original - [http://www.unexplainable.net/artman/publish/article_599.shtml#6]
Mar 12, 2004, 06:44
Did you know that astronauts are up to 2 inches taller while they're in space? As soon as they come back to Earth, though, they return to their normal height. What happens in space is not an optical illusion, but one more example of how microgravity affects our bodies.
Imagine that the vertebrae in your back form a giant spring. Pushing down on the spring keeps it coiled tightly. When the force is released, the spring stretches out. In the same way, the spine elongates by up to three percent while humans travel in space. There is less gravity pushing down on the vertebrae, so they can stretch out - up to 7.6 centimeters (3 inches).
To some degree, a similar stretching of the spine happens to you every night. When you lie down, gravity isn't pushing down on your vertebrae. You can do your own experiments with a meterstick. Measure your height carefully as soon as you get up or while you are still lying down. You will find that you're about a centimeter or two taller. That's not as much as astronauts change in space. The idea, however, is the same. As the day passes, your vertebrae compress through normal activities, and you'll lose those few centimeters you "grew" overnight.
There are two theories to explain why the spine gets longer, says Dr. Sudhakar Rajulu. He is a researcher at Johnson Space Center's Habitability and Environmental Factors Office. The first theory is that elongation only happens to the spine. It does not have a major effect on other areas of the body, such as the legs or arms. The reason is because those bones are not compressible like the discs in the spine. The natural curve of the spine is straightened somewhat in space. Without the usual force of gravity pushing down on it, the spine is freer to relax.
The second theory about spinal elongation says that the discs between each vertebra are pressed together in regular gravity. This compression is due to the pressure pulling the spine downward. When gravity is lowered in space, the discs are able to hold more spinal fluid. This makes them larger. It also puts more space between each vertebra.
Regardless of the reason for this spinal elongation, there are two types of change seen in an astronaut's height while in space. The first change is seen as soon as the space vehicle goes into the orbit. That is when most of the elongation takes place. The second is a smaller, more gradual change over time. This gradual relaxation of the spine eventually stops. If astronauts stayed in space forever, they would not continue to grow and grow.
For the most part, astronauts' extra growth doesn't cause a problem. But, since crew members wear custom-fitted pressurized suits when launching and landing, the suits need to be made large enough to have room for this change in height. Also, the Russian Soyuz vehicle has size limits in seating height. (Astronauts sometimes ride the Soyuz to launch from and return to Earth.) If crew members were too tall, it could pose a problem. "The spine is compressible, so astronauts might be able to squish to fit in, but if it's a tight fit to begin with, it might be too tight," says Rajulu.
So far, this has not been a problem. The Soyuz was designed to carry Russian cosmonauts. It was first used by American astronauts during the U.S.-MIR missions. The Russians had height limits smaller than those of American astronauts. Some Americans were too tall to fit in the seating areas. The Soyuz was chosen to be the emergency crew return vehicle for the International Space Station. So, the United States asked Russia to adjust the seats for taller American crew members.
The data about human spines in space was first gathered from the Skylab missions. This was more than 20 years ago. Six astronauts were studied. All six showed almost three percent growth. The spines returned to normal curvature as soon as the astronauts hit regular gravity. Then, they were back to normal within 10 days.