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"
Light and Glory over Crete
16 November 2020
The month was July, the place was the Greek island of Crete, and the sky was spectacular. Of course there were the usual stars like Polaris, Vega, and Antares -- and that common asterism everyone knows: the Big Dipper. But this sky was just getting started. The band of the Milky Way Galaxy stunned as it arched across the night like a bridge made of stars and dust but dotted with red nebula like candy. The planets Saturn and Jupiter were so bright you wanted to stop people on the beach and point them out. The air glowed like a rainbow -- but what really grabbed the glory was a comet. Just above the northern horizon, Comet NEOWISE spread its tails like nothing you had ever seen before or might ever see again. Staring in amazement, there was only one thing to do: take a picture. Coverage: NASA's SpaceX Crew-1 Mission