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"
Comet 220P in Outburst
24 August 2026
Comet 220P is unexpectedly bright. Normally, periodic Comet 220P/McNaught is so dim that to see it requires a telescope. Two surprising outbursts this year, however, have made it about 20,000 times brighter than usual, so that it is now visible with binoculars and long-duration camera exposures. As expected, Comet 220P continues to orbit the Sun between Mars and Jupiter with a period of over 5 years. The featured long duration exposure, taken 10 days ago from South Africa, shows the comet's bright green head and short dust tail. Reasons for Comet 220P's impressive outbursts are unknown but could be caused by the release of built-up subsurface gas or comet quakes. Comet 220P will pass about one Earth-Sun distance from the Earth in October, after which it is expected to fade quickly as it begins its return to the far part of its orbit. APOD's main NASA site is moving : From apod.nasa.gov to science.nasa.gov/apod