It seems to me that despite all the research that has been done on the subject, there are only four things we have really learned about human health in 0G:

1. It's bad for you
2. It gets worse for you over time
3. After a while, the damage is permanent
4. While some things (like vigorous exercise) can mitigate or slow the negative effects, there is no way to prevent them completely.

I would argue that all of the evidence gathered to date supports these conclusions almost as well as the connection between smoking and cancer. Better even, in that exposure to 0G over time is guaranteed to have certain negative effects, while some life-long smokers live quite a bit longer and healthier than we would otherwise expect.

My point is, with all the evidence indicating that long term 0G exposure is bad, why is there not more research into designing ships that could simulate gravity by spinning? If a ship that could simulate 1G is impractically large, then shouldn't we figure out a way to research the health effects of various lower levels of gravity, say between .2G and .5G? If we are ever going to launch a manned mission to Mars, and I realize that this may still be 20 or 30 years in the future or more, I think that more research needs to be done in simulating gravity, and less spent on science that only continues to prove what we already know about 0G.

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have to agree with you totally there! Isn't the ISS supposed to have a centrifuged crew sleeping compartment when it's finished? (If it's ever finished...)

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Yep. I absolutely agree. This is one of the things that is so frustrating about NASA's manned space flight program--it seems so blatantly a dead-end program.

Part of the reason why so little research into centrifuges is being done is because no research into the ships on which they would be useful is being done.

In any case, the negative effects are limited enough to allow a crew to exist in zero g for a trip to Mars, which is as far as we're considering for the moment.

Nine (well, maybe six) months to Mars, and then two years on Mars. What does two years at martian gravity do to an astronaut? Do they have to exercise on Mars? What sort of exercise eqiupment fits in best on the Martian base? What does another six months of zero g do to an already deconditioned astronaut on the way back?

The first lunar missions had, as i recall, an automatic return mode--if they made no burns after the initial burn to take them to the moon they would loop around the moon and be able to eject the SM and reenter. You could plot a similar orbit for Mars--this would be roughly a two-year orbit if you didn't brake into Mars orbit. So--what happens to your astronauts if you have to take the contingency escape and they end up with two years of zero-g?

At least the first set of questions should be answered before a Mars expedition is launched. This requires building a tethered space station simulating Mars gravity, and inhabiting it for a protracted period of time (well, to be fair, you could set up a lunar base and see what the long-term effects of lunar gravity were. If they weren't too bad, you could assume Mars gravity would be ok).

The lower g ships are still have to be really big. If enough money was dumped into research it might be able to be overcome but there just isn't the funding for NASA right now from the looks of the way they are building their probes right now.

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That's something else that you'd really like a spinning station to investigate--how much gravity is enough gravity, and how rapid a spin is too rapid. Maybe it would turn out that astronauts could get used to 10 rpm after a few weeks, but would need a "desaturation" period before they'd be productive in a non-rotating reference frame.

Human spaceflight has that annoying chicken/egg problem: it is terribly expensive, but to make it cheaper one has to find cheaper technology which is possible only by gaining experience, that is doing many flights.

If a spinning station or ship is expensive, you could build at least a rotating section (remember Discovery?), where the crew would exercise from time to time under 1G. I guess that future spaceship designers will have to get used to the idea that a "spinning lounge" in the craft will be as important as other life supporting systems.