In movies and on television, humans regularly travel in space far, far away. In reality, scientists don’t know how the human body would endure even a local trip to a neighboring planet in our own solar system.

A first-of-its-kind experiment to better understand how space travel affects the human body is beginning aboard the International Space Station (ISS).

U.S. astronaut Scott Kelly and Russian cosmonaut Mikhail Kornienko just joined the ISS crew. They’ll both spend almost a full year living and working aboard the station. Scientists will study how their bodies adjust to weightlessness, isolation, radiation and the stress of long spaceflights.

Russian cosmonauts Mikhail Kornienko and Gennady Padalka and U.S. astronaut Scott Kelly (NASA)

One element of this human experiment is unique. Scott Kelly’s twin brother, retired astronaut Mark Kelly, is back on Earth acting as a control subject for the experiment. By devoting a year to testing and evaluating the genetically identical brothers, scientists expect to produce comparative data that will offer insights on subtle bodily changes.

“We understand a lot about the engineering to get to Mars, but we don’t understand a lot about the physiology,” said Mark Kelly, as he spoke to his brother on the ISS in a March 30 check-in from Earth to the space station.

NASA Administrator Charles Bolden thanked the twins for committing to the study, especially important because the space agency aims to land humans on Mars by 2030.

A third space traveler launched on March 27 with Kelly and Kornienko. Russian cosmonaut Gennady Padalka also joined the ISS Expedition 43 crew; he will spend six months on the station.

NASA’s Curiosity rover has been photographing the landscape of Mars since its landing in 2012. (NASA/JPL)

Space agences of the United States, Russia, Europe, Japan and Canada are the leading ISS partners, but a total of 15 countries have been involved. Astronauts of different nationalities have been living and working in space around the clock, every single day, for more than 10 years. NASA describes the ISS as a place where pioneers set aside boundaries and differences to advance human knowledge on board the largest and longest-inhabited object ever to orbit the Earth.