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"
3D Barringer Meteorite Crater
23 June 2007
Barringer Meteorite Crater, near Winslow, Arizona, is one of the best known impact craters on planet Earth. View this color stereo anaglyph with red/blue glasses to get a dramatic sense of the crater's dimensions -- one mile wide, and up to 570 feet deep. (A cross-eyed stereo pair is available here.) Historically, this crater is the first recognized to be caused by an impact rather than a volcanic eruption. Modern research indicates that the impactor responsible, a 300,000 ton nickel-iron meteor, struck some 50,000 years ago. Estimates suggest that it was about 130 feet across and was traveling over 26,000 miles per hour. For comparison, the asteroid or comet impactor that created the Chicxulub crater 65 million years ago, and is thought to have caused the extinction of the dinosaurs, was 6 to 12 miles across.