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"
Impact: 65 Million Years Ago
26 February 2000
What killed the dinosaurs? Their sudden disappearance 65 million years ago, along with about 70 percent of all species then living on Earth, is known as the K-T event (Cretaceous-Tertiary Mass Extinction event). Geologists and paleontologists often entertain the idea that a large asteroid or comet impacting the Earth was the culprit. In such a cosmic catastrophe, the good(!) news would be that the impact would generate firestorms, tidal waves, earthquakes, and hurricane winds. As for the bad news ... debris thrown into the atmosphere would have a serious global environmental consequences, creating extended periods of darkness, low temperatures, and acid rains - resulting in a planet-wide extinction event. In 1990, dramatic support for this theory came from cosmochemist Alan Hildebrand's revelation of a 65 million year old, 112 mile wide ring structure still detectable under layers of sediment in the Yucatan Peninsula region of Mexico. The outlines of the structure, called the Chicxulub crater (named for a local village), are visible in the above representation of gravity and magnetic field data from the region. In addition to having the right age, the crater is consistent with the impact of an asteroid of sufficient size (6 to 12 miles wide) to cause the global disruptions. Regardless of the true cause of the K-T event, it is fortunate that such impacts are presently believed to happen only about once every 100 million years!