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"
Stars, Meteors, and a Comet in Taurus
7 January 2019
This was an unusual night to look in the direction of the Bull. The constellation Taurus is always well known for hosting two bright star clusters -- the Pleaides, visible on the right, and the comparatively diffuse Hyades, visible on the left. This night last month, however, was atypically the peak of the Geminid meteor shower, and so several meteors were caught shooting through the constellation with parallel trails. More unusually still, Comet Wirtanen was drifting through the constellation, here appearing near the image bottom surrounded by a greenish coma. The comet was near its brightest as it sped past the Earth. The orange star on the upper left is Aldebaran, considered to be the eye of the Bull. Aldebaran is the brightest star in Taurus and the 15th brightest star in the sky. The featured image is a combination of nearly 800 exposures taken from the Spanish village Albany�. Follow APOD on: Instagram, Facebook, Reddit, or Twitter