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 Ikeya-Zhang's Busy Tail
18 March 2002
One of the brightest comets of the past five years will likely reach its peak brightness this week. Comet Ikeya-Zhang, officially known as C/2002 C1, can now be seen without aide from a dark location above the western horizon shortly after sunset. Recent luminosity estimates for Comet Ikeya-Zhang place it between magnitudes three and four, making it brighter than most commonly visible stars. The above picture was taken on March 11 near Tucson, Arizona, USA. The image caught Comet Ikeya-Zhang showing a quite detailed ion tail, possibly disrupted by the magnetic field of the Sun's outwardly flowing particle wind. As the activity of comets is notoriously hard to predict, Comet Ikeya-Zhang may still hold some surprises as it rounds the Sun this month and starts back out of the Solar System in April.