American astronaut Scott Kelly, along with Russian cosmonaut Mikhail Kornienko, returned to Earth this week, after the duo spent 340 days in space. It's an endurance record for an American in space, but still less than the time it would take for a return trip to Mars.

The purpose of the extended stay aboard the International Space Station was to document changes to the body and the mind, when living without the influence of gravity inside a tin can for a year.

This experiment is picking up where the Russians left off when they made much longer flights on their space station Mir. During the 1980s and '90s, several cosmonauts spent more than a year aboard Mir, which was a smaller predecessor to the present ISS.

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U.S. astronaut Scott Kelly is seen inside the Cupola, a special module which provides a 360-degree viewing of the Earth and the space station, aboard the International Space Station on July 12, 2015. (Scott Kelly/NASA/Reuters)

The world record holder is Valeri Polyakov, who spent 438 days in orbit in 1994. He also spent 240 days in space in 1988, for a total of 678 days.

Those two flights combined is roughly the time it will take to journey to Mars and back. A return trip, including landing, takes about two-and-a-half years. A once-around flyby, without stopping, could be done in about 450 days.

In many ways, the Russian experience was closer to a Mars trip than what Kelly and Kornienko experienced aboard the ISS. Today, astronauts and cosmonauts live in groups of six, with regular visits and exchanges with new arrivals from the ground. They also have access to all forms of social media, so they can e-mail, Tweet and exchange selfies with family and friends back home pretty much any time of night or day.

Mir was more of a remote outpost, where the three-person crew spent most of their time isolated in space, only communicating with the ground once or twice a day. The rest of their time was spent entirely on their own, cut off from the rest of the planet and their families.  

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NASA astronaut Scott Kelly gives himself a flu shot for an ongoing study on the human immune system. (NASA via Getty Images)

Without the burden of gravity, the men suffered a number of physiological effects; they lost calcium in their bones, muscles atrophied, red blood cell counts went down. By the time they returned to Earth, they were unable to walk.

One cosmonaut told me that his body felt so heavy when he returned, even his eyelids took an effort to open.

Today, most of these negative effects are lessened by extensive exercise equipment on the ISS and the requirement for everyone to spend two-and-a-half hours a day working out on it. (Which is not a bad idea for anyone, whether in space or not.)

But the Russians also discovered the negative psychological effects of isolation.

While in Russia in the 1980s, I sat in on a weekly teleconference between the three-person crew on Mir and their families, which took place at mission control. About a dozen people crowded into the room, facing a black and white television with a camera pointing back at the group. Mir would pass overhead and be in range for less than 15 minutes. When contact was made, there was only enough time for each person in the room to basically say, "Hi. How are you? I'm fine." Then it was all over.

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Tim Kopra (left) talks to NASA after officially taking command of the International Space Station, as Scott Kelly and Mikhail Kornienko (right) take part in a farewell ceremony on Feb. 29. (NASA)

For the crew, it was a brief glimpse of their loved ones, then back into silent running. No e-mails to write, no personal phone calls, no-one to talk to but the same faces they had been looking at for months.

The psychologist responsible for the mental health of the crew said that after seven months some crew members became more irritable, preferring to spend time alone, and were more reluctant to carry out their duties.

After 10 months, lethargy and emotional problems began to appear.

One of the best remedies for depression in space was to ask the crew what famous Russian celebrity they would like to speak to, and that person would be invited to do a video uplink. Hockey players, dancers with the Bolshoi Ballet, and movie actors would turn up and spend time chatting with the crew, boosting morale for weeks afterwards.

This still happens today, as we saw when Canadian astronaut Chris Hadfield was Commander of the space station and spoke with William Shatner, fictional captain of the starship Enterprise.

Kelly and Kornienko prepare for ISS

Astronaut Scott Kelly and cosmonaut Mikhail Kornienko in pre-mission training in a mockup of the International Space Station. (Photographer: James Blair)

It is this type of psychological effect that will challenge a crew traveling to Mars.

As the ship gets farther from Earth, a time delay will come into the communication link, building up to 20 minutes in each direction. That means all messages, including voice and video, will have to be done one way, where they deliver their entire message, send it off and wait 40 minutes for an answer. That alone will add to the isolation and be a daily reminder of how far away from home they really are, just as the crews aboard Mir discovered.

Whenever crew on the ISS feel claustrophobic, they go to the cupola, a panoramic window looking down on the ever changing and stunningly beautiful Earth. A Mars crew will lose sight of the Earth after the first few days, so there won't be a lot to see out the window.

Even spacewalks will be limited because they will be outside the Earth's protective magnetic field, exposing the astronauts to harmful solar and cosmic radiation.

Scott Kelly is now being compared physically through medical tests to his identical twin brother Mark, a former astronaut, who remained on the ground as a control subject. The tests will show how much of a toll the spaceflight took on his body. That will help planners for future missions to compensate for those physical effects.

The two spacefarers are in remarkable shape back on Earth, compared to their predecessors, thanks to the exercise regimes and constant contact with the ground. They have demonstrated how our bodies can adapt to new environments quite well, with proper training.  

But adapting our minds to the demands of extreme isolation will be another challenge altogether.