My understanding is that space has powerful radiations in it, such as cosmic rays and high-speed charged particles. The earth's magnetic field and atmosphere largely protect earth dwellers from these emanations.

However, obviously anyone in space would be exposed to them because many such particles will pass through the thin steel of a space vehicle. In fact, the ISS is now testing a balloon room that would offer no protection at all to radiation.

I heard a rumor that when astronauts are selected, they deliberately pick those who have children and are not planning to have any more children for this reason.

Is that true? Do you become sterile if you go into space?

  • 3
    $\begingroup$ That is an old wives tale, they don't pick astronauts on that basis. The Spartans did at Thermopylae but not NASA. $\endgroup$
    – GdD
    Apr 8 '16 at 8:32
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    $\begingroup$ That "balloon room" has walls several centimeters thick and offers better radiation protection than the thin aluminium walls of most ISS modules. $\endgroup$
    – Hobbes
    Apr 8 '16 at 8:32
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    $\begingroup$ Regarding radiation: The ISS flies below the Van Allen belts and therefore receives their protection. So while ISS astronauts receive more radiation than people on the ground, it isn't nearly as bad as deep space. Many Apollo era astronauts had vasectomies, I believe because of fears of birth defects induced by their time spent beyond the Van Allen belts. Subsequently we re-assessed those risks and at least one had his vasectomy reversed and had another kid. $\endgroup$
    – Kengineer
    Sep 1 '17 at 20:39
  • $\begingroup$ There's space and then there's space. Apparently, the ISS is not such a severe radiation hazard. However, Mars is another matter; see this. $\endgroup$
    – Anthony X
    Sep 4 '17 at 17:22

Astronauts are not sterilized by radiation in space. According to a 2005 study, at least 17 children were born from female astronauts after they had been in space. Male astronauts were able to conceive children soon after returning from spaceflight.
That also suggests NASA has no policy in place of only picking astronauts who don't want to have more children.

Animal studies showed a decrease in fertility: sperm counts declined, ovaries shut down in mice.

Jennings has been involved in space medicine since 1987. He hasn't seen evidence that floating in a spacecraft affects astronauts' fertility, or their sex lives.


"We've had astronauts - after a two-week flight, they were able to get their spouses pregnant a day or two after landing," Jennings said. A 2005 study Jennings co-authored counted 17 babies born to female astronauts after they had been in space. The astronauts had a high miscarriage rate, but women who've gained the seniority to go into space tend to be near the end of their child-bearing years, Jennings said.

  • $\begingroup$ Radiation damage and weightlessness do reduce fertility, Radiation damage at the doses experienced in low earth orbit? Seems unlikely. radiation damage would increase the risk of birth defects. Not true at the kind of doses we're talking about -- see my answer. $\endgroup$
    – user687
    Sep 2 '17 at 0:58

Males produce new sperm cells continually, so it's hard to see how radiation exposure could make them permanently sterile.

Females draw from the same stock of egg cells for their whole lives. The empirical evidence, from Hiroshima survivors, seems to be that even with a pretty hefty radiation dose, there is no epidemiologically detectable additional risk of birth defects. This seems to me to make sense, because radiation has the biggest effect on cells undergoing rapid division -- that's why it works as a cancer treatment. Eggs cells aren't undergoing division at all until fertilization.

There is some empirical evidence that low doses of radiation increase fertility in female humans, trout, and mice. According to one study,

Lightly irradiated rodents were more fertile than controls through several generations (increased ovulation in dams, increased number, viability and growth rates of young, and faster physical development of young) with no evidence of mutations in the young exposed in utero. Irradiated colonies were maintained in good health through 21 generations.

This is an example of a more general type of effect known as radiation hormesis, which is hypothesized to occur because radiation triggers cells' mechanisms for damage control. The doses at which measurable hormesis occurs are usually fairly large, so I would guess that any pro-fertility effect on ISS astronauts is negligible, and in any case effects such as increased ovulation, even if significant, would be temporary.

  • $\begingroup$ The first paragraph (about males) makes no sense. Of course men can become permanently sterile if their spermatogonia are destroyed; and those are cells undergoing rapid division – after all, several hundred millions of spermatozoa are produced each day in a healthy male. $\endgroup$
    – chirlu
    Sep 2 '17 at 18:38

The carcinogenic risk of a year of spaceflight has been determined to be on a par with smoking a (very) few cigarettes over the same time frame. Not something to be done for no reason, but not something to change your choice of career over, either.

  • $\begingroup$ The question is about fertility, not about cancer. $\endgroup$
    – chirlu
    Sep 4 '17 at 1:33
  • $\begingroup$ Same difference, both consequences of radiation if there is any. If there were a fertility risk it would be cancer causing irradiation. $\endgroup$
    – M Weiss
    Sep 5 '17 at 4:39

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