Gravity hurts: you can feel it hoisting a loaded backpack or pushing a bike up a hill. But lack of gravity hurts, too: when astronauts return from long-term stints in space, they sometimes need to be carried away in stretchers.

Gravity is not just a force. It's also a signal -- a signal that tells the body how to act. For one thing, it tells muscles and bones how strong they must be. In zero-G, muscles atrophy quickly, because the body perceives it does not need them. The muscles used to fight gravity --like those in the calves and spine, which maintain posture-- can lose around 20 percent of their mass if you don't use them. Muscle mass can vanish at a rate as high as 5 percent a week.

For bones, the loss can be even more extreme. Bones in space atrophy at a rate of about 1 percent a month, and models suggest that the total loss could reach 40 to 60 percent.

Perhaps not if you plan to stay in space forever. But eventually astronauts return to Earth -- and the human body has to readjust to the relentless pull of gravity. Most space adaptations appear to be reversible, but the rebuilding process is not necessarily an easy one.

"Each of the parameters have their own normal recovery time," says Schneider. Blood volume, for example, is typically restored within a few days. "Astronauts get thirsty when they come back," Schneider explains, "because their body says, you don't have enough blood in your blood vessels, and that causes the messengers to say, drink more. [Also, the body doesn't] urinate as much."

Muscle, too, can be recouped. Most comes back "within a month or so, "although it might take longer to recover completely. "We normally say that it takes a day [of recovery on Earth] for each day that somebody's in space," says Schneider.

Bone recovery, though, has proven problematic. For a three to six month space flight, says Schneider, it might require two to three years to regain lost bone -- if it's going to come back, and some studies have suggested that it doesn't. "You really have to exercise a lot," says Schneider. "You really have to work at it."

According to Dr. Alan Hargens, recently of NASA Ames and now a professor of orthopedics at the University of California San Diego medical school, it is important to keep astronauts in good physical condition. "You want the crew members to function normally when they come back to Earth ... and not have to lie around for long periods of rehabilitation," he says.

And Earth isn't the only planet that astronauts might visit. One day humans will journey to Mars -- a six-month trip in zero-G before they disembark on a planet with 38 percent of Earth's gravity. "[We'll have to maintain] those astronauts at a fairly high level of fitness," explains Hargens. "When they get to Mars, there won't be anyone to help them if they get into trouble." They will need to be able to handle everything themselves.