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"
Hale-Bopp: A Continuing Tail
20 February 1998
Where is Hale-Bopp now? The Great Comet of 1997, one of the largest and most active comets ever, is outbound about 400 million miles from the sun. Too faint for viewing without telescopes or binoculars, Hale-Bopp is presently positioned in the very southerly constellation of Pictor. This "negative" image (black stars against white sky) of Hale-Bopp is the result of a 1 hour time exposure using the 1-metre European Southern Observatory Schmidt telescope on January 5. Clearly the comet still has a substantial tail, blown by the solar wind, that points generally away from the sunward direction. But look closely. A spiky "anti-tail" is also visible pointing toward the sun! It is likely that this anti-tail is composed of dust grains released from the comet nucleus which are too large to be easily pushed by the solar wind. As Hale-Bopp recedes from the sun its activity will subside but astronomers are still uncertain as to how long its tails will last. After a swing through the outer solar system and the Oort cloud, Hale-Bopp will pass through the inner solar system again ... around the year 5400.