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 on Schedule
25 July 1996
Late March and early April of 1996 marked a banner season for viewing the spectacular naked-eye comet Hyakutake. The spring of 1997 could well offer a similar cometary wonder, Comet Hale-Bopp. Discovered last year while approaching the inner solar sytem Hale-Bopp has been eagerly watched for signs that it will indeed brighten spectacularly. So far, things look good! This recent image of the comet was made on May 14 at the European Southern Observatory when the comet about 340 million miles from Earth. It is shown here using false colors to indicate relative brightness. Stars are visible through the expanding coma. Recent reports are that its level of activity, the rate of dust and gas production from the solar heating of the icy nucleus is as expected. An analysis of its orbit indicates that this comet will approach to within nearly 120 million miles of Earth on March 22, 1997. The orbital calculations also reveal that Hale-Bopp has visited the inner solar system before ... about 4200 years ago.