A single dose of radiation approaching  what will be faced by long-term space travelers to the Moon or Mars causes as much as a 39 percent spongy bone loss in mice, a new study shows.

The loss of connectivity in spongy bone ranged as high as 64 percent for one of the types of radiation tested, along the lines of an osteoporosis diagnosis.

The results say nothing directly about the effect of space radiation on people but it has implications for the future of human spaceflight, especially given the U.S. commitment to send astronauts on long trips beyond low-Earth orbit. Both mice and humans lose bone after radiation exposure.

"We were surprised that there was bone loss, and the degree was a lot more than we expected," said Ted Bateman of Clemson University, lead author of the research report. "We're seeing bone loss at much lower doses of radiation than we expected."

What's lost

Scientists already know that cancer patients who receive radiation treatments have a higher risk for spontaneous bone fractures down the line. Now it's more clear why.

For Bateman's experiment, 38 female mice were exposed to radiation, receiving about the same amount that would be a single day's dose for someone suffering from cancer. The gamma, proton, carbon and iron radiation used in the experiment is less damaging than the complex mix of radiation (protons and heavy ions, or ionizing radiation) that long-term space travelers will experience.

Bone is comprised of hard or cortical bone on the outside and marrow inside, as well as bone adjacent the marrow, called trabecular bone. This spongy part of the bone is key to bearing weight and preventing fractures. In the experiment, radiation had no real effect on cortical bone.

The result of Bateman's study, published in Journal of Applied Physiology, was a profound loss of trabecular bone-about 30 percent for all types of radiation, with carbon radiation inflicting 39 percent loss, the most of all. The loss of spongy connections in the four radiation groups ranged from 46 to 64 percent, with proton radiation inflicting the worst damage in terms of connectivity.

Fewer interconnected struts means more load on each of them, leaving bone structure less efficient and more vulnerable to fracture.

Trabecular bone connectivity is irreplaceable once lost. "You can regain bone mass," Bateman told SPACE.com, "but once the connections between struts is lost, the load is not being passed from strut to strut and that becomes permanent. Struts can become larger and thicker but loads are not transferred as efficiently once you've lost the connections between struts."

Implications

Currently, astronauts on the International Space Station and shuttle lose about 2 percent of their bone mass for each month in space as a result of microgravity much more than as a result of cosmic or solar radiation given their relatively short stays in space and protection by Earth's magnetic field.

The new study shows that on longer flights, such as a 6-month trip to the Moon or 30-month trip to Mars, the bone lost as a result of microgravity will be compounded by more extensive bone loss as a result of radiation exposure.  Up to now, NASA has focused on radiation's cancer-causing properties and effect on the central nervous and immune systems. The effect on bone health has been unexamined.

"Now we're concerned that radiation and reduced gravity are both going to contribute to bone loss," Bateman said.

Procter and Gamble, which makes an osteoporosis drug called Actonel, helped to support the research.