orbit, they have to fire the engine on the far side of the moon where there's no radio control. >> the engine burn will slow them down from 5,700 to about 3,700 miles an hour. they'll have put themselves into lunar orbit. the first men in history to have done so and the trickiest and most dangerous part of their flight. >> you could have any one of a number of things go wrong. end up in the wrong orbit or hit the moon. >> we have to have absolute faith in everything that has ever been done to develop this rocket motor. all they can do is point it in the right direction, press a button and hope it works perfectly. >> apollo 8 houston, over.