A Russian Soyuz rocket has docked with the International Space Station.
The capsule was transporting NASA astronaut Scott Kelly, and cosmonauts Mikhail Kornienko, and Gennady Padalka.
Kelly and Kornienko are slated to make the first year-long stay on the orbital outpost, double the current mission durations.
Scientists are interested in seeing how the human body fares during longer stays in space, as the United States and other countries begin planning for multi-year missions to Mars.
In addition to more exposure to radiation, astronauts experience bone and muscle loss and changes in their cardiovascular, immune and other systems.
Kelly and Kornienko will participate in a battery of experiments before, during and after their flight to assess psychological and physiological changes from being in microgravity for a year.
Based on reporting by Reuters and AP