Mouse sperm exposed to cosmic radiation on the International Space Station for nearly 300 days has been used to produce healthy offspring back on Earth, according to new research.

In August 2013, Japanese researchers sent samples of freeze-dried mouse spermatozoa to the space station on cargo launched by JAXA, Japan’s space agency. Researchers used freeze-dried sperm because the samples can survive in room temperature for up to two years, and be easily rehydrated back on Earth, sperm intact. They’re also cheaper to transport; cryogenically preserved cells would have required a freezer, and more weight on rockets makes the ride more expensive.

Once the sperm arrived on the ISS, the samples were stored at about minus 95 degrees Celsius, or minus 139 degrees Fahrenheit. For 288 days, they were exposed to the microgravity environment on the ISS, where the inhabitants, mouse sperm and astronauts alike, receive cosmic radiation about 100 times stronger than that on Earth.

The samples returned on a rocket in May 2014. Back in Japan, researchers compared the space spermatozoa to control samples that had been kept under similar conditions in a lab on Earth. They used to the sperm to fertilize the eggs of lab mice in vitro, and then implanted the embryos. After about 19 days of gestation, the researchers carried out c-sections and delivered healthy pups.

The research is described in a study published Monday in Proceedings of the National Academy of Sciences. The researchers had hoped, of course, for healthy offspring, said Teruhiko Wakayama, the lead author and professor at the University of Yamanshi in Kofu, who had watched along with his colleagues as the mouse sperm blasted off into the night sky from an island in Japan’s Kagoshima prefecture. The number of pups and the gender breakdown from births resulting from the space sperm was comparable to births that occurred in the lab using sperm from Earth. Examination of their brain tissue and organs, as well as genetic testing, showed only minor differences between the two groups. The researchers mated the space mice after they became adults and all had healthy offspring, suggesting that exposure to space hadn’t negatively affected fertility.

But even though the offspring were healthy, the sperm DNA was damaged after about nine months in space, Wakayama told me over email. In his earlier ground experiments simulating the space environment, DNA seemed to be affected only after much higher levels of exposure to radiation, higher than what’s found on the ISS. The researchers had expected any negative effect from this experiment to be unnoticeable. “Fortunately, those DNA damage did not affect the offspring,” Wakayama said. The presence of healthy offspring suggests that the DNA was somehow repaired during the embryonic stage.

Wakayama’s work builds on a growing body of research on reproduction of living organisms in space. Other scientists have studied the effects of spaceflight on salamanders, fish, sea urchins, and frogs. The results have been mixed. Some female mice brought to the ISS in 2010 and 2011 stopped ovulating, and others lost a key structure that forms inside the ovary to help maintain pregnancy in its early stages. Last year, a Chinese spacecraft took thousands of fertilized mouse embryos for a 12-day trip into space. When they returned, they had formed into blastocysts, clusters of different types of cells, and hit the same developmental milestones as embryos on Earth.

Mice are common stand-ins for humans in scientific and medical research, but space-related challenges like figuring out how to colonize Mars are slightly more complicated than, say, determining the side effects of cholesterol drugs. Scientists suspect the lack of gravity would negatively affect a human pregnancy, starting at its ability to even take hold in the uterus. They know some things, like that radiation exposure can increase the risk of cancer in astronauts, including in reproductive organs, and that men who’ve spent time in space seem to father more female children than male.

The damaged sustained by the mouse sperm DNA in the Japanese study illustrates the risks of long-duration space travel. The ISS orbits just inside Earth’s magnetic field, which blocks out the worst of cosmic radiation. If humans want to live beyond that bubble, the researchers note, they’ll need reproductive technology capable of preserving and protecting cells from the harsh environment of space, for themselves and the animals they might bring with them.