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"
Tails of Comet ZTF
9 January 2023
Comet ZTF may become visible to the unaided eye. Discovered early last year, this massive snowball has been brightening as it approaches the Sun and the Earth. C/2022 E3 (ZTF) will be closest to the Sun later this week, at which time it may become visible even without binoculars to northern observers with a clear and dark sky. As they near the Sun, comet brightnesses are notoriously hard to predict, though. In the featured image taken last week in front of a picturesque star field, three blue ion tails extend to the upper right, likely the result of a variable solar wind on ions ejected by the icy comet nucleus. The comet's white dust tail is visible to the upper left and much shorter. The green glow is the comet's coma, caused by glowing carbon gas. Comet ZTF is expected to pass nearest the Earth in early February, after which it should dim dramatically. Discovery + Outreach: Graduate student research position open for APOD