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.
2 November 2015
Most comets don't survive a close encounter with the Sun. Two years ago this month, though, Comet ISON was thought by some to be big enough to withstand its perilous sungrazing dive. The featured video shows the drama as it was recorded by the Solar and Heliospheric Observatory (SOHO), a joint mission of ESA and NASA. As many Earthlings watched in fascination, a bright area did emerge from closest approach, but it soon faded and dispersed. It is now assumed that no large fragments of Comet C/2012 S1 (ISON) survived. Besides the comet, the active Sun is seen to eject puffs of plasma known as coronal mass ejections. Launched in 1995, sun-orbiting SOHO has become a historic device in the discovery and tracking of comets known as sungrazers. Two months ago, a comet designated SOHO 3000 was named in honor of the record 3000th comet that was discovered on SOHO images, a total that amounts to about half of all known comets. Astrophysicists: Browse 1,100+ codes in the Astrophysics Source Code Library