10.2 The Day of the Comet by Jonathan Morris
The TARDIS brings the Doctor and Leela to the ruins of a once-great city racked by earthquakes, where they find a giant rocket on the side of a mountain. The planet is due to be destroyed by a comet in a year’s time and this ship is the only way off world.
But is everything quite as it seems? At least one person in the city thinks the comet is going to come to obliterate the planet much sooner than people believe.
Which would be bad enough - if there wasn’t a far worse secret hidden behind the scenes...
Note: This title is also available on CD as part of Doctor Who: The Fourth Doctor Adventures Series 10 Volume 01