visible to the unaided eye. It was imaged in spectacular fashion by the SOHO spacecraft as it rounded the Sun early last week.