A space station
Notice what makes a space station live:
Two Soyuzes, one Progress, many docking hatches....That's how a space station lives...Also many solar panels...
If the US wants Mars, there is something it could do right now: decide that one astronaut should stay aboard the station for 18 months, to start with.
I've read rumors Peggy Whitson would stay more than 6 months, but she appears to be part of the expedition 16 crew from october 07 to march 08
Well, for Mars, there isn't only the weightlessness to deal with, but solar radiation and dust!