A time cannon, also known as a temporal disruption pulser, was a Gallifreyan weapon which would send its targets far back in time while also transporting them through space. The Hedrons got ahold of a time cannon, and used it to jettison those who had been infected with genetic microbes that were fatal to their species.
A time cannon once hit the Doctor's TARDIS whilst the Sixth Doctor and Frobisher were aboard. (COMIC: Time Bomb)