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 NEOWISE over Vikos Gorge
3 August 2020
Did the Earth part to show us this comet? Of course not, even if this image makes it seem that way. Pictured far in the background is Comet NEOWISE as it appeared about two weeks ago over northern Greece. Above the comet are many stars including the bright stars of the Big Dipper (also the Sorcerer, in Aztec mythology), an asterism that many people around the world used to find the naked-eye comet as it hovered in the northern sky over the past month. In the foreground is Vikos Gorge, the deepest gorge on Earth, relative to its width. The gorge was slowly created by erosion from the Voidomatis River over the past few million years. Capturing this image took a lot of planning, waiting, luck, braving high winds, and avoiding local wolves. Comet C/2020 F3 (NEOWISE) continues to fade and is now best visible through binoculars as it coasts back to the outer Solar System. Notable Comet NEOWISE Images: July 31 30, 29, 28, 27, 26, 25, 24