As the Shuttle age draws to a close, there seems to be revived discussion in the media about where manned spaceflight is headed next.  The short answer is, of course, "nowhere," but we still see enthusiastic articles about returning to the moon, or visiting Mars.  The problem is, if you look at budgets and research programs, it quickly becomes clear that nobody's really interested in either of those objectives.

For instance, if NASA were actually interested in putting people on, say, Mars, for extended periods--or on the moon or indeed anywhere but low Earth orbit--they would logically have long ago embarked on a research program to learn what the biological effects of Martian or lunar gravity are.  Instead, they've invested decades and billions into learning how humans react to zero gravity--an almost useless scientific endeavor, because the clear lesson from the start of that program was that living in freefall is a bad idea.  Conclusion:  whenever people are going to spend more than a few weeks in orbit, provide them with artificial gravity in the form of a rotating spacecraft.  There's no reason not to; the technology involved in spinning things around is not actually rocket science.

No amount of data about how the human body reacts to zero-G is going to answer the important question, which is:  how does the human body react to extended periods under fractional gravity--like the moon's 1/6 G or Mars's .38 G?  If there's a potential show-stopper to colonizing other worlds, it's going to be how our physiology responds to fractional gravity, not zero gravity.

At what gravitational level does osteoporosis start in human bones?  What's the minimum level for maintenance of cardiovascular health?  At what level do embryonic and infant development begin to suffer?  Maybe these questions can be tentatively answered from studies in zero-G, but any conclusions reached that way need to be empirically confirmed.  In other words, what manned spaceflight needs as its next step is a variable-gravity research station.  The ISS is useless for learning what we really need to know; what's needed is a very simple, rotating station whose gravity can be tuned up or down to simulate life on worlds ranging from Mercury to the moon to Mars, or Ganymede or Titan.

It's pretty clear that NASA's not interested in doing such research.  There is an opportunity here, however, for the private sector to step in.  Once Robert Bigelow's inflatable space stations come onto the market, someone could attach one to a spent booster stage and rotate the ensemble.  They could then do the necessary experiments and sell the results to NASA or, say, the Chinese, who are sure to be interested.

I'm going to add this item to my list of things to do if I had a billion dollars.    But as long as the world's space agencies lack a variable-gravity station, you can be sure they're not actually serious about establishing a human presence on our neighboring worlds.