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"
STARDUST Flyby of Comet Wild 2
19 January 2004
Flying past a comet nucleus is dangerous. On January 2, the robot spacecraft STARDUST became one of the first to plow through the surrounding cloud of dust and grit to photograph the very heart of a comet. Pictured above is a short movie of the encounter showing unprecedented surface details of the icy center of Comet Wild 2. The STARDUST camera pivoted to remain pointed at the nucleus as the spacecraft passed. Heavily shielded from the onslaught of cometary debris, STARDUST survived the beating in excellent condition. Surprisingly, although the nucleus appeared to be solid, the the surrounding coma appeared to be highly fragmented into several distinct streams of particles. Souvenirs captured by the spacecraft will be ejected as the spacecraft passes the Earth in January 2006.