One of these twins is going to space—will he come back different?

NASA is sending astronaut Scott Kelly to the ISS for a year. Then they will compare him to his twin Mark Kelly, who will remain on Earth for the entire time. It's a fascinating experiment that may reveal some effects on how space affects the human organism.

Former Navy test pilots, the Kelly brothers joined NASA in 1996. Mark, the six-minute older brother, flew four space shuttle missions before retiring in 2011 to spend more time with his wife, Congresswoman Gabrielle Giffords. Scott has two space shuttle missions under his belt, and lived on the ISS for five months in 2010-2011.

When NASA decided to send Scott to the ISS for a year alongside Russian cosmonaut Mikhail Kornienko, the brothers mentioned that a twin study was bound to come up in press conferences. NASA decided it might as well run some tests, and called for study topic submissions from the scientific community. The space agency will decide which studies to conduct in January.

Of course, most of the studies will focus on the standard stuff: blood, saliva, and (euugh) stool samples, cheek swabs (to compare any DNA changes), and physical and psychological testing. Scientists will monitor how radiation and a zero-gravity environment affect the body's gene expression, metabolism, and mental state. Identical twin studies are the gold standard in determining genetic versus environmental influence on health and disease — since both twins have identical DNA, differences between them can be attributed to outside factors.

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