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"
Comet C/2002 T7 (LINEAR)
22 April 2004
Discovered by the the Lincoln Near Earth Asteroid Research (LINEAR) project in October of 2002, comet C/2002 T7 is now visiting the inner solar system, making its closest approach (see animation by L. Koehn) to the Sun tomorrow, April 23rd. Emerging from the solar glare, the comet is now just visible to the unaided eye in the constellation Pisces, near the eastern horizon in morning twilight. In this gorgeous telescopic view recorded before dawn yesterday, the clearly active comet has developed an extensive, complex tail extending over 2 degrees in the anti-sunward direction, and a pronounced anti-tail or anomalous tail. Later next month this comet should appear brighter, making its closest approach to planet Earth on May 19th. In fact, it could share southern skies with another naked-eye comet, also anticipated to brighten in May, designated C/2001 Q4 (NEAT).