Don: It's time to go again to the A Moment of Science mailbag.

Yaël: A listener writes:

     Dear AMOS,

     I heard that NASA is planning to send astronauts back to the moon for long      missions. How are the astronauts going to stay in shape in zero or low gravity?

D: Great question! First, let's explain. On earth, the pulling force of gravity makes our      muscles work to do things like lift weights or just walk around. Without gravity or in      very low gravity, though, there's not much work for muscles to do, so they shrink.      And because the heart doesn't have to work very hard to pump blood from the      lower parts of the body to the brain, it shrinks, too.

Y: That's why astronauts are often really weak when they return from several weeks or      months in space. Now, there are ways for astronauts to exercise in space---like      jogging by using bungee cords to hold them down on a treadmill. But treadmills are      bulky and heavy, which is not good for long missions.

D: So experts working on new exercise programs for astronauts have found that the      best exercise is rowing. The pull-back motion of rowing is unique because it very      efficiently exercises so many muscles at once. Researchers have found that rowers      have the largest hearts, the densest bones, and the biggest muscles.

Y: And rowing machines can be very compact, which is crucial for tight conditions on a      space ship or space station. Thirty to forty minutes of rowing per day, and astronauts      on the moon will be able to keep their hearts strong, and bones and muscles at full      strength.  

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