Exercise is the key. But exercising in space differs from exercising on Earth. Here, gravity's pull automatically provides a resistive force that maintains muscles and bones. "[In space] even if you do the same amount of work that you were doing down here on Earth, you miss that gravity component," says Schneider.

Various devices have been developed to mimic the help that gravity provides. One Russian experiment provides resistance by strapping jogging cosmonauts to a treadmill with bungee cords. But that particular combination has not yet proven effective in preventing bone loss -- perhaps because it cannot provide sufficient loads. "The straps are so uncomfortable that the cosmonauts can only exercise at 60 to 70 per cent of their body weight," says Hargens.

There's also IRED, a NASA-developed Interim Resistive Exercise Device. IRED consists of canisters that can provide more than 300 pounds of resistance for a variety of exercises. IRED's effectiveness is still being monitored, says Schneider.

The device, explains Hargens, prevents much of the loss of cardiovascular function and of muscle. It also seems to be effective in reducing some indices of bone loss. One reason is that the LBNP allows astronauts to exercise with an effective body weight between 100 percent and 120 percent of what they would feel on Earth. Another is that -- unlike any previous exercise device -- it restores the blood pressure gradient, increasing blood pressure to the legs.

There's growing evidence, Hargens says, that the body's systems interact with each other. For example, "you can't just put high loads on the bone and then expect it to recover if you're not taking care of the blood flow to that bone as well."

Scientists aren't yet sure how gravity "signals" the body to keep bones and muscles strong. "We know that, somehow, gravity is converted from a mechanical signal to a chemical signal -- and we know a lot about these chemical signals," says Schneider. The mechanical signals, though, remain a mystery.

Solving these problems, says Schneider, could lead to better therapies for people who aren't using gravity properly here on Earth. Aging is the perfect example. Zero-G living mimics closely the effects of old age. Like astronauts, the elderly fight gravity less. They're more sedentary, which triggers the loop of muscle atrophy, bone atrophy, and lower blood volume.

If researchers can identify the signals that generate strong muscles and bones, it might be possible "to get new pills and do exercises" that would trigger those signals here on Earth.

"We've just begun to do research ... looking at the changes that can happen to humans," says Schneider. "There are so many wonderful questions."

And the answers? They're waiting for us ... up there in space, where the absence of weight reminds us that gravitation isn't all bad. Sometimes it's a struggle, our daily contest with gravity, but now we know the struggle is good!