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"
Rosetta's Selfie
16 October 2014
This Rosetta spacecraft selfie was snapped on October 7th. At the time the spacecraft was about 472 million kilometers from planet Earth, but only 16 kilometers from the surface of comet 67P/Churyumov-Gerasimenko. Looming beyond the spacecraft near the top of the frame, dust and gas stream away from the comet's curious double-lobed nucleus and bright sunlight glints off one of Rosetta's 14 meter long solar arrays. In fact, two exposures, one short and one long, were combined to record the dramatic high contrast scene using the CIVA camera system on Rosetta's still-attached Philae lander. Its chosen primary landing site is visible on the smaller lobe of the nucleus. This is the last image anticipated from Philae's cameras before the lander separates from Rosetta on November 12. Shortly after separation Philae will take another image looking back toward the orbiter, and begin its descent to the nucleus of the comet.