Astronaut Scott Kelly is gearing up for a one-year space station stint next spring. And his twin brother will be offering more than his usual moral support.

Retired astronaut Mark Kelly will be joining in from Earth. He will undergo medical testing before, during and after his brother's American-record-setting flight.

It is part of an unprecedented study of identical twins, courtesy of the Kellys and Nasa. Researchers hope to understand better how prolonged weightlessness affects the body by comparing the space twin with the ground twin.

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