Comet

A small Solar System body made largely of ice, dust, and rock. Near the Sun, escaping gas and dust can form a glowing coma and tails that point generally away from the Sun.

Comet nucleus: The solid central body of a comet, composed mainly of ice, dust, and rocky material. Its surface supplies the gas and dust that make the comet visibly active.
Small Solar System body: A Solar System object that is neither a planet nor a dwarf planet and is not a natural satellite. Comets are one class of these smaller bodies.
Halley's Comet: A periodic comet visible from Earth roughly every 75 to 76 years. Its repeated returns made it central to recognizing that comets can follow predictable orbits.
Aristotle: An ancient Greek philosopher whose writings shaped European natural philosophy for centuries. His account treated comets as atmospheric phenomena rather than bodies in space.
Rosetta mission: A European Space Agency mission that orbited Comet 67P/Churyumov–Gerasimenko and deployed the Philae lander. It tracked a comet’s changing activity while measuring its nucleus and surrounding material.
Coma: The cloud of gas and dust surrounding an active comet’s nucleus. Sublimated material expands around the nucleus to create this temporary atmosphere.
Kuiper belt: A distant region beyond Neptune containing icy bodies orbiting the Sun. Many short-period comets are linked to objects scattered from this reservoir.
Comet Hale–Bopp: A bright comet discovered independently by Alan Hale and Thomas Bopp in 1995. Its unusually long naked-eye visibility made it a widely observed example of cometary activity.
Tycho Brahe: A Danish astronomer known for precise observations of celestial positions before the telescope. His observations of the Great Comet of 1577 showed it lay beyond the Moon.
Stardust (spacecraft): A NASA spacecraft that collected dust from Comet Wild 2 and returned it to Earth. Its samples enabled laboratory study of cometary particles.

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