A family frolicks on the surface of Mars in an illustration.
ILLUSTRATION BY ROBERT MURRAY / MARS SOCIETY

Fast-forward to several decades or a half-century from now, and it’s not inconceivable that humans could be living on Mars—building habitats, trundling around in rovers, mining the subsurface for resources, and producing the first generation of bipedal Martians.

Except, no one really knows if humans can successfully reproduce in space, whether that’s during spaceflight or on another planet. To be clear, having sex in (much) lower gravity is a simple physics problem. But a host of unknowns swirl around how space environments affect the actual biological sequences of events that must unfold with precision for a new human to grow, from fertilization to weaning.

It’s not as though we haven’t tried to sort it out. Mice, rats, salamanders, frogs, fish, and plants have been the subjects of experiments looking at how spaceflight affects reproduction. To put it simply, though, the results so far are mixed and inconclusive.

“All of our big tech gurus out there who want us to be a multiplanet civilization—this is a key question that no one has answered yet,” says Baylor College of Medicine physician Kris Lehnhardt, who specializes in space medicine.

“Everyone is focused on the hardware, and the hardware is great, but in the end it’s the squishy meat-sack that messes everything up. Ignoring the human system, if you will, in future plans and designs is only going to lead to failure.”