You may be looking for scavengers or the audio story of the same name.

Scavenger was a satellite that was knocked online by the Sixth Doctor trying to save Flip Jackson and Jyoti Cutler on 28 May 2071. It fired at Flip and Joyti. The Doctor worked out it was alien. It took what it needed from the spy satellite. Flip was used as its host. It went after the Doctor's TARDIS, when the Doctor went to collect her. It later started to clean up Earth. (AUDIO: Scavenger)

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