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"
That Night over Half Dome
2 May 2014
Captured one night last May this eight frame mosaic starts on the left, down Northside Drive through Yosemite National Park. It ends thousands of light-years away though, as the arc of the Milky Way tracks toward the center of our galaxy on the right, far beyond the park's rugged skyline. That night was still moonless when the storm clouds retreated, so the rocky faces of the surrounding mountains are lit by campfires and artificial lights. Yosemite Falls is at the left. The granite face of Half Dome juts above the far horizon, near the center of the view. The remarkable flash above it is a bright meteor. Part of the annual Eta Aquarid meteor shower the colorful streak is moving up, its trail pointing directly back to the shower's radiant, low in Aquarius. This year's Eta Aquarids should peak in the moonless early morning hours of May 6 as the Earth sweeps through dust from the tail of Comet Halley.