March 22, 2006: Consider a pair of brothers, identical twins. One gets a job as an astronaut and rockets into deep space. The other stays on Earth. When the traveling twin returns home, he discovers he’s younger than his brother.

This is Einstein’s Twins Paradox, and although it sounds strange, it is absolutely true. The theory of relativity tells us that the faster you travel through space, the slower you travel through time. Rocketing to Alpha Centauri — warp 9, please — is a good way to stay young.

Or is it?

Some researchers are beginning to believe that space travel could have the opposite effect. It could make you prematurely old.

Albert Einstein’s theory of Special Relativity says that time slows down for fast-moving space travelers, effectively keeping them young. Space radiation acting on telomeres could however, reverse the effect.

“The problem with Einstein’s paradox is that it doesn’t fold in biology — specifically, space radiation and the biology of aging”, says Frank Cucinotta, NASA’s chief scientist for radiation studies at the Johnson Space Center.

While the astronaut twin is hurtling through space, Cucinotta explains, his chromosomes are exposed to penetrating cosmic rays. This can damage his telomeres — little molecular caps on the ends of his DNA. Here on Earth, the loss of telomeres has been linked to aging.

So far, the risk hasn’t been a major concern: the effect on shuttle and space station astronauts, if any, would be very small. These astronauts orbit inside of Earth’s protective magnetic field, which deflects most cosmic rays.

But by 2018, NASA plans to send humans outside of that protective bubble to return to the moon and eventually travel to Mars. Astronauts on those missions could be exposed to cosmic rays for weeks or months at a time. Naturally, NASA is keen to find out whether or not the danger of “radiation aging” really exists, and if so, how to handle it.

Internet: (adapted).

Acccording to the text,

shuttle and space station astronauts are not subject to any risks.