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While billionaires like Jeff Bezos and Elon Musk were vying to launch themselves into space, scientists have been busy shuttling freeze-dried mouse sperm through the stars.

In new research published Friday in Science Advances, scientists studying the effects of space radiation on reproduction report that freeze-dried mouse sperm weathered a nearly six-year trip on the International Space Station without any damage and could ultimately last for 200 years in space. The finding may not be immediately applicable for civilians eying a trip to space, but researchers believe it could be a stepping stone for understanding risks to reproduction if humans ever colonize Mars, as SpaceX CEO Elon Musk plans to start doing by the year 2050.


“I read a lot of science fiction books,” Teruhiko Wakayama, a scientist at University of Yamanashi in Japan and author of the new study, said in an email. “I had an exam to be an astronaut (of course I failed), then I became a scientist, but still wanted to include something about space.”

Space radiation is one of the five major health hazards facing humans who lift off the planet, particularly for its potential to cause cancer, cardiovascular disease, and cognitive problems. For the short-term exploration missions that astronauts typically embark on today, the doses of space radiation that they’re exposed to are likely too low to cause reproductive problems.

“But as we move from space exploration to things like colony building and long-term living in space, doses from space radiation are going to accumulate and these may manifest as fertility and reproductive decrement,” said Zarana Patel, a senior scientist with the technology and engineering company KBR, Inc., and contractor for NASA’s Human Research Program, who was not involved in the study.


Most studies on space radiation and its biological effects up to now have taken place, counterintuitively, on Earth. Astronauts are busy people who don’t always have time to perform complicated or advanced experiments, and there’s not always a lot of space in space. Live animals or cell cultures that require regular maintenance, particularly samples like sperm that need to be kept in freezers, are not kept on the space station for long periods of time. Still, it’s difficult to reproduce the full force of space’s radioactive ions, which are different from radiation on the ground.

“To learn about space biology, we have to perform real space experiments,” said Wakayama.

So in 2013, Wakayama and his team sent 12 samples of freeze-dried sperm, which can be stored at room temperature, up to the International Space Station to study the long-term effects of space radiation on reproduction. The first sperm samples were returned to Earth after just nine months, to ensure everything was intact and working properly. More samples returned after almost three years, and the final samples were retrieved after almost six years in space.

The returned sperm were thawed and evaluated for DNA damage before being used to produce two generations of “space pups.” There were slight differences in the preservation of DNA and the rate of offspring produced between the 3-year and 6-year sperm, but they were both small enough that researchers concluded the space radiation did not cause damage. Healthy mice were born from each set of the freeze-dried samples. Wakayama also analyzed each generation’s gene expression and found no difference between the space pups and the control group, produced with fresh sperm.

Experts say that’s a novel finding, though there are caveats that make it difficult to draw conclusions about the relevance to humans.

The International Space Station orbits within Earth’s gravitational field, meaning that radiation isn’t as strong as it would be in deep space or on Mars. And the sperm is actually strengthened against space radiation in the freeze-drying process, according to the researchers, who also performed experiments on the ground to observe the effects of the freeze-drying process.

Additionally, the study only looked at mature sperm, but scientists have known for decades that radiation affects the entire process of sperm development. And when the sperm landed back on Earth to be thawed, re-hydrated, and used to fertilize female cells, the study only used one type of female mouse.

“It’s the diversity of the female’s ability to repair sperm damage that is the most important thing,” said Andrew Wyrobek, a senior scientist at Lawrence Berkeley National Laboratory who has studied radiation and the reproductive system since 1975, who was not involved in the study.

In the short term, experts said the study could bring needed attention to the potential dangers of space radiation to the reproductive system. Mars will need to be explored before the farms are built, and such a mission could take two or three years, said Ulrike Luderer, director at the Center for Occupational and Environmental Health at University of California, Irvine, who was also not involved in the study. To prepare, experts say that an integrated approach to ground and space research is needed, as both come with limitations.

“It is a small, very elite population of people, but nonetheless, those are people who are going to be exposed to these hazards,” said Luderer.

While more research is needed to learn the true dangers of space radiation for human reproductive systems, Wakayama hopes the new study may prove that if humans do move to Mars someday, other mammals could be brought along through freeze-dried reproduction cells.

“I believe that our research results will give the average person a sense of security about space,” said Wakayama.

When the freeze-dried sperm first launched to space in 2013, Wakayama and his team had been working on the proposal and preparation for four years. At the launch, the whole team cried with joy. Wakayama took a video that day and plays it regularly for students.

“I think I’ve watched the movie more than 100 times, but still every time, it makes me want to cry.”

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