Astronauts at the International Space Station and scientists on Earth are embarking on a bit of a quirky experiment: growing bones in space to test new bone loss treatments.

A group of 40 rodents will call the International Space Station home for two months, during which scientists will test a bone-growth molecule on them in a microgravity environment. Microgravity, which happens when an object (or rodent) is in free fall. It lets scientists produce changes in bone and organ systems that can't be replicated on Earth.

Microgravity is helpful for studying bone loss because of a bone's calcium balance is thrown off in space. A bone's calcium balance, or the difference between how much calcium is absorbed and how much is excreted, is about zero on earth. It decreases significantly during a long stay in space.

UCLA researchers are leading the experiment, and they're testing a bone-forming molecule called NELL-1. NELL-1 directs stem cells to create bones and prevent bone decay. Stem cells don't have a specific function, but can give rise to specialized cells, like these that form bone.

NELL-1 promoted bone regeneration when tested on mice on Earth. The molecule also helped increase bone volume and density when used with bone grafts in sheep.

Scientists hope this experiment will give them a better understanding of how to prevent bone loss. The biggest cause of bone loss is osteoporosis, which affects about 10 million people in the US. It's a disease marked by weak bones and decreased bone mass, and makes people more susceptible to bone fractures.

Bone loss is actually a problem for astronauts themselves. It's a cause for concern on long flights in microgravity conditions. Astronauts aren't as physically active as people walking around on a planet with gravity, and because of that, they're not forming bone mass like we are.

Crew members on those flights work out for 2.5 hours per day, six days a week, to prevent significant bone loss. Physical exercise can't combat the problem alone, though. The study hopes to gain insight into better ways to prevent bone loss for astronauts on long flights and for patients back on Earth.

The research, which will begin on the ground sometime soon, is funded by grants from the Center for the Advancement of Science in Space and National Institutes of Health.