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"
Perseids of Summer
10 August 2001
Like falling stardust, cast off bits of comet Swift-Tuttle hurtle through the upper atmosphere about this time each year as planet Earth passes near the comet's orbital path. For the northern hemisphere, this regular celestial display is known as the annual Perseid meteor shower -- so named because the meteor trails all appear traceable to a common "radiant point" in the constellation Perseus. This gorgeous wide-angle photo from the 1997 shower captures a 20-degree-long fireball meteor and another, fainter Perseid meteor trail in a rich area of the northern summer Milky Way. A labeled version is available identifying the shower's radiant point, surrounding deep-sky objects, and constellations. Easy to view (just go outside and look up!), the Perseid meteor shower will peak this weekend with maximum rates anticipated early Sunday morning, August 12, for eastern North America. Despite interfering moonlight, last year's faithful Perseid watchers were rewarded with bright meteors and extensive displays of the northern lights.