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"
Philae Lander Found on Comet 67P
12 September 2016
A little spacecraft that was presumed lost has now been found. In 2014, the Philae lander slowly descended from its parent Rosetta spacecraft to the nucleus of Comet C67/P Churyumov-Gerasimenko. At the surface, after a harpoon malfunction, the lander bounced softly twice and eventually sent back images from an unknown location. Earlier this month, though, Rosetta swooped low enough to spot its cub. The meter-sized Philae is seen on the far right of the main image, with inset images showing both a zoom out and a zoom in. At the end of this month, Rosetta itself will be directed to land on 67P, but Rosetta's landing will be harder and, although taking unique images and data, will bring the mission to an end. Free Download: APOD 2017 Calendar: NASA Images