Halley's Comet was a comet of the Sol system. It was one of the few that could be seen by the naked eye from Earth. However, it was only visible in this way when it approached the planet every 75 to 76 years.
Time travelling to 1985, the Cybermen 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)