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"
Horizon to Horizon
21 December 2007
Scroll right and journey from horizon to horizon as your gaze sweeps through the zenith in the night sky over Beg-Meil, France. Recorded on December 13th, the entertaining panorama (image key) covers 210 degrees in 21 separate exposures, beginning on the beach with bright star Sirius rising in the southeast. Look up (pan right) to encounter the nebula rich constellation of Orion and continue on to find the lovely Pleiades star cluster. Farther along, higher in the sky, is the famous Comet Holmes, still gracing the northern hemisphere's night with its remarkable expanding coma. Finally, just before diving into the urban glow from city lights along the northwestern horizon (far right), check out the double star cluster in Perseus and take in the cosmic streak of a bright Geminid Meteor. APOD presents: Astronomy Pictures of the Year for 2007