NASA astronaut Scott Kelly is getting ready for a mission to the International Space Station, an increasingly routine happening, except for one thing — he's going to be up there for a full year.

If he manages it, he will break the U.S. record for the longest continual time spent in space, currently standing at seven months.

Kelly and Russian crewmate Mikhail Kornienko are set to launch on Friday afternoon, along with Russian cosmonaut Gennady Padalka, heading for the ISS aboard a Russian Soyuz spacecraft.

Kelly and Kornienko are both scheduled to spend a year on the orbiting science lab.

"It's not going to be easy to spend a year in that kind of isolated environment," Kelly said while training for the mission. "But I think I'm up for that challenge."

There are both physical and emotional challenges involved in long-term stays in space.

In weightless conditions, vision can deteriorate and bone mass is lost, while the confinement and isolation can cause psychological problems like depression.

While medical experts say there's a considerable body of knowledge about what happens to humans who spend around six months in space, the effects of a full year in microgravity are less clear.

Only four people have spent that long in space, all Soviet or Russian cosmonauts on the MIR space station in the 1980s and 1990s.

The record-holder is Valery Polyakov, who spent 14 months aboard MIR in 1994-95.

However, "one person being able to do this doesn't mean everyone will," says Mathias Basner from the University of Pennsylvania School of Medicine, experienced in the study of effects of space missions on crews. "We know next to nothing about what will happen once we put people into space for prolonged periods of time."

In order to gain valuable knowledge, researchers will have the help of Kelly's twin brother and former astronaut Mark Kelly, who will participate in studies here on Earth to help learn more about how Scott Kelly's time in orbit might be affecting him and his body.

The results are considered vital as NASA turns its eyes to a possible crewed mission to Mars, which would see astronauts spending as much as eight months in a small spacecraft heading to the Red Planet — and again on the return.

Kelly is well aware of that.

"If we're ever going to go to Mars someday, the International Space Station is really a great platform to learn much more about having people live and work in space for longer durations," he has said.