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"
Philae Attempts Comet Nucleus Landing
12 November 2014
Today humanity will make its first attempt to land a probe on the nucleus of a comet. As the day progresses, the Philae (fee-LAY) lander will separate from the Rosetta spacecraft and head down to the surface of Comet 67P/Churyumov�Gerasimenko. Since the texture of the comet's surface is unknown and its surface gravity is surely low, Philae will then attempt to harpoon itself down, something that has never been done before. Featured here is an artist's illustration of dishwasher-sized Philae as it might look on Comet Churyumov�Gerasimenko's surface, along with explanation balloons detailing onboard scientific instruments. Many people on a blue planet across the Solar System will be eagerly awaiting news and updates. Whether Philae actually lands, whether it lands on a smooth patch, whether the harpoons take hold, and how far the robotic lander sinks into the surface should all become known as events unfold today. Comet lander updates: From ESA