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"
Comets Bradfield and LINEAR Rising
3 May 2004
Comet Bradfield is easy to see on the left, but can you find Comet LINEAR on the right? Last week, just before sunrise from the northern hemisphere, two bright comets were visible in the same part of the sky at the same time. The above long-exposure image was taken on the morning of April 25 from Joshua Tree National Park in California, USA. Comet C/2004 F4 (Bradfield) is giving an unexpectedly good show as it recedes from the Sun and Earth and fades from view. Its tail is estimated by some to be about 10 degrees long. Having just rounded the Sun itself, Comet C/2002 T7 (LINEAR) is now moving toward the Earth. Although intrinsically fading, T7 will appear to brighten until about mid-May and so continue to be visible to the unaided eye before sunrise to southern hemisphere observers into June. Q4, the third coincidental naked eye comet, will become visible in mid-May to northern hemisphere observers.