Mouse sperm that spent nine months in space has developed into healthy offspring, this paving the way for possible human colonisation of other planets.
The freeze-dried sperm samples were launched to the International Space Station in 2013 and returned to Earth a year later.
The intense radiation of space - which is about 100 times stronger than that on Earth - caused slight DNA damage to the sperm, according to scientists.
However, after in-vitro fertilisation on the ground, healthy offspring resulted. The baby mice grew into adults with normal fertility of their own.
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.
They envision missions lasting several years or multiple generations, during which assisted reproductive technology might be used for domestic animals and people.
The findings were published in the Proceedings of the National Academy of Sciences.
"When we think about the difficulties of colonising another planet, the last thing we probably worry about about is sex. However, for our species to survive beyond Earth it's a fundamental issue," said Adam Watkins, research fellow at Aston University.
"While these studies indicate that the first stages of reproduction in space are possible... we have yet to see the entire process of an animal in space getting pregnant, going through a normal pregnancy and having healthy babies up there.
"While sex in space has not been high on the priority list for NASA, the survival of the human race could well depend on us getting a better understanding of how to reproduce in zero gravity."
Mammals are more difficult to maintain and handle in space so testing has been limited. More extensive testing on sperm preservation is needed in space, according to the researchers.
Besides looking ahead to long-term space crews and societies, the researchers see other reasons for saving sperm in space, including in the event of disasters on Earth.
The moon would be ideal for underground sperm storage, they said, in particular lunar lava tubes because of "their very low temperatures, protection from space radiation by thick bedrock layers, and complete isolation from any disasters on Earth".