NASA has conducted research in what sexual positions are possible in outer space:

Twenty positions were tested by computer simulation to obtain the best 10, [science writer Pierre Kohler] says. "Two guinea pigs then tested them in real zero-gravity conditions. The results were videotaped but are considered so sensitive that even NASA was only given a censored version."

Only four positions were found possible without "mechanical assistance". The other six needed a special elastic belt and inflatable tunnel, like an open-ended sleeping bag.

Mr Kohler says: "One of the principal findings was that the classic so-called missionary position, which is so easy on earth when gravity pushes one downwards, is simply not possible."