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"
Gamma-rays and Comet Dust
20 August 2016
Gamma-rays and dust from periodic Comet Swift-Tuttle plowed through planet Earth's atmosphere on the night of August 11/12. Impacting at about 60 kilometers per second the grains of comet dust produced this year's remarkably active Perseid meteor shower. This composite wide-angle image of aligned shower meteors covers a 4.5 hour period on that Perseid night. In it the flashing meteor streaks can be traced back to the shower's origin on the sky. Alongside the Milky Way in the constellation Perseus, the radiant marks the direction along the perodic comet's orbit. Traveling at the speed of light, cosmic gamma-rays impacting Earth's atmosphere generated showers too, showers of high energy particles. Just as the meteor streaks point back to their origin, the even briefer flashes of light from the particles can be used to reconstruct the direction of the particle shower, to point back to the origin on the sky of the incoming gamma-ray. Unlike the meteors, the incredibly fast particle shower flashes can't be followed by eye. But both can be followed by the high speed cameras on the multi-mirrored dishes in the foreground. Of course, the dishes are MAGIC (Major Atmospheric Gamma Imaging Cherenkov) telescopes, an Earth-based gamma-ray observatory on the Canary Island of La Palma.