A Russian Soyuz spacecraft with three crew on board successfully docked with the International Space Station on Friday after blasting off from Kazakhstan, Nasa said.
The Soyuz-TMA16M spacecraft's crew included a US astronaut and a Russian cosmonaut who will be the first to spend an entire year on the orbiting outpost.
The successful docking occurred after the Soyuz took off without a glitch from the Baikonur cosmodrome in Kazakhstan at 22:42 pm (1942 GMT) Friday, Russian space agency officials said earlier.
Nasa flight engineer Scott Kelly and Russian Mikhail Kornienko will together spend a total of 342 days aboard the ISS, while the third crew member, Russian cosmonaut Gennady Padalka, will spend the usual period of six months.
Kelly, 51, and 54-year-old Kornienko will take part in the extended mission to test the effects of long-term space missions on the human body.
The trip marks the longest amount of time that two people will live continuously at the ISS, though a handful of Russian cosmonauts spent a year to 14 months at the Russian space station Mir in the 1990s.
"This is the first time we're doing it as an international partnership, which I think is one of the great success stories of the International Space Station," Kelly said at a news conference in Baikonur ahead of the launch.
He said that the experiment could prove vital towards planning future international missions, including to Mars.
"If we ever go beyond Low Earth orbit again, perhaps to Mars, because of the cost and the complexity it will most likely be an international mission, so I see this as a stepping stone to that."
The physical effects of a year in space will be closely monitored by doctors on the ground in an unprecedented study of how the human body withstands the rigours of spaceflight before humans plan to journey to Mars.
Kelly said he was concerned about the impacts of radiation and living in zero gravity, particularly in terms of compromised immunity and bone and vision loss.
"I'm hopeful that there is not a big cliff out there with regards to our ability to stay and live and work in space for longer periods of time," he said.
"But we are not going to know that until we have actually done it," he added.
"The jury is out."
Kelly's twin brother Mark, a retired US astronaut who flew to the ISS four times, is part of the experiment and will undergo regular health checks on Earth so doctors can compare the brothers' vital signs.
Kelly and Kornienko, both of whom have flown multiple missions to orbit and have each previously spent about six months on the ISS, are set to stay on the space station until March 2016.
Three crew members -- Russian Anton Shkaplerov, Italian Samantha Cristoforetti and American Terry Virts -- are currently onboard the space station, due to leave in May.
The arriving crew members are due to coincide with the first space tourist since 2009, British soprano Sarah Brightman, who has paid $52 million for her 10-day stint planned to start on September 1 and has said she will perform in space.
"We'll sing together," Kornienko pledged at a news conference ahead of the launch.
Space exploration remains a hugely symbolic area of cooperation between Russia and the US despite the freeze in relations over Russia's alleged actions in Ukraine.
Russia last month confirmed that it will continue using the International Space Station in partnership with Nasa until 2024, after threatening to pull out and stop financing it in 2020.
Watch: The moment Soyuz space capsule blasts off, with US and Russian astronauts in it