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 ZTF: Orbital Plane Crossing
27 January 2023
The current darling of the northern night, Comet C/2022 E3 ZTF is captured in this telescopic image from a dark sky location at June Lake, California, USA. Of course Comet ZTF has been growing brighter in recent days, headed for its closest approach to Earth on February 1. But this view was recorded on January 23, very close to the time planet Earth crossed the orbital plane of long-period Comet ZTF. The comet's broad, whitish dust tail is still curved and fanned out away from the Sun as Comet ZTF sweeps along its orbit. Due to perspective near the orbital plane crossing, components of the fanned out dust tail appear on both sides of the comet's green tinted coma though, to lend Comet ZTF a visually striking (left) anti-tail. Buffeted by solar activity the comet's narrower ion tail also streams away from the coma diagonally to the right, across the nearly three degree wide field of view. Comet ZTF Gallery: Notable Submissions to APOD