Space travel is a dangerous business—everyone knows that. But even those astronauts who return to Earth safely may not be in the clear. A new study of International Space Station (ISS) veterans reveals a significant loss of bone strength, potentially upping their risk for injury later in life.

Scientists have long known that extended residency in microgravity can wreak havoc on bone density; the new study, published online by the journal Bone, adds to the dossier by quantifying how that decrease in density affects bone strength. The 13 astronauts studied, who had spent anywhere from 4.3 to 6.5 months aboard the ISS, showed an average loss of 14 percent in strength of the femur, or thighbone. (The researchers note that bone strength appeared to decline even more precipitously than bone density.)

"If preventive measures are not taken, some of our astronauts may be at increased risk for age-related fractures decades after their missions," lead study author Joyce Keyak, a professor of orthopedic surgery and biomedical engineering at the University of California, Irvine, said in a statement. Some subjects lost as much in a few months of spaceflight as would be expected over a lifetime under normal circumstances, with femoral strength dissipating a rate of up to 5 percent each month.

Photo of ISS courtesy of NASA