Clancy's comet
From Tardis Wiki, the free Doctor Who reference
Clancy's comet was a comet which passed Earth every 157 years, as discovered by the astronomer Theodore Clancy. It was visible at points in 1504, 1661, 1818, on 17 May, 1975, and in 2132.
While in Italy, in May of 1975, Sarah Jane Smith, the Third Doctor and Jeremy Fitzoliver discovered that the return of the comet amplified the ability of people to cross over into N-Space. (AUDIO: The Ghosts of N-Space)
