Elon Musk doesn’t want to simply send humans to Mars. The SpaceX CEO has bigger ambitions. He wants us to be an “interplanetary species,” which means creating a self-sustaining civilization on Mars, which means living and dying on Mars — which at some point might mean sex and pregnancy on Mars.

So how would that work?

Given that Musk hasn’t figured out how to keep people alive on the trip to the Red Planet, it’s unlikely he has details on how people will make more people once they’re there. We don’t have any data on how human bodies will work on Mars specifically, but we have enough information to know that sex in space could be a real hassle.

No one has had sex in space yet (as far as we know), though there are astronauts married to each other. Mark Lee and Jan Davis were secretly married before they went into space in the early 90s. (Technically, NASA forbids married couples to go together, mostly because they think it might negatively affect the team dynamic.)

Having only married couples is a bad idea

But for those who want to try, be warned that intimate relations without gravity will be physically complicated. Space-sickness is possible, and that’s definitely not romantic. Microgravity makes it very hard to hold onto each other; you’d keep floating away unless you held onto each other or somehow anchored yourself. It can also create blood-flow problems that make it difficult to get and sustain an erection.

And then, because of the way zero-G works, bodily fluids could just float away in small droplets. The logistics are so complicated that NASA physician Jim Logan told NBC that sex in space would likely have to be “choreographed.”

If we get all that figured out, should we send only married couples to Mars? Musk is optimistic that the trip could take as little as 80 days, but a more realistic estimate is around six months, and sexual frustration could be a serious problem. One SpaceX rival, Inspiration Mars, has tried to recruit older married couples for its trips for this reason.

But having only married couples can complicate things, according to Sheryl Bishop, a researcher who studies human performance in extreme environments. “People always think that you can solve the problem of sexual conflict by sending married people, and I’m saying, really?” she says. If you put 10 married couples on a boat together and sent them off for two years, it’s not always the same 10 pairs that will come back.

On long missions, married couples could have a positive and stabilizing role, and of course polyamorous relationships are also possible. But if one of those marriages breaks down, it’ll be more than just the two who are affected — and that could be disastrous on a long flight where you can’t leave. Human sexuality is complicated, especially in these unique situations.

A better way to combat sexual tension is to think very carefully about the makeup of the crew. When it comes to teams getting along on long expeditions, the biggest source of sexual conflict is — unsurprisingly — putting together single young men and single young women, according to sports psychologist Elisabeth Rosnet. This group is prone to what she calls “seduction behaviors,” as well as frustration, rivalry, and sexual harassment. With any other combination of people, the risk of sexual jealousy or sexual relations isn’t as high. So maybe the answer is simply not to send young, single folks on the first trips.

“the only ethical thing is that every single person on board would have to agree to be sterilized”

Even if the logistical problems of space sex are solved, other obstacles remain. Radiation, which Musk dismissed as “not too big of a deal,” can make people infertile, or have disastrous consequences on the genome. That means that astronauts who are exposed to radiation might later have babies with severe birth defects — which might undermine the idea of a self-sustaining civilization.

Even without the radiation, pregnancy in space is not a good idea now, or maybe ever. Gravity is important to make sure babies develop properly in the womb, and studies on mice have shown that microgravity has very bad effects. “Since we have already indications of that, it’s completely irresponsible for anybody to consider experimenting with this before we have a lot more information and a lot more data,” says Bishop. “If we meet Elon Musk’s schedule and we’re rolling out a team to Mars in the next decade, I think that the only ethical thing is that every single person on board would have to agree to be sterilized because the risk to that fetus is catastrophic and you can’t do that ethically.”

If we one day make it to Mars and want to experiment with pregnancy, one option could be to create an artificial gravity lab where pregnant women live for nine months, according to Bishop. But until then, best not to think too ambitiously about being the first person to give birth to a baby on Mars.

A look at SpaceX's Mars colonization plans