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"
A Brighter Comet LINEAR
25 June 2001
Brighter than ever expected, comet LINEAR -- you know, the one designated C/2001 A2 -- is a sight to see in southern skies. This comet LINEAR first brightened impressively in late March as its active nucleus began to fragment, prompting some speculation that the comet might soon break up completely. But still hanging together after its closest approach to the Sun, C/2001 A2 suddenly brightened again and was reported last week to have reached nearly 3rd magnitude, easily visible to the unaided eye. This delightful telescopic picture of the brighter coma of comet LINEAR was recorded from Australia on June 20. Stars seen through the tenuous coma and filamentary tail appear as a series of short trails in this three-color composite image registered on the comet. North is up and the scene covers about half the width of the full Moon. Now moving through the constellation Cetus, comet LINEAR will be north of the celestial equator by July 4 as it comes into view for eager northern sky-gazers.