Officially, NASA denies any such thing and there are no official, confirmed, yes-they-definitely-got-it-on reports about anyone doing it in space. (The idea of space sex is so good, though, that it has inspired at least one hoax. Pierre Kohler, the author of The Last Mission, claimed that NASA had commissioned a study on sexual positions in space back in 2003, complete with special two-person sleeping bags to make getting together a bit easier in low-gravity situations. The claim was debunked, but that hasn't stopped people from continuing to be fascinated with the idea of getting it on in orbit.)
But as we get closer to making the dream of sending astronauts on the very long trip to Mars a reality, it's understandable that people are wondering if a little space love (whether of the personal kind or via a duo) has ever happened or is even that likely.
There's been gossipy excitement over this idea ever since the first woman went to space in 1963, (followed by the second woman in 1982, also a Russian and then the first American woman in space, Sally Ride, in 1983) but the odds are good that if it's ever happened the space sex was more about the novelty of nookie in space and less about the actual joy of it. Logistically, it doesn't seem like having sex in space would be that easy or terribly fun.
For one thing, there are tons of physiological changes that happen during spaceflight that could play a part in sex and (if things were really serious with the partner or if it was a really long trip) procreation. When you're in space you experience gravity changes, radiation, vibration, noise, isolation, disrupted circadian rhythms and a whole bunch of stress. All of this can make it difficult to actually get together and do the deed.
There's also the little question of gravity. Gravity helps blood flow rapidly to certain places on the body and without gravity, men may not be able to get erections and women may also not get quite as physically aroused by sex.
So yeah, even if they can do it, so to speak, it's not necessarily a terribly satisfying encounter.
The idea of human sexual activity in the weightlessness or extreme environments of outer space – sex in space – presents difficulties for the performance of most sexual activities because of Newton's third law. According to the law, if the couple remain attached, their movements will counter each other, which means their actions won't change velocity unless they are affected by another, unattached object. There could also be problems from drifting into other objects (which makes sense when you think about it) since if the couple have a combined velocity relative to other objects, collisions could occur.
So far, NASA isn't down with having formal space sex experiments to let us know how any of this is for sure, but Pornhub is definitely interested. The company in 2015 announced it intends to make a space sex film and opened up its efforts to crowd funding. You'd think that would have translated to porn in sex immediately being made, but Pornhub has collected only a fraction of the more than $3 million required to make the movie.
So for now, sex in space is going to have to continue to live where it does best, the popular imagination. Unless, of course, there are any astronauts out there with some fun stories to tell about how exactly they defied the problems of gravity and Newton's third law to get it on. In which case, we all definitely want to hear what they've got to say.