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 Paranal
28 December 2011
Comet Lovejoy (C/2011 W3) survived its close encounter with the Sun earlier this month, taking its place among wonders of the southern skies just in time for Christmas. Seen here before sunrise from Paranal Observatory in Chile, the sungrazing comet's tails stretch far above the eastern horizon. Spanning over 20 degrees they rise alongside the plane of the our Milky Way galaxy. A breathtaking spectacle in itself, Lovejoy performs on this celestial stage with southern stars and nebulae, including the Large and Small Magellanic clouds right of the telescope dome, and the glow of zodiacal light along the left edge of the frame. With Paranal's Very Large Telescope units in the foreground, this wide-angle scene was captured on December 23. Receding from the Sun, Comet Lovejoy's tails have continued to grow in length even as the comet fades.