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"
Comet C/2001 Q4 (NEAT)
23 April 2004
Inbound from the distant solar system, comet C/2001 Q4 will soon pass just inside planet Earth's orbit and should be one of two bright, naked-eye comets visible in southern skies in May. First picked up nearly three years ago by the Near Earth Asteroid Tracking (NEAT) project Q4 appears in both of these stunning telescopic views recorded only a few days ago, on April 18th (left) and 19th, from a site near Alcohuaz, Chile. Remarkable changes in the structure of the long, graceful tail can be seen by comparing the two photos, including the dramatic kink seen near the tail's midpoint on April 19th. The apparent motion of the comet sweeping across the sky is evident when you compare the position of the tail relative to background galaxy NGC 1313, visible as a smudge near the top of each image. Q4's closest approach to the Sun will be on May 15th while its closest encounter with planet Earth will be on May 7th (see animation by L. Koehn).