A centrifuge aboard ISS
Yes, you're right about the centrifuge, but that would only have been used for small animals.
Humans would need to train aboard a much bigger space station (to avoid effects of the Coryolis force), with a centrifuge simulating 38% of a G, for a year and a half, between 2 six months 0G sessions, before we're sure it's ok.
And this is only about the body sustaining gravity or none, and not even about solar radiations, boredom..etc..