There's all sorts of concerns after a year in space.

http://www.theverge.com/2016/3/1/11138102/scott-kelly-year-i...

I guess they aren't well characterized yet.

Risks are relative.

Early travelers will face very significant risks. Radiation and zero g are indeed risks, but a minor compared to the other risks.

Risks are relative. is a very different calculation than People have lived in zero g for a year at a time, without significant health impact.

Different, but both true.

If you have a 5% of dying in transit or at Mars due to equipment failure, as well as a 2% increased chance of getting cancer at some time in the future, the first obviously is the one to worry about.

It's going to be dangerous enough on Mars, without adding those factors in. Worse, it's going to be difficult if the person who dies is in a critical position. You would have some redundancy in personnel, but that's costly in terms of Delta-v too, for them, and for everything it takes to keep them alive.

The effects are really pretty bad after a year and significant even after 6 months with exercise. That's why it was so important to get the transit time down to 3 months.

I've always wondered why there haven't been any efforts to generate artificial gravity via spinning spacecraft ala 2001.

Have there ever been any serious plans or proposals to do that?

Nautilus-X was pretty serious.

You really want a pretty large diameter so that you don't have a huge gravity gradient causing your astronauts discomfort. Tethers might be more practical than the classical doughnut designs for the near future.

JAXA recently brought up a small wheel into the ISS that can be used to simulate varying levels of spin on mice. Research into .4G gravity is particularly important if we're going to be sending people to Mars, but in general if lower levels of gravity keep mice and people happy those'll be easier to generate on future spacecraft.