10. Astrid Peth, "Voyage of the Damned"
A one-off companion played to a bright-eyed quizzical delight by songstress Kylie Minogue, Astrid was never going to be a full-time companion but her exit is still pretty shocking, given that it happens as a heroic sacrifice while she and the Doctor are busy trying to stop a nuclear-powered cruise ship from crashing into the Earth. Astrid, manning a forklift truck like no-one's business, drives evil conglomerate head-in-a-jar Max Capricorn over the edge into the engines, saving the Doctor and the human race as a result. It's sad and tragically heroic but her molecules, frozen in time, are partially restored and she moves on into the universe as sentient particles. A bittersweet ending for the girl who wanted to see the stars.