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"
Active Comet ISON
16 November 2013
Falling through planet Earth's predawn skies toward its close encounter with the Sun on November 28, Comet ISON is coming to life. The much anticipated comet has now been reported to have substantially increased in activity, surging to naked-eye visibility for dark sites and sprouting a more complex tail. ISON's tail stretches over two degrees in this telephoto skyview from southern Kenya, captured on the morning of November 14. Shown in two panels, the enlarged negative version on the right makes details of the long tail easier to trace, including the tail's separated filaments toward the top of the frame. A sungrazer and first time visitor to the inner solar system, the possibility of ISON's survival to become a bright comet in planet Earth's December skies remains a question.