They are calling it the 200-mile-high club - after decades of avoiding the subject, space travel experts are preparing for the first orbital docking between man and woman.
The arrival of an American astronaut and two Russian cosmonauts at the International Space Station earlier this month heralded a new era in which people will learn to live and work in space for long periods. The next crew, due to arrive in February, will include an American woman.
Nasa has always avoided the topic of sex in space, and Russia's explorations in this area have been cloaked in Cold War secrecy. But the agencies involved in the ISS have been forced to confront the issue after the publication in a specialist journal last week of an exhaustive study on the effects of isolation on astronauts.
Professor Stephen Johnson, editor of Quest, a quarterly journal on the history of space flight, said one passage of the study dealt with how astronauts coped with sexual stress in space. 'We don't know in long duration space flight whether cosmonauts or astronauts masturbate and relieve their sexual tension that way,' Dr Victor Schneider, chief physician for the Nasa/Mir mission, told the study's author. 'As long as they do it in private, it's a potential relief of sexual problems that may have occurred.'
Johnson, who is also assistant professor of space studies at the University of North Dakota, said that while some aspects of the study may grab people's attention for the wrong reasons, they were relevant to the subject. 'The article breaks new ground.' The 15,000-word, peer-reviewed study, The Psychological and Social Effects of Isolation on Earth and in Space , was written by science journalist Peter Pesavento, who investigated allegations of adultery in space by two Russian cosmonauts, the watching of pornographic videos on board spacecraft, and plans to film a couple having sex on Mir.
Pesavento said: 'There are issues that both the Russian and American space medicine communities have not yet definitively resolved, including depression during orbit, less-than-encouraging social interactions with mixed-gender and mixed-foreign national groups, sensory deprivation, as well as the emotional and sexual effects of close confinement on crew members.'
American comment on the ultimate contact has been limited to unofficial testimony. Harry Stine, a former Nasa technician, recalls in his book Life in Space how weightless sex was simulated - after hours - in a buoyancy tank used for astronaut training. 'It was possible but difficult,' he said, 'and was made easier when a third person assisted by holding one of the others in place.'
Pesavento said the Russians were now more forthcoming about the problems of isolation, revealing in diaries from the time their depressions and anxieties. 'No need to say what we're longing for,' wrote Dr Valery Polyakov, who holds the space endurance record for his 14-month stay on board Mir in 1992. 'Men think about those things. One can't turn away from them. But these thoughts somehow fade with time. Perhaps there is some tension building up, but "pollution" helps to unload it for a while. Then all these feelings become suppressed once again. When preparing for return to Earth, we try to regain them. Psychological Support Service sent us some nice, "colourful" movies which help to recover our will, to act like a normal adult male. There is nothing to be ashamed of. We all have wives...'
Polyakov is rumoured to have been the first member of the 200-mile-high club, along with his fellow - and married - cosmonaut, Yelena Kondakova. But allegations that the two had sex during a mission have been brushed off by them and denied by officials.
Polyakov was once asked if any 'experiments' in human reproduction had taken place. He replied: 'Only in animals. And only by the Americans. Any information that such experiments were made in the Russian program is not true.'
The cosmonaut did, however, reflect on human sexuality in space: 'It would be desirable to have a normal sex life in long-term space flights. Some people have asked me about ways of compensating for sexual abstention in space, particularly about using a doll one can buy in a sex shop.
'I strongly opposed such a solution. A person who is using such things may develop so-called "doll syndrome" or, in other words, start preferring the doll to their own spouse or loved one, even after they return to Earth.
But tensions do arise. Last January a Canadian scientist taking part in a 136-day visit to a Mir simulation module was sexually assaulted by a cosmonaut while two of his colleagues, who had been drinking vodka during New Year celebrations, had a fist fight.