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 PanSTARRS is near the Edge
16 February 2018
The comet PanSTARRS also known as the blue comet (C/2016 R2) really is near the lower left edge of this stunning, wide field view recorded on January 13. Spanning nearly 20 degrees on the sky, the cosmic landscape is explored by well-exposed and processed frames from a sensitive digital camera. It consists of colorful clouds and dusty dark nebulae otherwise too faint for your eye to see, though. At top right, the California Nebula (aka NGC 1499) does have a familiar shape. Its coastline is over 60 light-years long and lies some 1,500 light-years away. The nebula's pronounced reddish glow is from hydrogen atoms ionized by luminous blue star Xi Persei just below it. Near bottom center, the famous Pleiades star cluster is some 400 light-years distant and around 15 light-years across. Its spectacular blue color is due to the reflection of starlight by interstellar dust. In between are hot stars of the Perseus OB2 association and dusty, dark nebulae along the edge of the nearby, massive Taurus and Perseus molecular clouds. Emission from unusually abundant ionized carbon monoxide (CO+) molecules fluorescing in sunlight is largely responsible for the telltale blue tint of the remarkable comet's tail. The comet was about 17 light minutes from Earth.