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"
Near Comet Hyakutake's Nucleus
21 March 1996
This March 19th false-color picture of Comet Hyakutake from one of the most sophisticated ground based telescopes captures the area surrounding the comet's nucleus. A comet's nucleus - not directly visible here - is a solid dirty iceball probably no more than 10 kilometers across. This image shows, for the first time, features of irregular brightness in the coma surrounding the nucleus. The beginning of the ion tale is visible as the bright feature emanating from the right, approaching about 1000 km from nucleus. At the distance of the comet, the whole field captured here is roughly the size of the Earth. If it's clear, Comet Hyakutake may be easily visible tonight! Information: The Scale of the Universe Debate in April 1996