Comets

A comet is a celestial body, usually orbiting a star, composed of rock and ice. If a comet approaches close enough to a star, the increased heat causes it shed mass away from the star, creating a coma and/or "tail".

One of the best known comets in the Sol system is Halley's Comet. (TNG: "Time's Arrow"; VOY: "Future's End")

In June 2151, the Earth starship Enterprise NX-01 discovered the largest comet yet observed by humans. It was named for Enterprise's captain, Jonathan Archer. (ENT: "Breaking the Ice")

Comet Burke on approach to Mars.In the 22nd century, many comets in the Sol system were diverted to Mars by the verteron array for the Martian terraforming project. At one point, fourteen comets were scheduled to impact with Mars in a thirty-month span, starting with the arrival of comet Burke in January of 2155. (ENT: "Terra Prime")

The comet Icarus IV in the Romulan Neutral Zone was the locus for part of the battle between the USS Enterprise and a Romulan Bird-of-Prey in 2266. (TOS: "Balance of Terror")

The USS Enterprise-D tracked a rogue comet through Sector 1156 in 2370. Its core, composed largely of fortanium, proved to be a D'Arsay cultural archive, built over 78 million years previous. (TNG: "Masks")

In 2371, the wormhole relay station experiment conducted by the USS Defiant in the Gamma Quadrant diverted a comet laden with silithium towards the Bajoran wormhole. Some interpreted this comet as the "Sword of Stars", an element in Trakor's Third Prophecy. (DS9: "Destiny")

In 2373, the USS Enterprise-E detected a class-2 comet while on patrol in the Romulan Neutral Zone. (Star Trek: First Contact)

In 2377, Harry Kim, Seven of Nine, and the Doctor undertook a mission to collect cometary bio-matter, so that the Doctor could create new medicines. (VOY: "Body and Soul")

In the Sol system, many comets originate from the Oort cloud, a collection of ice, dust, and other debris particles that marks the outer boundary of the solar system. Other comets are known to come from the Kuiper belt.