Mars500, a 520-day simulation conducted by the European Space Agency and Russia's Roscosmos, wrapped up late last year. Mars500 took place here on Earth; NASA, looking to conduct a similar test, is thinking of taking the project onto the International Space Station for more accurate conditions.
In super simple terms, a modern mission to Mars would have three distinct parts: the trip there, a period in orbit/on the surface of the Red Planet, and the trip back. Russia and Europe's Mars500 spread these three distinct segments out over 520 days to see the effects the trip would have on the six-man crew, all of whom spent the bulk of that time in a capsule with a brief stint on the simulated surface of our ruddy neighbor.
Those effects studied were mostly psychological or logistical, such as how the crew would deal with limited communication with Earth, what it's like being cooped up in an enclosed area for over a year and a half and what it'd take to supply a real shot at Mars.
On the International Space Station, NASA would instead be able to gauge the physical impact long stretches in low-to-nearly-nil gravity would have on an astronaut. For example, we know that time in orbit can cause muscle shrinkage and bone deterioration, but it could also have a negative effect on an astronaut's eyes, too.
That's one area where the Russian space program has a leg up on everyone else, at least in terms of time logged. The longest amount of time a NASA astronaut spent in orbit in a single flight is held by now-retired, Spanish-born Michael López-Alegría, who spent 215 days on the ISS in 2007.
That record is half what Russian cosmonaut Valeri Polyakov can boast of, who spent over 437 days on a Mir mission, or about 14 months, over 1994 and '95. Second behind Polyakov is another Russian, Sergei Avdeyev, who spent just shy of 380 days in orbit. Behind Polyakov and Avdeyev, the two other humans who have spent over a year in space are both Russian cosmonauts as well: Vladimir Titov and Musa Manarov.
When Polyakov returned from his mission, he said "it is possible to preserve your physical and psychological health throughout a mission similar in length to a flight to Mars and back," according to this 2009 article in the New York Times.
If it goes forward, NASA's take on Mars500 won't happen for another two or three years at least, and it'll be racing against the International Space Station's retirement sometime around 2020.
Via ABC News