As plans take shape to send the first people to Mars, there remains a pressing question about the future of humankind in a place that isn't Earth: Just how does human sperm survive the ravages of space travel to let us reproduce on other planets?
The subject of space reproduction recently arose after Japanese researchers showed that freeze-dried mouse sperm could produce live baby mice after spending 288 days on the International Space Station, where they were exposed to microgravity and extreme radiation.
"Before, nobody knew whether mammalian sperm can be preserved in space, and we showed that is possible," Teruhiko Wakayama, lead researcher and biologist at the University of Yamanashi in Kofu, Japan, told Mic.
According to what little knowledge we have on the subject — NASA has never conducted official experiments on animal reproduction in space, a spokesman told Slate in 2007 — live sperm could simply not handle the levels of radiation in the ISS, let alone in deep space.
"I would imagine that there are less free radicals formed with freeze-dried sperm," Dr. James Logan, a physician who spent more than two decades working for NASA, said in an interview.
Cosmic rays act as "nano-bullets," as Logan explained. Some, known as "heavy ions," have more mass than others and do the most damage. When those "nano-bullets" come in contact with water, they turn into free radicals that can damage the DNA structure of cells.
Since the mouse study used freeze-dried sperm, which does not contain water, researchers don't know yet if the results could be replicated with live sperm.
While the ISS, where the mouse sperm was stored, offers a small amount of protection from space radiation in space, going to Mars may prove more problematic.
"Going to Mars is like getting a full-body CT scan every five to six days," Logan said.
Even if you could safely protect human sperm from the harmful effects of radiation, sex may not be particularly enjoyable for the participants.
"It requires three," Linda Plush, a family nurse practitioner and member of the Space Medicine Associates, told Mic about the very awkward mechanics of sexual intercourse in zero gravity. To actually engage in intercourse, there would need to be a third person available to hold the two in place — or at the very least, straps or a device allowing the encounter to occur. Additionally, we still don't know the gravity requirements for having sex, conceiving and carrying to term in space.
"Could somebody get it done? Yeah, probably," Plush said. "The bigger question is, is the human body going to accept it and are you going to conceive? We don't know."
Sex in space would get kind of nasty, and not in a good way. Astronauts tend to get sweatier in space due to extreme temperature swings, Plush said. Those bodily fluids could then float away and potentially damage onboard equipment.
As LiveScience reported, some experts have suggested that "intimacy chambers" could help our friends in space. They'd need to be filled with "droplets of cool water or scented oil" to keep things fresh.
Logan said there are simply no data points about sex in space because nobody has ever had it — on the record, at least. But Logan is less concerned about whether we can have intercourse and more with whether we can reproduce at all.
"There seems to be a lot of interest in sex in space and not a lot of interest in reproductive biology in space," Logan said.
What the study on freeze-dried mouse sperm can provide to humans is a chance at life beyond our own planet. At the very least, Wakayama said, his study offers the chance to eat a good meal in space, as domestic animals will likely be produced by frozen sperm.
But still, the study is promising, at least to people like Logan.
"It needs to be repeated, because this is one study that showed this result, but it's still promising," he said. "But it does not mean in any way shape or form that reproduction in space, or on the surface of the moon or Mars, is going to be any less problematic than we already think it will be."