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"
Deep Space 1
3 December 1998
Going gently into the night, Deep Space 1's ion drive has been running smoothly since it was restarted on November 24. How powerful is this high-tech spacecraft's ion propulsion system? At full throttle the engine will consume about 2,100 watts of electrical power generated by solar panels and produce about 1/50th of a pound of thrust. This is roughly equivalent to the force you exert when holding up a single sheet of paper! While clearly not suitable for vehicles which need rapid acceleration, ion propulsion is advantageous to use for missions involving asteroid and comet rendezvous. For these long space voyages which ultimately require a lot of energy, the continuous, gentle nudge of an ion engine easily wins out over brief, powerful, but less efficient blasts from chemical rockets. Pictured here during ground tests, the Deep Space 1 craft is now about three million miles from planet Earth.