While humans have been a spacefaring species for more than 50 years, it's quite possible we have yet to perform that most basic of acts — sex — beyond terra firma. Yet.

Rumors have long swirled that astronauts may have hooked up in orbit, perhaps even as part of secret sex-in-space experiments run by the Russian or American governments. But those stories are likely the product of overactive — and overheated — imaginations, experts say.

A Russian space official, for example, has categorically denied any such weightless shenanigans by his countrymen, the news agency AFP reported Friday (April 22).

"There is no official or unofficial evidence that there were instances of sexual intercourse or the carrying out of sexual experiments in space," Valery Bogomolov, the deputy director of the Moscow-based Institute of Biomedical Problems, told the news agency Interfax, AFP reported. "At least, in the history of Russian or Soviet space exploration, this most certainly was not the case." [10 Surprising Sex Statistics]

What about NASA astronauts?

Bogomolov also addressed the rumors of American hanky-panky, though with considerably less authority.

"As for American space exploration, well, I just don't have the information to categorically deny that," Bogomolov told Interfax, according to AFP. "There are just anecdotal rumors, which are not worth trusting."

But those in the know say NASA astronauts have likely been as chaste as their Russian counterparts while zipping around Earth at 17,500 mph (28,164 kph). While NASA apparently doesn't explicitly forbid sex in orbit, its astronaut code of conduct calls for "relationships of trust" and "professional standards" to be maintained at all times.

"I'm not aware of an official NASA policy on this," said former astronaut Leroy Chiao, a veteran of four space missions between 1994 and 2005. "It was not discussed when I was there, it was simply understood. Nobody brought it up — it simply wasn't a consideration."

Despite the advent of mixed-gender crews in 1983, NASA astronauts seem to have behaved themselves in orbit, according to Chiao.

"As for any couple having had sex in space, I seriously doubt it," Chiao told SPACE.com in an email interview.

Chiao, who spent more than 229 days in space, explained some of his reasoning in a blog post for the tech website Gizmodo back in 2009.

"Guys are guys. If a guy had sex in space, he would not be able to stand not bragging about it," Chiao wrote. "Sorry to disappoint you, but there it is. We would all know about it. Or, I should say, we will all know about it when it happens."

Other astronauts have backed Chiao's viewpoint, saying that NASA's spaceflyers have thus far been too focused on their missions to risk any romantic entanglements in orbit.

That's not to say that such entanglements don't unfold back on Earth, however, as the messy love triangle involving then-astronauts William Oefelein and Lisa Nowak demonstrates. Nowak was arrested in 2007 for allegedly attacking a woman she viewed as a rival for Oefelein's affections. She ultimately received probation.

It will happen

Sex in space will happen eventually, if it hasn't already. It's one thing for a space shuttle crew to contain themselves for a few weeks, or astronauts aboard the station to remain chaste for five or six months. But manned missions to Mars would last years, so abstinence for that long would be a tall order for most people.

And sex would likely be a natural part of life at a lunar or Mars base, especially if the aim is to one day establish a self-sustaining colony.

The rise of private spaceflight should open the door even more to sex in space. Space tourists would not be bound by NASA's code of conduct, or as restricted by the demands of a complicated mission.

And some people will probably even fly to space just to join the "220-mile-high club." Virgin Galactic, which hopes to start flying tourists to suborbital space as early as next year, has already turned down a $1 million offer from an unidentified party to aid in the production of a sex-in-space movie.

You can follow SPACE.com senior writer Mike Wall on Twitter: @michaeldwall. Follow SPACE.com for the latest in space science and exploration news on Twitter @Spacedotcom and on Facebook.