Comet
A comet is an icy body that orbits the Sun on a stretched path and develops a glowing coma and tails when sunlight vaporizes its ice.
Definition
A comet is a ball of ice, dust, and rock orbiting the Sun on a long, stretched path; near the Sun its ice turns to gas and forms a glowing cloud and a long tail, which is why comets are called dirty snowballs. A typical nucleus is a few kilometers across on a highly elliptical orbit, frozen when far out, but near the Sun solar heat sublimates the ice, releasing gas and dust that form a coma up to hundreds of thousands of kilometers wide and one or two tails millions of kilometers long; short-period comets come from the Kuiper belt and long-period comets from the Oort cloud. The nucleus is a low-density aggregate ($0.3$ to $0.6\ \text{g/cm}^3$) of water ice, frozen carbon dioxide and carbon monoxide, organics, and silicate dust, and sublimation beginning near $3$ to $5$ AU drives outgassing jets that measurably perturb the orbit. Radiation pressure shapes the curved dust tail, and the solar wind's magnetic field drapes around the ionized coma to form the straight ion tail.
Example
Halley's Comet returns about every $76$ years, was last seen in 1986, and will be back in 2061; its nucleus is about $15\ \text{km}$ long and its orbit runs from inside Venus's orbit out past Neptune. ESA's Rosetta spacecraft orbited comet 67P/Churyumov-Gerasimenko from 2014 to 2016 and measured a density of about $0.53\ \text{g/cm}^3$, a porosity above $70\%$, and a deuterium-to-hydrogen ratio in its water about three times the value in Earth's oceans.
Key Insight
A comet's tail does not trail behind it like exhaust; it always points away from the Sun, so an outbound comet travels tail-first. Comets carry two tails, a curved yellowish dust tail along the orbit and a straight blue ion tail pushed by the solar wind. Comets are frozen samples of the outer solar nebula, largely unaltered for $4.6$ billion years, and their water isotope ratios test whether comets delivered Earth's oceans; Rosetta's result suggests asteroids, not comets like 67P, supplied most of it.