"The objective of our project is to know whether mammalian reproduction is possible in space or not", said Wakayama. Wakayama adds that, "It is possible that if we lengthen the storage time, then the damage caused by radiation will be worse, and the DNA fix mechanism would not be able to cope".
According to National Geographic, these findings not only show that space sperm is viable, but also offer hope for potential space fertility (since scientists still don't know whether human reproduction in space is actually safe at this point). While the DNA of the sperm did show signs of damage, the mouse pups born from it were healthy and fertile.
However, this damage appears not to affect the health of the fetus or offspring.
The birth rate and sex ratio of pups derived from the sperm stored in space was comparable to those of pups derived from the control samples.
Still, there are limits to this research.
While humanity is already pretty obsessed with the idea of getting off this rock and exploring the big, wide Universe out there, the fact is we understand very little about how sexual reproduction will fare once we leave Earth behind, which is why the results of this Japanese "Space Pup" experiment are so promising. Our results demonstrate that generating human or domestic animal offspring from space-preserved spermatozoa is a possibility, which should be useful when the "space age" arrives. Considering the current technological limit for sperm preservation is only about two years, this seems fine - for now, anyway.
"Unfortunately, to bring live mice and take care [of them] in space is too hard", Professor Teruhiko Wakayama, one of the lead authors of the study, told The Guardian.
"If sperm samples are to be preserved for longer periods in space, then it is likely that DNA damage will increase and exceed the limit of the [egg] oocyte's capacity for fix", write the authors, in a report by BBC News.
The researchers - led by Sayaka Wakayama of the University of Yamanashi - said it is a step towards reproducing other mammals, even humans, using space-preserved sperm. "The most damaging radiation is found outside the Earth's geomagnetic shielding", far beyond the orbit of the ISS, he says.
That's good news for the space pups, but also for the many human astronauts who have gone on to become parents after spending time in space.
Once they've cracked that, they can set their sights on the Moon sperm banks.
Following their return to Earth, the sperm samples were compared with control samples preserved on Earth during the same period under similar conditions.
But radiation from the sun and cosmic rays is 100 times stronger in space than on Earth, which is protected by a layer of ozone and the Van Allen radiation belts. These were also kept at a temperature of -139 degrees Fahrenheit for nine months. The slight damage from radiation had no impact on the birth for or normality of the babies because it was mostly repaired while the embryos were developing, the study said.