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.”
Gravity of the situation
On Earth, evolutionary processes are fine-tuned to work in an environment characterized by one of our planet’s most basic forces: gravity. In space, gravity is essentially nonexistent, and on Mars it’s about 38 percent the strength of Earth’s downward pull. So far, no one has even come close to figuring out how a partial gravity environment could affect mammalian reproduction.
As well, radiation in space is stronger and potentially more damaging than down here on the ground, because Earth’s magnetic field helps shield the planet from energetic cosmic particles. High radiation doses are already a serious concern for adult space travelers, and space agencies carefully track their astronauts’ exposures in orbit. What that radiation could do to a much more developmentally sensitive fetus is a real worry.
The effects of those two things—gravity and radiation—on reproduction are, so far, the major issues scientists are trying to address. And because of ethical qualms associated with studying medical risks in humans, scientists have spent decades launching various other animals and associated tissues into space.
Early experiments done by the Soviets in the late 1970s included sending several rats into orbit aboard the Cosmos 1129 satellite. When they returned, there was evidence that they’d mated in space, but none of the females ever delivered, which might not be surprising to anyone who studies rodents, given their sensitivity to environmental perturbations.
Later, NASA scientist April Ronca sent pregnant rats into orbit and observed how spaceflight affected the later stages of pregnancy; back on Earth, the birthing process was more or less normal, but other work suggests that rat pups exposed to microgravity develop abnormal vestibular systems, or the inner-ear machinery associated in sensing movement direction and orientation.
Spaceflight also appears to decrease total rat sperm counts, while increasing abnormalities; still, Ronca has written that “the available data suggest that numerous aspects of pregnancy, birth and early mammalian development can proceed under altered gravity conditions.”
In mice, the story is similarly complicated. Research suggests that the two rodent species respond differently to changes in gravity. Two-cell mouse embryos sent into space aboard the shuttle Columbia failed to develop further, even as Earth-based controls matured normally. Later, work in simulated microgravity (achieved using a rotating piece of machinery called a clinostat) showed that while in vitro fertilization could occur normally, microgravity-cultured embryos transferred to female mice failed to implant and develop at normal rates.
Most recently, a Japanese-led study found that freeze-dried mouse sperm could produce embryos after spending nine months in space. Other work shows that crickets, nematodes, and fruit flies can successfully reproduce when spaceflight is involved. And Japanese medaka fish mated and produced offspring while on board the space shuttle Columbia.
Meanwhile, salamander eggs from Pleurodeles waltl fertilized aboard the Russian space station Mir produced embryos that developed into larvae, although with some alterations. Experiments in sea urchins similarly suggest that fertilization in space can occur, but microgravity dramatically affects how their sperm move. And quail eggs kept in an incubator aboard Mir failed to develop normally.
Taken together, these experiments and others aren’t exactly constructing a cohesive image of how spaceflight affects reproduction.
“If you were to take reproduction and break it down into all of its various parts … there’s never really been a dedicated scientific program that looks at how each of those steps is affected by the space environment,” Lehnhardt says. “It’s one thing to know that it’s even possible, it’s another thing to know that it can be done safely and have a good outcome.”
In general, though, it’s not looking good for mammals, in which successful embryonic development starts with a complicated interchange between mom and fetus and just gets more complex from there.
“Across the board, almost every study has shown that in space, either things don’t work at all or they’re not as good—and so as we move forward, we need better and bigger studies, and human studies,” says James Nodler of the Houston Fertility Clinic, who reviewed the links between gravity and embryonic development.
Of mice and humans
In an attempt to address some of the concerns associated with long-term human habitation on the surface of Mars, a team based primarily at NASA’s Langley Research Center designed an experiment that would allow scientists to study the long-term effects of partial gravity on mammalian reproduction.
“Before significant investment is made in capabilities leading to such pioneering efforts, the challenges of multigenerational mammalian reproduction in a partial gravity environment need be investigated,” the scientists write. “Humans may encounter reproductive challenges in gravity environments different than Earth’s, as gravitational forces may disrupt mammalian life cycle processes and actively shape genomes in ways that are inheritable.”
As envisioned, the experiment would involve placing a mouse colony into lunar orbit, enclosed in a rotating habitat that could be observed and operated almost completely autonomously, courtesy of 600 cameras and telerobotic animal care.
Called MICEHAB, for Multigenerational Independent Colony for Extraterrestrial Habitation, Autonomy and Behavior health, the experiment would study the effects of spaceflight and partial gravity on at least three generations of mice a year, with scientists keeping track of birth rates and overall animal health.
About once a year or so, the autonomous mouse colony would rendezvous with a planned human habitat in cis-lunar space, allowing astronauts to retrieve samples from the experiment and perform any necessary maintenance, with the goal being to run the experiment for 10 years.
“Partial gravity mammalian reproduction research should be conducted prior to the late 2020s in order to inform design decisions on future human Mars missions,” the scientists write. “Permanent surface settlements may be infeasible if partial gravity reproduction challenges are too great to overcome.”
But as of now, there’s no indication that MICEHAB will be launching any time soon – and even if it were, some scientists worry it wouldn’t actually answer the questions we’re keen to know about ourselves. Human reproduction differs dramatically even from that of other primates, and none of the organisms studied so far are effective surrogates, says Nodler, a reproductive endocrinologist who specializes in assisted reproductive technologies.
“If you look at early IVF studies, they skipped over a lot of mouse and primate studies—it’s just not the same,” Nodler says. “It can’t be overstated that at some point, we have to do human studies to see what’s really going on here.”
Ethics and embryos
But deciding which experiment to perform depends on the goal posts, Nodler says, and whether we’re thinking a bit outside the frame of “normal” reproduction and potentially leaning on assisted technologies to produce a generation of Martians.
“Is our end point to see if we send up a man and a woman, and they have sex, can they have a baby?” he asks. “Or do we want to say, can we take a whole bunch of embryos, freeze them on Earth, send them to Mars and thaw them?”
Performing that first experiment is technically simple enough, although mired in potential ethical snags. And while studying the precise effects of a space environment on human embryos is more difficult, it could feasibly be done today, except for an even bigger pile of moral and ethical snags.
For example, scientists could send human sperm and eggs to the International Space Station and attempt in vitro fertilization to see if it would even work, and then compare how many embryos were produced compared to controls on Earth.
“The problem is, those are potentially viable embryos, and people would have a heyday with that,” Nodler says.
Scientists could also send already fertilized embryos to the ISS and look at how the space environment affects development, DNA damage, and repair. This could be done, Nodler says, with embryos that already have no chance of developing normally—which might remove some of the ethical challenges—but the real test would be to look at the effects of spaceflight on viable embryos.
“Let them stay frozen on ISS for six months or a year, then bring them back to Earth, and use them to try to have a live birth. That would be really, really difficult to get approval for, but at one point you gotta do it,” he says, noting that “we have thousands of discarded embryos that patients have said we can use for scientific research. The problem is getting someone to let me use them for scientific research.”
Lehnhardt agrees that it’ll be tricky to study human reproduction in space without actually studying humans, and that means being willing to tackle not only the scientific challenges, but the ethical quandaries as soon as possible.
“The moral and ethical challenges are not going away,” he says. “So, we’re going to have to face those head-on as we work on stuff like this in the future.”
Editor's Note: A previous version of this article incorrectly stated that Kris Lehnhardt is at Baylor University; he is at Baylor College of Medicine.