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"
A Swift Look at Tempel 1
3 July 2005
Comet Tempel 1 is targeted for a collision with the impactor probe from NASA's Deep Impact Spacecraft at about 1:52am EDT on July 4th (other time zones). Cameras on the impactor probe and the flyby spacecraft will capture close-up images of the event - expected to produce a crater on the surface of the comet's nucleus. Of course, Earth-orbiting and ground-based telescopes will be watching too, including instruments on the Swift satellite normally used to spot gamma-ray bursts in the distant universe. Swift's ultraviolet telescope recorded this picture of Tempel 1 on June 29th. Because the image is registered on the comet, the background stars appear as short trails. Want to follow the encounter? Media coverage chronicling the event, and the possibilities for viewing the comet with small telescopes can be found through the Deep Impact website. Updated images will also be available from the Kitt Peak National Observatory.