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NASA One Year Mission will send two people to ISS for one year to help us to get closer to Mars
Valeri Polyakov Is current record holder for longest space flight.
So, there is one of most obvious questions: We have people who already have been in zero-g for year and even longer (record is 437.7 days). Why cannot we use medical data obtained from these missions?
The Russian medical monitoring of long term flight participants was not considered well done by the rest of the worlds medical establishment. The ISS has much more equipment, resources, and crew time to monitor than did Mir (Crew of 6 vs 3).
This is a repeat with a larger (2 vs 1, yay!) sample set with better monitoring in place, using lessons learned from the past.
Additionally this study has an incredibly fortuitous add on. The Kelly brothers are twins. Both are astronauts with very similar careers. Scott Kelly will be on the station, and his twin (Mark) will spend the year performing similar tasks on Earth. Twin studies are great, since their genome is pretty darn close (compared to any other two random people) and helps remove genetic predispositions from the results. Very fortuitous!
Most importantly, repeatability and sample size are the keys in experimentation. You should repeat it often to ensure you get the same results. Sample set of 1 is mostly meaningless. Or as commenter Federico notes, sample size of 1 is basically a Mythbusters episode. (Credit to PHD Comics)
The data from previous missions is largely valid and technically should be usable. They can act as a very good guide to what can be expected. However, there are always issues and it always comes down to details:
Ownership of the Data
Valeri Palyakov is a Russian, so Russia owns the data for his mission. Russia may or may not want to release all the data for Valeri's mission or reveal all of the conditions that Valeri had to deal with during his mission.
The Russians can use their data to advance their programs in Space. America is a competitor to the Russians in Space. What does Russian gain by helping America advance its Space programs?
The American would like to have their own data based on their personnel and their test conditions.
Mission Conditions & Operating Procedures
The American mission may want to test for different conditions than those of the Russian mission that Valeri was on. The Americans may want to emphasise some things in their conditions that the Russians didn't in theirs. Likewise they may want to de-emphasise some things that the Russians did.
The Russians and Americans have different cultures, different attitudes to authority, different attitudes to hardship and extremes and some of their operating procedures will be different. This may influence the results. Russians like to emphasise that no other people on Earth can tolerate cold conditions like they can. But is it really true? Similar statements are made by people who live in very hot environments.
Conducting such missions with your own personnel and with you own procedures gives you greater control over the results and how useful the results will be. You can also change the program as you go along, depending on circumstances.
The other thing is that in science results from experiments need to be tested, repeated and verified before they can be fully accepted. An American long duration test program will provide a second set of results that can be compared to those of Valeri's mission, if the Russians release all of the data for that mission.
Politics
Remember, there once was a Space race in which the Russians and American competed against each other and there was a cold war between the two. A new cold war appears to be re-immerging as a result of what's happening in Ukraine. The attitude of sharing, between the two nations has changed and it is not as open as it has been in the recent past.
