American astronaut Scott Kelly and Russian cosmonaut Mikhail Kornienko were busy preparing to return to Earth on Monday after spending a year aboard the International Space Station.

With a handshake, Kelly handed over command of the ISS to fellow American Tim Kopra at about 3:10 p.m. ET.

By the time the mission wraps, Kornienko and Kelly will have spent 340 consecutive days in space, sharing lodgings with 13 others.

It's a U.S. record for most consecutive days in space and the longest, by 125 days, for NASA. The overall duration record goes to Valeri Polyakov, who spent 437 days on the former Russian Mir space station in 1994 and 1995.

NASA will release video of the hatch closure, as well as undocking and landing activities at 1:30 a.m. ET Wednesday. Kelly and Kornienko will be joined in the Soyuz space capsule by cosmonaut Sergey Volkov.

Korneinko and Kelly have served with eight different crewmates, unpacked six cargo ships, weathered two botched supply runs and participated in dozens of science experiments.

Their time on the ISS will give agencies even more information on how space travel affects the body, and the kind of physiological and psychological performance challenges astronauts face during long-duration missions.

As soon as Kelly returns, NASA's first and only yearlong spaceman will try to pop up from a lying position and stand still for three minutes.

He'll take a crack at a mini-obstacle course and attempt to walk a straight line, heel to toe — all so researchers can see whether he'd hit the ground running if this were Mars instead of Earth.

NASA considers it crucial prep work for future Mars explorers who will have to spend much longer in space and won't have the help of a welcoming committee. In fact, this mission is all about Mars.

"I think we'll learn a lot about longer-duration spaceflight and how that will take us to Mars someday," Kelly said Thursday in his final news conference from orbit. "So I'd like to think that this is another of many steppingstones to us landing on Mars sometime in our future."

What could new arrivals do on Mars, asks Dr. Stevan Gilmore, the lead flight surgeon who will be at the landing site to receive Kelly. Could they jump up and down? Could they open a hatch? Could they do an immediate spacewalk?

The tests on Kelly and Kornienko should provide some answers. There will also be blood draws, heart monitoring and other medical exams. The testing will continue for weeks if not months once they're back home in Houston and at cosmonaut headquarters at Star City, Russia.

Checkups will also continue for Kelly's identical twin, retired astronaut Mark Kelly. The 52-year-old brothers joined forces to provide NASA with a potential gold mine of scientific data: one twin studied for a year in orbit — twice the usual space station stay — while his genetic double underwent similar tests on the ground.

As of Thursday — Day 335 — Kelly professed to feeling pretty good. Indeed, flight surgeon Gilmore doesn't expect any alarming results at touchdown.

Kelly's vision has degraded a bit as it did during his last mission, a normal outcome for some astronauts because of increased pressure inside the skull in weightlessness. He anticipates his bones and muscles have weakened as well, despite daily exercise in orbit.

Johnson Space Center physiologist John Charles puts the psychological side of long-duration spaceflight right up there with radiation, as well as in-flight medical care and even food preservation and packaging for the long haul.

"Just about everything is a big problem for Mars," Charles said in a phone interview.

Mars expeditions planned for the 2030s will last 2½ years; the anticipated crew size will be four to six. The astronauts will almost certainly have to grow some of their own food; that's the reason for an experimental greenhouse aboard the space station.

Kelly and his crewmates grew red romaine lettuce in the mini-hothouse last summer and sampled some of the crop.

Even more impressive, Kelly nursed zinnias back to health in January, displaying a lush orange and yellow floral bouquet on Valentine's Day. He had to "channel my inner Mark Watney" — the lone astronaut played by Matt Damon who survives on potatoes in last year's blockbuster movie The Martian — to save the zinnias from mold.