Astronauts on long-term missions in space show a marked decline in bone strength once they return to earth, a new study finds.

Researchers found that 13 astronauts who spent four to six months on the International Space Station showed significant dips in bone strength around the hip. A few had strength reductions comparable to those seen over a lifetime in people who keep their feet on earth.

It's not clear whether the bone strength can be regained over time, the researchers report in the journal Bone.

But the results raise the possibility that these astronauts will be at high risk of bone fractures later in life.

"If preventive measures are not taken, some of our astronauts may be at increased risk for age-related fractures decades after their missions," lead researcher Joyce Keyak, a professor of orthopaedic surgery and biomedical engineering at the University of California, Irvine, said in a written statement.

It's well-known that the weightless conditions of long-term space flight cause astronauts to lose some bone mass. Bone is living tissue that is constantly being reshaped in response to stress, and the earth's gravity provides the stress needed to trigger new bone formation.

These latest findings indicate the actual strength of the bone may be greatly diminished as well.

For the study, Prof. Keyak's team took CT scans of the astronauts before and after their months on the space station; the researchers focused on bone strength at the top of the thigh bone, where it inserts into the hip socket.

On average, the researchers found, the astronauts' bone strength decline by 14 per cent during the mission. And three showed strength loss of 20 to 30 per cent, which is similar to what's seen in older women with osteoporosis.

Here on earth, people who are bedridden due to illness or injury experience a similar situation - their bones no longer being stressed by gravity.