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 Lovejoy Over a Windmill
9 December 2013
Lovejoy continues to be an impressive camera comet. Pictured above, Comet C/2013 R1 (Lovejoy) was imaged above the windmill in Saint-Michel-l'Observatoire in southern France with a six-second exposure. In the foreground is a field of lavender. Comet Lovejoy should remain available for photo opportunities for northern observers during much of December and during much of the night, although it will be fading as the month progresses and highest in the sky before sunrise. In person, the comet will be best viewed with binoculars. A giant dirty snowball, Comet Lovejoy last visited the inner Solar System about 7,000 years ago, around the time that humans developed the wheel. Budget Universe: Free APOD 2014 Calendar in PDF format