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That's what gives astronauts the puffy-looking faces while they're in orbit. Some are bothered by this, and complain or stuffy sinuses or headaches; other don't notice it-- until they look at the pictures later!

There are some physiological effects that are not important on the short (week-long) Space Shuttle flights, but which could be important on longer Space Station or interplanetary flights. For examples, scientists learned from earlier space projects that astronauts gradually lose calcium from their bones while they're in orbit. This is largely a result of the fact that there is no stress on the bones, and stress is an important stimulant to calcium production. Calcium loss is not important for astronauts aboard the shuttle, because we're not in orbit long enough to lose a significant amount. Even on an extended interplanetary flight, the bone loss would not be important to the astronaut while he or she is weightless--because strong bones aren't important in weightlessness. It only becomes a problem when the astronaut who has been weightless for a long time (many months or years) returns to Earth and once again needs strong bones. So far as we know, this condition is reversible--that is, upon return to Earth, the calcium in the bones returns to normal levels. But this problem requires considerable further study, and preferable also an exchange of data with the Soviets, who have much more experience with long duration spaceflight than we do.