David's leaving soon (sad) and here is the airdate for the next special:
Doctor Who: The Waters of Mars airs on BBC AMERICA, Saturday December 19, 9:00pm ET/PT.
It stars Tennant and British stage and screen actress Lindsay Duncan (Rome, Langford) as Adelaide, his cleverest and most strong-minded companion.
All will be revealed as the Doctor and his companion Adelaide face terror on the Red Planet in one of the scariest adventures yet. Peter O'Brien (Casualty, Neighbours) guest stars as Ed, Adelaide's second-in-command at the base.