Halley's Comet, also known as Comet 9/12/44, was a comet of the Sol system. Humans on Earth associated Halley's Comet with impending disaster. (TV: Attack of the Cybermen)

Halley's Comet passed Earth in 1066, where it crashed into a Persopolisian ship. (AUDIO: The Time Capsule) When the First Doctor visited Edith in 1066, she recalled that since the comet's appearance that April, "things had been not quite right in this part of the world", "strange things had happened", and "disturbing rumours had reached the greenwood." (PROSE: The Time Meddler)

Time travelling to 1985, the y planned to divert Halley's Comet so that it would destroy Earth. This way, the Cyberman home planet of Mondas would not be destroyed when it approached Earth in December 1986. The plan was foiled by the Sixth Doctor. (TV: Attack of the Cybermen)