The Russians have been pushing to do this on ISS for years. Unfortunately, NASA has always resisted.
Yet, as I wrote in Leaving Earth, we will never be able to send humans to any other planets until we have flown at least one simulated mission, in zero gravity in Earth orbit, beforehand. Wernher von Braun pointed out this reality out back in the 1950s, and that reality has not changed in the ensuing half century. Not only will such a mission tell us a great deal about the medical issues of living in weightlessness for years at a time — issues that are far from trivial — it will give us the opportunity to find out the engineering problems of building a vessel capable of keeping humans alive during interplanetary flight, far from Earth.
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