US astronaut Scott Kelly and Russian cosmonaut Mikhail Kornienko are set to return to Earth after spending almost a year in space.

Their extended stay is part of an effort to study the effects of long-duration spaceflight on the body.

But scientists will gain further insights by comparing Scott Kelly with his Earthbound identical twin, Mark.

Scott Kelly and Kornienko will have spent 340 days on the space station by the time they return on 2 March.

The mission is a record for the International Space Station (ISS) and will give Cmdr Kelly a tally of 520 cumulative days in space, over four flights.

Cmdr Kelly will also bring back rocket seeds that were taken up to the orbiting outpost by UK astronaut Tim Peake in December. They will be distributed to schools to allow pupils to compare their growth with rocket (salad) plants that have stayed on Earth.

"I could go another 100 days, I could go another year if I had to, it would just depend on what I was doing and whether it made sense," Cmdr Kelly told the media during a link-up with the ISS.

But he said: "I am looking forward to getting home", and described his home for the past year as a "magical place" and an "incredible science facility".

The idea behind the twin study is to help minimise extraneous factors as scientists research how genetically identical individuals respond to different conditions over a year.

The harsh space environment takes its toll in the form of muscle atrophy, sleep problems, bone loss, vision impairment and radiation exposure. But some of the most pronounced effects are on the mind.

"Physically I feel pretty good... but the hardest part is being isolated in the physical sense from people on the ground who are important to you," said Cmdr Kelly, adding: "There's a loss of connection."

"Even after being here for a year, you don't feel perfectly normal," he explained, adding: "Having no running water is kind of like being in the woods camping for a year in terms of hygiene."

After he touches down in Kazakhstan in the early hours of Wednesday morning, Scott Kelly will be flown to Houston where Nasa researchers will begin their battery of scientific checks. Following this, Mr Kelly explained, he would "go home and jump in my pool".

Mark Kelly, who was an astronaut, now retired, has been undergoing tests of his own on Earth during his brother's sojourn on the ISS.

Comparisons between the twins should allow scientists to deepen their understanding of the subtle physical and psychological shifts which occur during long stays in orbit. Nasa says the research should inform its stated aim of sending humans to Mars.

"We'll learn a lot about long-duration spaceflight... and I'd like to think that this is one of many stepping stones that will take us to Mars some time in the future," said Cmdr Kelly.

Monday saw Kelly will hand over command of the ISS to his Nasa colleague Timothy Kopra. In the early hours of Wednesday 2 March (GMT), Kelly, Kornienko and Russian cosmonaut Sergey Volkov will board their Soyuz spacecraft and undock from the station.

The Soyuz will perform an engine burn at 03:34 GMT to begin its plunge through the atmosphere. The final moments of its descent will be slowed by parachute, with touch-down on the Kazakhstan steppe expected at 04:27 GMT.

Despite the length of their flight, Kelly and Kornienko are still some way shy of the record for the longest single spaceflight, which is held by the Russian Valery Polyakov, who spent 437 consecutive days aboard the Mir space station in the mid-1990s.