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 NEOWISE over Lebanon
7 July 2020
A comet has suddenly become visible to the unaided eye. Comet C/2020 F3 (NEOWISE) was discovered in late March and brightened as it reached its closest approach to the Sun, inside the orbit of Mercury, late last week. The interplanetary iceberg survived solar heating, so far, and is now becoming closer to the Earth as it starts its long trek back to the outer Solar System. As Comet NEOWISE became one of the few naked-eye comets of the 21st Century, word spread quickly, and the comet has already been photographed behind many famous sites and cities around the globe. Featured, Comet NEOWISE was captured over Lebanon two days ago just before sunrise. The future brightness of Comet NEOWISE remains somewhat uncertain but the comet will likely continue to be findable not only in the early morning sky, but also next week in the early evening sky. Comet NEOWISE from Around the Globe: Notable Images Submitted to APOD