BAIKONUR, Kazakhstan: An American astronaut and two Russian cosmonauts blasted off early Saturday to begin a year away from Earth.

Of the three-man team, American Scott Kelly and Russian Mikhail Kornienko are to stay at the International Space Station (ISS) until March 2016.

Kelly’s identical twin Mark, a retired astronaut, is taking part in many of the same medical experiments as his orbiting sibling to help scientists see how a body in space compares with its genetic double on Earth.

The trip by the three astronauts is NASA’s first stab at a one-year spaceflight, anticipating Mars expeditions that would last two-to-three years.

Kelly, Kornienko and another Russian, Gennady Padalka, began their marathon mission with a Soyuz rocket launch from Kazakhstan and are to arrive at the orbiting outpost six hours later.

Experts believe differences between the twin brothers could reveal how the body copes with extreme environments. But results from the study may never see the light of day.

The Kellys are having their entire genomes sequenced, and if they discover sensitive medical information they do not want shared — such as susceptibility to particular diseases — the results may never be published, nature.com reported.

“This is such new territory, we can’t anticipate what will happen,” says Craig Kundrot, deputy chief scientist of the human research program at NASA’s Johnson Space Center in Houston, Texas.