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 Iwamoto and the Sombrero Galaxy
9 February 2019
Comet Iwamoto (C/2018 Y1), shows off a pretty, greenish coma at the upper left in this telescopic field of view. Taken on February 4 from the Mount John Observatory, University of Canterbury, the 30 minute long total exposure time shows the comet sweeping quickly across a background of stars and distant galaxies in the constellation Virgo. The long exposure and Iwamoto's rapid motion relative to the stars and galaxies results in the noticeable blurred streak tracing the the comet's bright inner coma. In fact, the streaked coma gives the comet a remarkably similar appearance to Messier 104 at lower right, popularly known as the Sombrero Galaxy. The comet, a visitor to the inner Solar System, is a mere 4 light-minutes away though, while majestic Messier 104, a spiral galaxy posing edge-on, is 30 million light-years distant. The first binocular comet of 2019, Iwamoto will pass closest to Earth on February 12. This comet's highly elliptical orbit around the Sun stretches beyond the Kuiper belt with an estimated 1,371 year orbital period. That should bring it back to the inner Solar System in 3390 AD.