"Project Gemini" made awesomely literal: Scott Kelly will be floating a whole year on the ISS, starting March 2015. Scott's twin, Mark Kelly, will remain on Earth, bound by gravity. Two genetically identical humans in two radically different environments? The brothers volunteered for detailed studies.

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NASA’s Human Research Program is now requesting and evaluating proposals for the study, officially called "Differential Effects on Homozygous Twin Astronauts Associated with Differences in Exposure to Spaceflight Factors."

While the Kellys present a unique opportunity for study, they are not the perfect lab rats:

The time Mark has already spent in space muddies the study up a little since he’s been exposed to the same exotic physical environment Scott has. The ideal subjects would be a twin who is flying in space and one who has never left the planet. But after Scott’s return he will have amassed 540 days in space, or precisely 10 times Mark’s total, which ought to be a big enough difference to give the findings real credibility.