American astronaut Scott Kelly and his Russian counterpart Mikhail Kornienko have blasted off on a mission to spend an entire year away from the Earth.

The trip is Nasas first attempt at a one-year space flight, anticipating Mars expeditions that would last two to three years.

Their Soyuz space capsule set off from Russias manned space launch facility on the steppes of Kazakhstan at 1.42am local time on Saturday and was to dock with the International Space Station (ISS) about six hours later after making four orbits of the planet.

Cosmonaut Gennady Padalka of Russia was also aboard their Soyuz capsule. He is scheduled for the standard six-month tour of duty aboard the space station.

Kellys identical twin Mark, a retired astronaut, agreed to take 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.

Kelly and Kornienko will remain on board until next March. During that time, they will undergo extensive medical experiments, and prepare the station for the anticipated 2017 arrival of new US commercial crew capsules. That means a series of spacewalks for the 51-year-old Kelly.

They also will oversee the comings and goings of numerous cargo ships, as well as other Russian-launched space crews.

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Trio blast off to space station