Astronaut Ellen Baker bikes to nowhere at 18,000 miles per hour on a bike mounted on a platform that insulates sensitive microgravity experiments from vibration caused by exercise.
Courtesy NASA

Dark lines indicate bone mineral; lighter areas are gaps between mineral. Left: heavy lines mark stronger bones. Middle: mild osteoporosis. Right: severe bone loss.

The incredible anti-gravity machine
In space, the most nettlesome medical problems stem from weightlessness -- the lack of gravity. (Gravity does not disappear, but orbiting astronauts are falling freely. They don't hit Earth because their momentum causes them to stay in orbit.)

Still, it feels like gravity has been cancelled, and since our balance system relies on gravity to tell us which way is up, the result can be space motion sickness. Like motion sickness on Earth, the symptoms include nausea, "stomach awareness," headaches, dizziness and lethargy.

While many astronauts suffer this sickness, it generally passes in one to three days, says space medicine researcher Ronald White, sometimes with the help of motion-sickness drugs.

All animals evolved to deal with gravity, developing systems to prevent fluids like blood and lymph from moving toward the feet. Absent gravity, these mechanisms push fluids toward the head, resulting in bulging necks, swelling and a feeling of congestion. The fluid comes from points south -- White says a leg can lose one-tenth of its volume on the first day in space.

Bone heads in space
The most tenacious problem associated with space travel is bone loss -- AKA osteoporosis. When bones are no longer stressed by gravity, they demineralize, losing calcium and strength.

And the longer the mission, the greater the loss. "If you go on a prolonged mission, it may be possible to lose 10, 20 or 30 percent of bone mass," says Michael Holick, a bone specialist at Boston University Medical Center. As a spacecraft slows for landing while reentering the atmosphere, the exaggeration of gravity could have "devastating consequences," Holick says.

On the relatively short missions to date, bone loss has generally been mild -- although one Soviet cosmonaut did lose a dramatic and dangerous 20 percent of bone mass. Even exercise, which maintains bones on Earth, does not help in space, since it's impossible to simulate gravity effectively (even exercising with bungee cords doesn't help).

Nor do bones recover fully after return to Earth. "For most space travel, where you are losing only a few percent, 75 percent recovery may be fine," says Holick. "But we don't know if that would be true if you lost 15 to 20 percent."

Holick expects, however, that drugs will control the bone-destroying process of osteoporosis in time for a Mars mission. And that's just as well, he adds, since the liberated calcium in the blood can become deposited as kidney stones, which are described as more excruciating than childbirth. "You have to wonder what you'd do if astronauts got kidney stones" while en route Mars, says Holick.

Counting countermeasures
One way to prevent bone loss may be to create artificial gravity. Tethering two capsules together and rotating them about their center of gravity would create centrifugal force that would feel like gravity (although it might look like a giant dog bone spinning through space).

Furthermore, space travel isn't always weightless. During a trip to Mars, for example, the crew would be weightless for six or more months on each interplanetary journey, but would spend a year or more on the 37 percent gravity of Mars. Animal studies in a centrifuge on the space station may answer whether this level of gravity would prevent bone thinning.

A larger centrifuge may help Homo sapiens during long weightless periods. That possibility arises from studies of bed rest, which thins bone much like space travel. Surprisingly, just 15 minutes of daily walking can prevent osteoporosis in bed-rest patients, and White suggests a brief daily spin in a centrifuge might do the same for astronauts.

Granted, spinning around can be highly nauseating on Earth. White notes that making certain head movements while spinning in a chair is an instant recipe for barfing on Earth. In space, it ain't necessarily so.

Can you have an ounce of prevention without gravity?
With bone loss, prevention is better than cure, since even if bones do recover their full density, they may not return to factory-fresh condition. "The skeleton is a three-dimensional structure," observes Holick, and while bone-loss studies assume that the new calcium will rebuild the skeleton to original specifications, that remains to be seen. ''How do you know you have reestablished the architectural integrity?" Holick asks. "Are you going to have the same mechanical strength after you return?"

Indeed, White says returning to Earth is "probably more difficult than entering space. When you first come back to Earth, the perception is that your body weighs three times as much. It's more difficult to stand up when you first come back, you walk wobbly, have difficulty going up and down steps, walking around corners." Aside from bone loss, however, he says most problems clear up within a week or two.

Overall, the experts we spoke with thought all problems -- even bone loss -- would be solved before anybody buys a round-trip ticket to Mars, a decade or two from now. In the long term, White says, "I believe we'll figure out how to deal with these."

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