Astronauts sent to colonize Mars would be well advised to avoid getting pregnant en route to the Red Planet, according to a review of radiation hazards by three scientists.

High-energy particles bombarding the ship would almost certainly sterilize any female fetus conceived in deep space, making it that much more difficult to establish a successful Mars colony once the crew lands.

"The present shielding capabilities would probably preclude having a pregnancy transited to Mars," said radiation biophysicist Tore Straume of NASA Ames Research Center, lead author of the review published in the Journal of Cosmology.

Sex in space is a touchy subject for NASA, whose code of conduct for astronauts dictates that "relationships of trust" and "professional standards" are to be maintained at all times. But the logical outgrowth of human space exploration is colonization, write Straume and his co-workers, with Mars being our closest, best bet – and that would entail reproduction.

The DNA that guides development of a fertilized embryo and the functioning of all the cells in the body is easily damaged by the kind of radiation that would bombard astronauts on a Mars voyage and ultimately on the planet itself.

Radiation spoils space sex

One hazard comes from solar flares, which spew energetic protons across the solar system. Although the timing and intensity of such outbursts is difficult to predict in advance, these particles would be relatively easy to shield against, Straume told SPACE.com.

"A few centimeters of a material can knock them way down in intensity to acceptable levels," Straume said.

Posing a tougher problem would be radiation streaming in from outside the solar system.

So-called galactic cosmic rays consist largely of very high-energy protons, but they also include charged atomic nuclei running up the periodic table all the way to iron, which is quite heavy, atomically speaking. Such charged particles can blow apart biological molecules such as DNA and would easily rip through the aluminum shielding of a spacecraft traveling through interplanetary space.

Researchers' understanding of the reproductive hazards of ionizing radiation come primarily from sudden exposures such as radiotherapy for cancer and atomic bomb blasts.

However, studies in nonhuman primates have found that even relatively low doses of ionizing radiation are sufficient to kill most of the immature oocytes, or egg cells, in a female fetus during the second half of pregnancy. If those results apply to people as well, then a girl conceived in interplanetary space might well be born sterile because of damage to her eggs.

"One would have to be very protective of those cells during gestation, during pregnancy, to make sure that the female didn't become sterile so they could continue the colony," Straume said.

Space pregnancies are risky

A child conceived in space would also be likely to suffer from other problems as well.

Cells divide and differentiate very rapidly during gestation, and damage to a single cell destined to become the brain or another organ could easily be amplified. Straume said the dose of radiation received by a fetus on a trip to Mars could likely result in severe mental retardation or other deficits.

Similar problems could result from damage to sperm, said radiation biologist and geneticist Andrew Wyrobek of Lawrence Berkeley National Laboratory, who was not part of the study. Although the effects of chronic space radiation are unclear, low doses of radiation can kill or damage sperm, which might render a man infertile or lead to birth defects.

And in rodents, radiation damage can affect offspring born long after the initial exposure to their fathers.

"We know that ionizing radiation can induce permanent genetic damage in stem cells" – the cells from which sperm arise, Wyrobek said.

Assuming everyone's gonads made the voyage unscathed, the prospects for successful reproduction should start to look up once they actually reach the Red Planet. There the atmosphere, and Mars, itself would soak up or slow down a good portion of the incoming radiation.

Colonists could further shield themselves by putting a layer of ruddy Martian soil, or regolith, between themselves and the sky.

Straume and his colleagues note that an intriguing site for a base near Mars– at least in terms of radiation exposure – might be the Martian moon Phobos, specifically Stickney crater, which is on the side of Phobos facing Mars. The high walls of the crater along with the Red Planet might obstruct up to 90 percent of cosmic rays in certain locations.

Of course, the low gravity of Phobos would present its own difficulties. But radiation is such a thorny problem it's worth exploring our options for successful reproduction.

"This is an issue that really needs to be resolved if we ever plan to have a colony on Mars," Straume said.