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"
LHS 475 b: Earth-Sized Exoplanet
24 January 2023
If you could stand on exoplanet LHS 475 b, what might you see? No one knows for sure but pictured here is an interesting guess made by an Earth-based artificial intelligence (AI) engine. The existence of the exoplanet was indicated in data taken by the Earth-orbiting TESS satellite but confirmed and further investigated only this year by the near-Earth Sun-orbiting James Webb Space Telescope. What is known for sure is that LHS 475 b has a mass very similar to our Earth and closely orbits a small red star about 40 light years away. The featured AI-illustrated guess depicts a plausibly rugged Earth-like landscape replete with molten lava and with the central red star rising in the distance. Webb data does not as yet reveal, however, whether LHS 475 b has an atmosphere. One of Webb’s science objectives is to follow up previous discoveries of distant exoplanets to better discern their potential for developing life. Comet ZTF Gallery: Notable Submissions to APOD