Identical twin brothers have volunteered for a NASA project that will provide key information about how the human body responds to weightlessness in space.

Of the pair, Mark Kelly is a retired astronaut, while Scott Kelly plans to break the American record for spending the longest amount of time in space with a year-long stint that begins next March, The Associated Press reported.

"No second thoughts--I'm actually getting kind of excited about the whole idea as we get closer," Scott told the AP in a recent interview.

As for people's reactions, he said he's gotten everything from "'Oh, that would be really cool to be in space for a year' to 'What, are you out of your mind?'" 

Since the two brothers are genetically identical, scientists will be better able to compare the effects of a year on Earth for Mark to a year on the International Space Station for Scott. Their volunteering for the mission is a unique contribution for other reasons as well.

"Not only are they the same genetically, but one is an astronaut, one's a retired astronaut. So they've followed very similar career paths," Craig Kundrot, deputy chief scientist for the human research program at NASA's Johnson Space Center in Houston, told the AP.

"After Scott's mission is done, he'll have 540 days of spaceflight (in four missions). Mark will have 54. So exactly a 10-fold difference. That's just an uncanny opportunity that we're taking advantage of."

Scott, who is divorced and lives in Houston, will say goodbye for a year to his daughters, ages 19 and 10, as well as his 74-year-old father. During the flight, he will likely see Sarah Brightman, an English soprano famous for playing the original Christine in "The Phantom of the Opera." The singer plans to fly up to the ISS as a paying passenger.

Not one to shy away from medical experimentation, Scott has additionally volunteered to have a pressure sensor drilled into his skull so researchers can evaluate how long flights result in impaired vision.

"As a test pilot, I like to push the envelope on things and, in this case, I feel like I'm maybe trying to push the envelope on data collection as well," Scott told the AP.

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