9.1 Purgatory 12 by Marc Platt
Still searching for a way out of E-Space, the TARDIS crew land on an isolated space rock... and immediately find it drawn towards a nearby asteroid.
The asteroid has air and gravity unequal to its size and is strewn with the wrecks of spaceships. Veins and pools of rust are everywhere.
Stuck on the asteroid away from his friends, Adric discovers that it’s a penal colony housing a gang of alien convicts - but resources are low, and they’re starting to starve.
But escaping the prisoners is only the first part of the traveller’s troubles. Because there’s a sinister presence at the heart of the asteroid... and it won’t release them quite as easily.
Note: This title is also available on CD and download on Doctor Who - The Fourth Doctor Adventures Series 09 Volume 01