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.
5 October 2022
What happens if you crash a spaceship into an asteroid? In the case of NASA's DART spaceship and the small asteroid Dimorphos, as happened last week, you get quite a plume. The goal of the planned impact was planetary protection -- to show that the path of an asteroid can be slightly altered, so that, if done right, a big space rock will miss the Earth. The high brightness of the plume, though, was unexpected by many, and what it means remains a topic of research. One possibility is that 170-meter wide Dimorphos is primarily a rubble pile asteroid and the collision dispersed some of the rubble in the pile. The featured time-lapse video covers about 20 minutes and was taken from the Les Makes Observatory on France's Reunion Island, off the southeast coast of southern Africa. One of many Earth-based observatories following the impact, the initial dot is primarily Dimorphos's larger companion: asteroid Didymos. Most recently, images show that the Didymos - Dimorphos system has developed comet-like tails. DART Impact on Dimorphos: Notable images submitted to APOD