American astronaut Scott Kelly and Russian Cosmonaut Mikhail Kornienko took off Friday for a year-long mission aboard the International Space Station. Meanwhile, Kelly’s identical twin brother, and former astronaut, Mark Kelly will be monitored on the ground in the first twin study ever done on the effects of long-term space flight.

This will be the first year-long mission aboard the space station, and a first for the Americans, but not the first in the history of humans in space.

The Russians had several year-long missions in the late 1980s and early 90s, aboard the Russian space station Mir. The record-holder for a continuous stay in space is Valery Polykov, at 437.75 days in 1995.

Sergei Krikalev, who was stranded on Mir when the Soviet Union collapsed in 1991, and then went on to fly on American space shuttles and the International Space Station, has a total accumulated time in space of 803 days.

The Russians laid the groundwork for long duration spaceflight, learning a great deal about the physical and mental health effects of removing oneself from planet Earth for extended periods.

Kelly and Kornienko

Astronauts Scott Kelly (left) and Mikhail Kornienko will spend a year on the international space station. (Bill Stafford/NASA)

Cosmonauts experienced a loss of calcium in their bones, a shift of fluids into the upper body, swollen faces, anemia (the loss of red blood cells), and lowered immune systems.

They also faced the mental stress of being isolated, away from family, friends and the great outdoors, for such a long time. 

Adding to that isolation, crew members on Mir communicated with mission control only when the space station passed over Moscow, and, at times of low activity, that would only be for a few minutes, two or three times a day. The rest of the time in orbit, they were completely on their own.

They also had to get along with each other during their mission. Mir was a much smaller space station than the ISS, and usually had a crew of only three. That alone increases the "in your face" effect of seeing the same people, day after day, in a confined space.

During the year-long missions, crew performance began to degrade around eight months. They were slower to do their work, and, as time went on, even became reluctant to do some of it. The astronauts came to a point where the most enjoyable activity was eating, because it was a pleasant pastime that did not require conversation with the others.

Finally, when the Russians returned to Earth after a year in space, they had to readjust to Earth's gravity. They had a very difficult time walking, for example, because of weak muscles and severe dizziness. One cosmonaut told me, "Every part of my body felt heavy, even my eyelids."

Kelly and Kornienko prepare for ISS

Astronaut Scott Kelly (right) and cosmonaut Mikhail Kornienko are seen here training for their year-long mission in a mockup of the International Space Station. (Photographer: James Blair)

It is from that earlier experience on Mir that mission planners for the International Space Station decided to restrict occupation to no more than six months. But this new year-long mission will be different from those of the past in several ways. 

First of all, Kelly and Korienko will have plenty of company among  the six-person crew of the space station, which will be changed at least once during their flight.

The space station offers much more room to get away from the others, and it has a 360-degree panoramic window on the Earth, which all crew members find endlessly entertaining.

The pair will have exclusive channels to make private phone calls to their families, along with all the other modern communications devices that were simply not available 30 years ago for Mir missions.

They will also be on a strict exercise regime of two hours every day, to fight off the physical effects of weightlessness.

But, there are still questions to be answered. Since Scott Kelly's identical twin brother Mark will remain on the ground, study of the twins will offer opportunities to look in more detail at the effects of prolonged stays in space on the immune system, digestive tract, vision, and mental health issues.

This is, of course, in preparation for trips to Mars, which will require roughly 500 days for a journey there and back. The technical challenges of keeping a crew well fed and happy in an extreme environment for that long are significant enough. How the crew takes care of themselves is a challenge just as great.