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"
Sail On, Stardust
25 January 2001
Spacecraft on long interplanetary voyages often use the planets themselves as gravitational "sling shots" to boost them along their way. Launched in February of 1999 on a historic voyage to a comet, the Stardust spacecraft is no different. On 15 January 2001 Stardust made its closest approach to planet Earth since launch, coming within about 6,000 kilometers of the surface. It used this gravity assist maneuver to increase its speed and alter its trajectory toward an encounter with comet Wild 2, which it should reach in 2004. Shortly before its time of closest approach, astronomer Gordon Garradd recorded this exposure of Stardust sailing through the skies above Loomberah, Australia. Nearby and moving fast, the spacecraft appears as a streak against a background of faint stars in the constellation Cetus. Stardust cruised within just 98,000 kilometers of the Moon about 15 hours later. After collecting dust from the tail of comet Wild 2, Stardust's voyage will continue -- as it returns the samples to Earth in 2006.