Astronaut Mark Kelly has an identical twin brother named Scott. Together, they will undergo a unique NASA study that will focus on “the effects of long-term space missions on the human body,” according to TODAY. The sub-focus is this: A “what if?” Mars mission.

In 2015, Scott will travel with a Russian cosmonaut and spend a year on the ISS, while Mark will stay in Arizona with wife Gabrielle Giffords, a former U.S. House representative.

Blood, saliva, cheek swab, and stool samples will be taken from both brothers at the same times before, during and after the one-year mission to study the effects of long space missions on the human body. Psychological and physical performance tests will also be conducted.