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, Moon, and Venus
23 July 2015
It is the object to the left of the big tree that's generating much recent excitement. If you look closely, there you can see Comet PanSTARRS, complete with two tails. During July, this comet has increased markedly in brightness and has just passed its closest approach to Earth. The statuesque tree in the center is a Norfolk Island Pine, and to either side of this tree are New Zealand Pohutukawa trees. Over the trees, far in the distance, are bright Venus and an even brighter crescent Moon. If you look even more closely, you can find Jupiter hidden in the branches of the pine. The featured image was taken a few days ago in Fergusson Park, New Zealand, looking over Tauranga Harbour Inlet. In the coming days and weeks, Comet C/2014 Q1 (PANSTARRS) will slowly move away from the Sun and the Earth, drift deep into southern skies, and fade. Follow APOD on: Facebook, Google Plus, or Twitter