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"
A Double Ringed Basin on Mercury
7 October 2009
What caused the smooth floor inside the double ringed basin on Mercury? No one is sure. The unusual feature spans 160 kilometers and was imaged during the robotic MESSENGER spacecraft's swing past our Solar System's innermost planet last week. Double and multiple ringed basins, although rare, have also been imaged in years past on Mars, Venus, Earth, and Earth's Moon. Mercury itself has several doubles, including huge Caloris basin, Rembrandt basin, and enigmatic Raditladi basin. Most large multiple ringed basins on planets and moons are caused initially by a forceful impact by a single asteroid or comet fragment. One interesting feature of the above-imaged double ring is that the basin center appears much smoother than the region between the rings. Possibly, the internal floor was smoothed by later volcanic activity. Also possible, however, is that the floor was smoothed by melting and flowing of material upon impact. MESSENGER has now completed its last flyby of Mercury but will return and attempt to enter orbit in 2011 March. Note: The text of this APOD has been updated.