The second source of danger is less well-understood. Microgravity — you know, the whole thing where astronauts float around the International Space Station like a Cirque du Soleil troupe with a penchant for polo shirts — seems to alter biology too. It’s well-known that astronauts lose muscle mass while in space. Your body gets weaker when it doesn’t have to bear its own weight every day. But the effects of microgravity are weirder and more complex than what can be addressed by a modified treadmill. Some of the female mice that traveled to the space station in 2010 and 2011 stopped ovulating, and others lost their corpus luteum, an important structure that forms in the ovary after the release of an egg. The corpus luteum is responsible for producing hormones that maintain a pregnancy until the placenta can grow enough to do that job itself. Without it, you might get pregnant, but the pregnancy would be unlikely to stick.

This connects to data from older experiments. Back in 1979, Russian scientists launched a satellite carrying male and female rats and gave them the opportunity to mingle beginning a couple of days into their 18-day trip. The experiment didn’t result in any babies. Two rats had apparently gotten pregnant, but both miscarried. There’s consistent evidence that microgravity affects hormone levels in both males and females, Tash said. It’s possible those rats had estrogen levels so low that most of them weren’t even interested in mating. These effects persist after the animals return to Earth, but things eventually reset after they spend enough time in normal gravity. Mars’s surface doesn’t have normal gravity, however. It’s about 38 percent of the gravity of Earth. And we don’t know if that’s enough to reverse the effects.

We also don’t know whether humans experience these same effects. We only have a very small sample of female astronauts to begin with — as of 2015, only 11 percent of people who have gone to space have been female. These women are also typically in their late 30s when they first go to space, and most of them choose to take hormonal birth control to stop menstruating while they’re there.