Many people interested in both outer space and space exploration have wondered about the effects of space travel on astronauts. In particular, we wonder about the impact of weightlessness on the human body. Several decades of physiological data have been gathered from the manned space missions to date, but the long term effects of weightlessness are not fully known or understood.
The effects of weightless on the skeleton, organs, and soft tissues are important areas of study. For example, what effect does weightlessness have on human growth? Does weightlessness make a person grow taller or shorter? Are there any other related effects on the human body?
Human astronauts have been shown to lose bone-mass while in space, yet, by contrast, they actually grow taller! Astronauts will become two inches taller during a space mission of 12 weeks duration or more and this fact is why space suits are made two inches taller than the astronaut's normal height.
The best theories that scientists have about both bone loss and adding the extra two inches of height point back to gravity's absence. In the case of bone loss, one theory is that the loss of gravity causes the bones to reconfigure themselves from a "gravity resistant" state to a weightless state. In the case of height increase, gravity's absence prevents compression of the spine which acts as a counter-balance to fluid absorption in the spinal discs. The height gain in space is a natural reaction to the loss of gravity's pull on the spine, which allows the spine to lengthen.
Want some proof? Try measuring your own height at the beginning and end of the day and you'll notice that you'll lose close to an inch of height during the day. Why? Gravity compresses your spine during the day. At night, when you're lying down, the compression will reverse itself until you get up the next day. The effects of weightlessness further eliminate the compression of the spine.
Is this height gain from weightlessness permanent? No! Just like your own spine elongates after lying down for a number of hours, an astronaut will eventually return to their normal height within days of their return. So those of you who dream of being astronauts can take assurance in one fact: you won't have to make a permanent change to your wardrobe after each mission!
National Space Biomedical Research Institute, "Human Physiology in Space" http://www.nsbri.org/HumanPhysSpace/indexb.html
NASA, "Space Bones" http://science.nasa.gov/headlines/y2001/ast01oct_1.htm
Proinversion, "About Back Pain" http://www.proinversion.net/about-backpain.html