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"
Lovejoy in the New Year
3 January 2014
A rival to vanquished Comet ISON in 2013, Comet Lovejoy (C/2013 R1) still sweeps through early morning skies, captured in this starry scene on New Year's day. The frame stretches some 3.5 degrees (about 7 full moons) across a background of faint stars in the constellation Hercules. Only just visible to the naked eye from dark sites before dawn, Lovejoy remains a good target for the northern hemisphere's binocular equipped skygazers. But this deep exposure shows off Lovejoy's beautiful tails and tantalizing greenish coma better than binocular views. Not a sungrazer, this Comet Lovejoy made its closest approach to the Sun around December 22, looping high above the ecliptic plane. Now headed for the outer Solar System, Lovejoy began the new year about 6.7 light-minutes from planet Earth. Free lecture: APOD editor to speak in NYC Tonight