Pierre Kohler, a French scientific writer, claimed in another book that NASA had tested 20 positions by computer simulation and then arranged for two people to try the best 10 in zero gravity.

Only four were possible to reach without "mechanical assistance", according to Kohler. An elastic belt and an inflatable tunnel, like an open-ended sleeping bag, were needed for the other six.

"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," he wrote.