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 Garradd and M92
4 February 2012
Sweeping slowly through the constellation Hercules, Comet Garradd (C2009/P1) passed with about 0.5 degrees of globular star cluster M92 on February 3. Captured here in its latest Messier moment, the steady performer remains just below naked-eye visibility with a central coma comparable in brightness to the dense, well-known star cluster. The rich telescopic view from New Mexico's, early morning skies, also features Garradd's broad fan shaped dust tail and a much narrower ion tail that extends up and beyond the right edge of the frame. Pushed out by the pressure of sunlight, the dust tail tends to trail the comet along its orbit while the ion tail, blown by the solar wind, streams away from the comet in the direction opposite the Sun. Of course, M92 is over 25,000 light-years away. Comet Garradd is 12.5 light-minutes from planet Earth, arcing above the ecliptic plane.