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 with M44
9 November 2013
While anxiously waiting for Comet ISON to brighten further as it falls toward the Sun, northern skygazers can also find three other bright comets in the east before dawn. In fact, Comet Lovejoy C/2013 R1 is currently the morning sky's brightest. Only discovered in September and not a sungrazing comet, this Comet Lovejoy is nearing the edge of naked-eye visibility and might be spotted from very dark sky sites. Sporting a greenish coma and tail in this telescopic view taken on November 7, Comet Lovejoy is about 0.5 AU from our fair planet and 1.2 AU from the Sun. The comet is having a photogenic Messier moment, sweeping past well known star cluster M44, the Beehive in Cancer. Yellowish bright star Delta Cancri is near the bottom of the frame.