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"
The Cone Nebula Neighborhood
12 April 2007
Cosmic clouds of hydrogen gas and dust abound in this gorgeous skyscape, stretching through Monoceros in the neighborhood of The Cone Nebula. A dark, obscuring dust cloud, the simple, sculpted shape of the Cone Nebula is near the lower left edge. Surrounded by the red glow of hydrogen gas, the cone points up, toward bright, blue-white S Monocerotis, a quadruple system of very massive, hot stars. S Mon itself is also surrounded by intriguing red emission nebulae characteristic of star forming regions while above and to the right of S Mon, expansive dark markings on the sky are silhouetted by a larger region of fainter emission. Yellowish open star cluster Trumpler 5 is below picture center, with the striking blue reflection nebula, IC 2169 near the center right edge. The curious compact cometary shape near the bottom edge is Hubble's Variable Nebula. Over 31 hours of exposure time went into this truly remarkable, 2.5 degree wide, color mosaic.