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"
700 Kilometers Below Comet Hartley 2
8 November 2010
What kind of comet is this? Last week, NASA's robotic EPOXI spacecraft whizzed past Comet 103P/Hartley, also known as Comet Hartley 2, and recorded images and data that are both strange and fascinating. EPOXI was near its closest approach -- about 700 kilometers away -- when it snapped the above picture. As expected, the comet has indeed shown itself to be a tumbling iceberg orbiting the Sun between Earth and Jupiter. However, unexpected features on the images have raised many questions. For example, where are all the craters? Why is there a large smooth area around the middle? How much of Comet Hartley 2 is a loose pile of dust and ice shards? Future analyses and comparisons to other comet nuclei may answer some of these questions and, hopefully, lead to a better general understanding of comets, meteors, and the early Solar System.