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 McNaught-Hartley
14 March 2001
Outbound and climbing above the plane of our solar system, comet McNaught-Hartley (C/1999 T1) is presently soaring through northern skies. This telescopic picture, a composite of many 30 second exposures made through three color filters, recorded the delicate colors in its diminutive coma and faint tail on February 26th. Combining the exposures to produce the final image registered on the comet causes stars to appear as "dotted trails", evidence of the comet's motion relative to the distant stellar background. Discovered by southern hemisphere observers, this comet's closest approach to the Sun occurred in December last year as it passed just outside planet Earth's orbit. For now the brightest comet in the sky, this primordial chunk of solar system is crossing from the constellation Hercules to Draco and will continue to fade. Never visible to the unaided eye, McNaught-Hartley is still at about 10th magnitude and can be viewed by comet seekers using small telescopes.