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Why one of NASA's twin astronauts is younger than the other

NASA will conduct an experiment using its twin astronauts to assess the effects of space travel on humans. Here's why one of the twins is older than the other.

US astronaut Scott Kelly prepares for launch to the ISS in October 2010. He will return to the station in March 2015, where he will be the subject of a NASA experiment assessing the effects of space travel on humans.

Shamil Zhumatov/Reuters

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Beginning in March 2015, NASA astronaut Scott Kelly will spend one year at the International Space Station. His twin brother, a former astronaut, will spend that time at home in Arizona.

So, at the twin’s request, NASA plans to use the opportunity to measure the effects of space flight on Scott’s body, using his twin’s earthbound body as a baseline for assessments. Last month, the organization opened a public call for research proposals under the topic "Differential Effects on Homozygous Twin Astronauts Associated with Differences in Exposure to Spaceflight Factors."

NASA’s experiment – more public relations than ground-breaking science, as Scott has already logged significantly more time in space than Mark and, technically, the two could already be test subjects without one of them going back to space – will likely show that Scott is biologically "older" than Mark, given the toll that spaceflight is believed to take on astronaut’s bodies.

But even if space travel has made Scott biologically older than Mark, it has also make him in a different sense younger – thanks to special relativity. 

In 1911, the French physicist Paul Langevin put forward a thought experiment: What if one twin flies away from Earth at 99.99 percent of the speed of light? When the twin returns two years later, he expects that his twin, like himself, is two years older. But his twin isn’t there anymore – in the traveler’s absence, 200 years have passed on Earth, and his Earth-bound twin is long dead.

Langevin called it a paradox, but in fact it’s not so paradoxical.

In 1905, Albert Einstein upended the notion that time is fixed and absolute. According to his Special Theory of Relatively, time is relative. It’s the speed of light that’s fixed.


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