The Doctor:  Those shields wo n't hold them there for long .
Briggs: They 'll last until we 're back to Earth .
The Doctor: And then ?
Briggs: There 's only a few of them : our security men will cope with them .
The Doctor: They 're an invasion force , Earth is where they want to go . There are considerably more than a few on board .
Adric: How many of these silos are you carrying ?
Briggs: Oh , _ fifteen _ thousand or ...
Briggs: No
Briggs: . It 's not possible !