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"
Comets and Bright Star
6 January 2016
This timely, telescopic, two panel mosaic spans about 10 full moons across planet Earth's predawn skies. Recorded as the year began from Tenerife, Canary Islands, near the top of the frame are the faint coma and tail of Comet Borrelly (P/19). A comet with a seven year orbital period, Borrelly's nucleus was visited by the ion propelled spacecraft Deep Space 1 near the beginning of the 21st century. Anchoring the scene at the bottom is brilliant star Arcturus (Alpha Bootes) and Comet Catalina (C/2013 US10) a first time visitor from the Oort Cloud. Catalina's yellowish dust tail extends below and right. Buffeted by winds and storms from the Sun, the comet's complex ion tail sweeps up and toward the right, across most of the field of view. Remarkably, one of the composition's 30 second exposure subframes also caught the trail of a bright meteor, slashing toward the left between comets and bright star.