If there’s one thing people are good at, it’s making more people. We’re such prolific baby-makers that 7 billion of us now roam the planet. But our reproductive skills may soon be put to the test. Whether from overpopulation, environmental destruction or just for the sake of exploration, we’ll want to reach out and settle alien planets, too. And Mars will need moms — and dads. But given the harsh radiation prevalent throughout space, and the challenges of reproducing in low gravity, that’s easier said than done.

Researchers have known that space travel can be hazardous to human health since NASA’s Gemini missions in the 1960s. Scientists noted that bone density in astronauts’ feet dwindled by some 6 percent after less than two weeks in space. Muscles shrank at an even higher rate. And we've since learned that cosmic rays, ubiquitous in space and within space stations, cause cancer and tissue disease — and would eventually prove damaging to DNA and the nervous system, too.

Space is a hostile place, even for trained astronauts. What will the harsh environment do to fragile embryos, fetuses and newborns? How does it affect our reproductive systems in the first place? The answers remain largely unknown. Scientists don’t know if women can even get pregnant — let alone stay pregnant — in space.

With biology working against us, researchers are still trying to answer the crucial question: How will we make happy, healthy babies in space?

Sperm, Meet Egg

Every facet of baby-making in space is difficult.

No one’s ever had sex in space, so far as we know. Without gravity, just keeping hold of your partner to do the deed might prove difficult. And then functional sperm and egg cells still need to meet up to kick-start a pregnancy, which requires a slew of cellular actions working perfectly.

For decades, scientists have been working to solve the reproductive mysteries of space. After the space race, astronauts started hauling fish, roundworms, frogs and salamanders into orbit to test their off-world breeding abilities. In a series of promising surprises, they all managed to produce healthy offspring. But amid the success, scientists hit a wall. Aboard a Russian satellite in 1979, male and female rats either failed to fertilize during an 18.5-day mission or chose not to have sex — a display of rodent abstinence practically unheard of on Earth. Subsequent experiments with mice have raised other concerns.

Unlike their slimy, fertile friends, rodents are mammals, whose anatomy, physiology and genes are similar to ours. Teresa Woodruff, vice chair for research at Northwestern University’s Department of Obstetrics and Gynecology, says mice are so similar that any problems we’d see in their early pregnancies are likely the same we’d see in our own. Other researchers agree.

Only mammals have a placenta. So to know the effects of placentadevelopment in space, we have to use mammals, says Teruhiko Wakayama, director of the University of Yamanashi’s Advanced Biotechnology Center in Japan. He’s spent the past decade studying how mammals — mostly mice — could reproduce in space. Without Earth’s gravity pushing everything down, the need to walk, run and lug around heavy things goes away in orbit. Consequently, so do our strong bones and muscles. Since this strength is important for pregnancy, and essential for a growing fetus, Wakayama wanted to see if mice could even become pregnant under these conditions.

In 2009, Wakayama extracted eggs and sperm from mice, then set them loose in a device that simulates microgravity (the technical term for orbiting astronauts’ weightlessness). He watched the sperm swim into the eggs, and after a few days of microgravity, he and his colleagues implanted the embryos into female mice in standard gravity. The results were mixed: While many normal, healthy pups were born, a lot of the embryos didn’t develop properly after implantation. As a result, the mice had fewer offspring than mice in regular gravity.

To see if these results hold up in space, where high radiation levels accompany microgravity, Wakayama reached out to the Japan Aerospace Exploration Agency about replicating his study on the International Space Station (ISS). But this time, astronauts would extract the sperm and eggs from living mice, then transfer the fertile embryos back into mice, all in microgravity. That plan proved too difficult, so the mice never made it to space. Their sperm, on the other hand, did.

Wakayama, who’s now the principal investigator of NASA’s Space Pup mission, freeze-dried mouse sperm and preserved it at room temperature. Three sets of these freeze-dried samples went to the ISS in 2013, and he’ll study their viability after different lengths of time on the space station. It’s not the same as studying fertilization and pregnancy under microgravity, but this work allows astronauts to analyze the effects of space radiation on male reproductive cells.

The ISS is exposed to strong space radiation that may break sperm DNA, and the resulting offspring may be changed, Wakayama says. Not much research has looked at the health of mice made with damaged DNA, but Wakayama is slowly answering those questions.

After staying on the ISS for nine months, some of that sperm showed signs of slight DNA damage, yet went on to produce normal, healthy pups. Wakayama’s team is just now analyzing samples that flew on the ISS for three years; the final batch, in space for six years, is set to return to Earth this spring.

If his freezing technique works, Wakayama plans to haul frozen mouse embryos to the ISS to investigate the next part of the problem: finding out why they’re not fully developing in space.

Space Radiation 101 Beyond the protection of Earth’s atmosphere, Martian residents will be pelted with dangerous, high-energy radiation. It blazes from our sun during fiery solar flares, and is also thought to blast from distant supernova explosions. These cosmic rays are made up of atomic nuclei (protons and neutrons) and electrons, which get knocked out of the atoms as they fly through space at nearly the speed of light. After the electrons are gone, the remaining particles become ionized, ready to transfer energy to nearby objects. This powerful radiation can penetrate a spacecraft or an astronaut’s body without pause. When it comes in contact with humans, the rays can wipe out the electrons in their cells. This bombardment is known to damage the structure of DNA and cause cells to mutate or die off altogether, increasing astronauts’ risk of disease.

From Mice to Martians

Mice may be one of the best models of human reproduction in space, but despite the similarities, they’re still a far cry from an actual human.

Joseph Tash, a reproductive biologist at the University of Kansas Medical Center, points out that without functional human sperm and eggs, our settlements won’t last long in space. He’s been working with NASA since 1996, and until a few years ago, his research mainly focused on the effects of spaceflight on mice and other animals. But in April 2018, he used a method similar to Wakayama’s and launched frozen sperm into space — this time, it was human.

The experiment, dubbed Micro-11, collected top-notch sperm from 12 healthy, fertile men. In a lab aboard the ISS, astronauts thawed the frozen samples and mixed them with a chemical cocktail that spurred the sperm to start swimming. Essentially, the mixture’s chemical signals tricked the sperm into thinking they were heading toward an egg. Then astronauts filmed the movements using a high-powered microscope, trying to capture whether the sperm were physically capable of fertilizing eggs in space.

“Successful survival in space may require radical, currently unacceptable and/or controversial decisions and actions.”

“There are various changes in the appearance of the sperm when you look under a microscope,” Tash says. “You can see very specific changes to the head of the sperm that are required for the sperm to fertilize an egg.” The swimmers need to gain speed as they get closer to their target, and the cells in their heads need to merge together so they’re strong enough to break through the egg. If they don’t, pregnancy is a no-go.

The samples are now back on Earth, but Tash says it will take another year to comb through observations of their tiny reproductive motions and determine if the space sperm would make the cut. Once Micro-11 is complete, though, he’s jumping right into the next cosmic conundrum: female fertility in space.

He’s already found some reasons to worry. Tash studied female mice that went on NASA space shuttle trips back in 2010 and 2011. He discovered that their corpora lutea — short-lived glands in the ovaries responsible for producing sex hormones and nurturing nascent embryos — changed for the worse.

“We found that in all the female mice that were exposed to spaceflight from 12 to 15 days, the corpora lutea were either totally absent or there were very, very few,” Tash says. That means potential reproductive problems emerged after just two weeks in orbit.

Next year, he plans to send live female mice on a 30- to 37-day trip to the ISS. By examining their reproductive cycle, which takes mere weeks to go from conception to birth, Tash aims to finally figure out why mice have trouble reproducing in space.

Woodruff, who’s also the director for the Center for Reproductive Sciences at Northwestern, is going one step further and planning research with human embryos. If she can find funding, the experiment would move beyond mice and send frozen human eggs and sperm to space to see if they can lead to a pregnancy.

It would be easy to tell if it happens. Woodruff and a team of researchers recently discovered the zinc spark — a flash of zinc that shines from an egg the moment it’s fertilized. As an indicator of pregnancy, the zinc spark could let Woodruff actually see the first stages of human life in space.

Home Sweet Home Earth and Mars might be neighbors, but conditions on the two worlds differ drastically. Humans are accustomed to Earth’s abundant sunshine, hefty gravity and the protection of our atmosphere and magnetic field. The Red Planet, on the other hand, has frigid temperatures, a mere fraction of Earth’s gravity and is not protected from hazardous radiation. When stacked up, life on Mars might make you rethink your dream of becoming an astronaut.

(Credits, Earth and Mars: Golden Sikorka/Shutterstock)

Distorted Martian Reality

Woodruff and Tash are starting to work with human reproductive cells; however, they agree that full-fledged human trials, involving viable pregnancy and childbirth in space, lie in the distant future.

Even if it’s decades down the road, Konrad Szocik, an assistant professor of philosophy at the University of Information Technology and Management in Poland, thinks we should already start bracing ourselves for the harsh realities of reproduction in space. In a paper published last year in the journal Futures, he spells out the non-scientific implications of having babies in a settlement on Mars.

“We may expect that Martian population will be task- and goal-oriented,” Szocik says. “It is possible that particular, individual interests and benefits will be totally subordinated to the benefits of the group. It may be seen as problematic in such areas like individual freedom and personal decisions, but also in sexual and reproductive life — something that goes against ethical standards and moral intuitions of the West.”

To create a healthy population in a low-gravity, radiation-riddled world, Szocik says that mission planners shouldn’t rule out things like gene editing, genetic pre-selection of sexual partners and a structured reproduction policy — ideas that qualify as eugenics on Earth. Even with these precautions, of course, babies could still be born with disabilities and, with very limited resources, a martian society might not be able to care for them.

“Abortion and sexual policy in general may be applied in space in a much broader extent than on Earth,” Szocik says. “Successful survival in space may require radical, currently unacceptable and/or controversial decisions and actions.”

However contentious its policies may be, creating a settlement beyond Earth would mark a revolutionary step for humanity. The road to becoming an interplanetary species is a hazardous one, and it’s been taken as a cautious crawl rather than a reckless run. We’ve inched our way toward innovative yet incremental experiments, but we’re still miles away from the risky research that separates us from our lustful martian dream.

Amber Jorgensen is an assistant editor at Discover. This feature originally appeared in print as "Sex in the Cosmic City."

This story is part of "The Future of Fertility" a new series on Discover exploring the frontiers of reproduction.

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