I've actually been posting about this a bit recently on various threads. I think a permanent rotating space station with 1G artificial gravity in low earth orbi

I've actually been posting about this a bit recently on various threads. I think a permanent rotating space station with 1G artificial gravity in low earth orbit could be the next stepping stone to opening access into outer space. Facilities with gravity would enable permanent habitation without bone and muscle loss, and manufacturing could be done with existing processes. As to why we haven't built one already, the main reason is because early research indicated that such structures would have to be prohibitively large to avoid nausea (300+ meter diameter) and require thousands of tons of shielding. The Space Shuttle's lift capacity was also limited (only 25T of payload) and expensive ($450M/launch). This paper suggests that humans are more tolerant of a rotating environment than early studies led us to believe, and that shielding is unnecessary for some low-Earth orbits: http://www.nss.org/settlement/space/GlobusRotationPaper.pdf http://www.nss.org/settlement/space/GlobusRotationPaper.pdf (In particular, see pages 19-23) The figures it suggests are ~600 tons of mass, 25m to 60m radius for the station. Falcon Heavy can launch 63T for $95M, and the BFR is aiming for 150T for $7M.