A U.S. astronaut and a Russian cosmonaut have arrived at the International Space Station (ISS) to begin a mission that will see the pair spend 350 days in space. It will be the longest continuous mission for any NASA astronaut and the longest mission that space travelers from any nation have completed on the ISS.

The purpose of the mission is to study the effects of prolonged exposure to a weightless environment on the human body, ahead of mooted manned missions to Mars in coming years.

America's Scott Kelly and Russia's Mikhail Kornienko departed Earth from Russia's spaceflight facility in Kazakhstan at 3:42 p.m. ET on Friday and entered the ISS at 11:33 p.m. ET. They were joined on their flight by Russian cosmonaut Gennady Padalka, who will complete a regular, six-month mission to the station.



As part of the experiments to be conducted on the mission, Kelly and his identical twin brother, Mark, also a former U.S. astronaut, will conduct a series of medical experiments, with one brother in space, and one on Earth. This will allow scientists to compare the effects of the microgravity environment on a human body with a genetically identical counterpart on Earth.

Prolonged spaceflight has, in the past, resulted in bone density loss and muscle weakness, but an exercise and nutritional regimen is hoped to combat this.

Recent discoveries also suggest that spaceflight can cause damage to the eyes. Many astronauts return to Earth complaining that their vision is not as good as when they went up, the BBC reported. The weightless environment is thought to affect the distribution of fluid in the body, causing pressure to build in the skull and on the optic nerve, damaging eyesight.

Despite a mission of this length being a first for U.S. astronauts, it will not be the longest continuous period a human has spent in space. That record belongs to Russian Valery Polyakov, who spent just under 438 consecutive days aboard the Russian Mir space platform. The longest mission for a U.S. astronaut was seven consecutive months.

In addition to biomedical experiments, the crew of the 43rd expedition to the ISS will help prepare the station to receive new U.S.-built spacecraft, which are currently being developed by Boeing and SpaceX.