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 Lemmon and the Milky Way
25 November 2025
What did Comet Lemmon look like when it was at its best? One example is pictured here, featuring three celestial spectacles all at different distances. The closest spectacle is the snowcapped Meili Mountains, part of the Himalayas in China. The middle marvel is Comet Lemmon near its picturesque best early this month, showing not only a white dust tail trailing off to the right but its blue solar wind-distorted ion tail trailing off to the left. Far in the distance on the left is the magnificent central plane of our Milky Way Galaxy, featuring dark dust, red nebula, and including billions of Sun-like stars. Comet C/2025 A6 (Lemmon) is already fading as it heads back into the outer Solar System, while the Himalayan mountains will gradually erode over the next billion years. The Milky Way Galaxy, though, will live on -- forming new mountains and comets -- for many billions of years into the future.