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"
Castle Eye View
7 April 2017
The best known asterism in northern skies, The Big Dipper is easy to recognize, even when viewed upside down, though some might see a plough or wagon. The star names and the familiar outlines appear framed in the ruined tower walls of the French Chateau du Morimont if you just slide your cursor over the image or follow this link. Dubhe, alpha star of the dipper's parent constellation Ursa Major is at the lower left. Together with beta star Merak the two form a line pointing the way to Polaris and the North Celestial Pole, hidden from view by the stones. Since the image was captured on March 30, you can follow a line from dipper stars Phecda and Megrez to spot the faint greenish glow of Comet 41P/Tuttle-Giacobini-Kresak below center, still within the castle eye view. The periodic comet made a remarkable close approach to planet Earth on April 1.