This note was added on 30 January 2010:

In the story below, the Nasa report cited by Pierre Kohler proved to be a hoax. See this later explanatory Guardian blog: theguardian.com/news/blog/2007/dec/06/sexinspaceornot

US and Russian astronauts have had sex in space for separate research programmes on how human beings might survive years in orbit, according to a book published yesterday.

Pierre Kohler, a respected French scientific writer, says in The Final Mission: Mir, The Human Adventure that the subject is taboo both at Nasa and at mission control in Moscow, but that cosmic couplings have taken place.

"The issue of sex in space is a serious one," he says. "The experiments carried out so far relate to missions planned for married couples on the future International Space Station, the successor to Mir. Scientists need to know how far sexual relations are possible without gravity."

He cites a confidential Nasa report on a space shuttle mission in 1996. A project codenamed STS-XX was to explore sexual positions possible in a weightless atmosphere.

Twenty positions were tested by computer simulation to obtain the best 10, he 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."