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.
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APODs including "Comet"
Asteroid 1998 WT24 Passes Near Earth
24 December 2001
Last week, an asteroid approached unusually close to the Earth. Passing well outside the orbit of our Moon, Asteroid 1998 WT24 posed no danger, but became bright enough to see with binoculars and to track with radar. Pictured above, the kilometer-sized asteroid was imaged crossing the sky on December 14, two days before closest approach. Every few years, an asteroid will actually pass inside the orbit of the Moon. Large impact features on the Earth are testaments to asteroids or comets that actually impacted the Earth in the distant past. Astronomers continue to discover, track, and study potentially hazardous asteroids with a goal of making planet Earth a safer place.