Comet

A comet was a ball of ice and rock that existed in space. When their orbit took them close to a star, the solar winds melted the ice and formed a tail behind it. Many comets existed in the Oort cloud. (AUDIO: Murmurs of Earth) Debris from comets formed meteorites. (TV: Spearhead from Space)

Comets
Halley's Comet was one of the best-known comets in Earth history. Its orbit brought it close to Earth approximately every seventy-five years. It collided with a Persopolisian ship in 1066. (AUDIO: The Time Capsule) The Cybermen time travelled to 1985, planning to divert Halley's Comet to crash into Earth, thus preventing the destruction of their home planet Mondas the following year. The plan was foiled by the Sixth Doctor. (TV: Attack of the Cybermen)

A comet was discovered by the astronomer Theodore Clancy in 1661. It returned to Earth every one hundred fifty-seven years. (AUDIO: The Ghosts of N-Space)

The rogue planet Skardal was created when a comet travelled through a cloud of space dust, causing each dust particle to become magnetised and join together. (COMIC: The Rogue Planet)

In 250,339, the comet Schuler-Khan was on a collision course with the Gyre. (PROSE: Night of the Humans)

Other references
Nemesis, a validium of Gallifreyan origin, appeared as a comet. (TV: Silver Nemesis)

The Terileptil escape pod that crashed in England in 1666 was thought to be a comet. (TV: The Visitation)

Hobo Kostinen got annoyed whenever people would use the terms "comet" and "asteroid" interchangeably, knowing that they were two different things. (PROSE: Lucy Wilson & the Bledoe Cadets)