The Wormhole wrote:
Just watched Four to Doomsday. How the hell do those helmets work anyway? Forcefields?
Well, they're not in a vacuum when they venture out into the ship, it's just that there's no 02 to breathe (IIRC). So it'd be a bit inefficient, but if the spacepack just pumped a lot of oxygen into the area in front of their mouth they'd work OK as partial pressure breathers (the sort of thing you might wear on Mars or at the top of Everest, for instance).
None of that explains how the Doctor copes during the spacewalk, of course. But that's Time Lord miracle stuff!