why astronauts are taller when they return home?

Answer 1

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 yardstick. If you 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 significant as what astronauts experience in space, but the principle is the same. As the day passes, your vertebrae will be compressed through normal activities, and you’ll lose those few centimeters you “grew” overnight.

There are two theories about why this spinal elongation happens, says Dr. Sudhakar Rajulu, a researcher at Johnson Space Center’s Habitability and Environmental Factors Office. The first is that elongation focuses on the spine. It does not significantly affect other areas of the body, such as the legs or arms, because those bones are not compressible like the intervertebral discs. The natural curve of the spine is straightened somewhat in microgravity. Without the usual force of gravity pushing down on the spine, Rajulu says, it’s freer to relax.

The second theory about spinal elongation says that the discs between each vertebra are compressed in regular gravity, due again to the pressure pulling the spine downward. When gravity is reduced in space, the discs are able to retain more of the spinal fluid. This makes them larger and naturally puts more space—and more height—between each vertebra. When they are affected by Earth’s gravity, they bulge out the sides slightly.

“Regardless of the reason for this spinal elongation,” Rajulu says, “there are two types of change seen in an astronaut’s height while in space. The first is immediate, 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 continuation over time. This gradual relaxation of the spine eventually levels off. If astronauts stayed in space indefinitely, they would not continue to grow and grow.”

For the most part, astronauts’ extra growth doesn’t cause a problem, Rajulu says, but since crew members wear custom-fitted pressurized suits when launching and landing, the suits need to be made large enough to accommodate this change in height. Additionally, the Russian Soyuz vehicle, which astronauts sometimes ride to launch from and return to Earth, has size restrictions in seating height. 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, which was initially used by American astronauts during the U.S.-MIR missions, was designed to carry Russian cosmonauts, who had height restrictions smaller than those of American astronauts. Some Americans were too tall to fit in the seating areas, so when Soyuz was designated the emergency crew return vehicle for the International Space Station, the United States asked Russia to modify the seats to accommodate taller American crew members.

The data about human spines in space was originally gathered from the Skylab missions flown more than 20 years ago. Six astronauts were studied, and all six experienced approximately three percent growth. The spines returned to normal curvature as soon as the astronauts hit regular gravity, and stabilized totally within 10 days.

Answer 2

Because they haven't had the effect of gravity acting on their spines: most people lose about 1cm a day if they stand a lot during the day. Don't worry, you get it back at night when you lie horizontally.

Answer 3

There is less gravity and pressure in space and hence cells can grow more freely.