Cressida was a moon of Uranus.
History
It was turned into a space station in the 41st century. One of its main functions was to adjust the transmissions going through the Hub, which was the Galactic Federation's communications system. In 4010, the asteroid Maleficent collided with Uranus. The chemical reaction between the rare ores in the asteroid and the planet's atmosphere resulted in an amazing display. Merculite tried to avert the collision, but was stopped by the Seventh Doctor. However, in the process Cressida was destroyed. (PROSE: Uranus [+]Craig Hinton, Short Trips: The Solar System (Short Trips short stories, 2005).)
Behind the scenes
Since Cressida is named after the mythological figure, in Doctor Who universe lore, per TV: The Myth Makers [+]Donald Cotton, Doctor Who season 3 (BBC1, 1965)., it's actually named after First Doctor companion Vicki Pallister.
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