Marc Kaufman of Washington Post finds that in zero g, "Microbes May Threaten Lengthy Spaceflights[;] Immune Systems Of Astronauts Found To Be Weakened". I guess the whole spinning-like-a-carnival-ride thing is far too obvious a solution even if the problem is not worthy of a "crash program or anything like that".

Why not consider G. Harry Stine's Pilgrim Observer: http://culttvman.com/andreas_haase_s_pilgrim_one_ob_0.html
that spins AND has a NERVA engine so the trip doesn't take that long?

Because NERVA uses a nuclear reactor, and "nuclear" = "evil"! Plus, we wouldn't want to contaminate the ecosystem of outer space...there might be endangered organisms out there...

The SARJ on the ISS is the largest rotary joint in space that I'm aware of. It's probably a good thing one of them broke down now so we can learn more about the behavior of such things. A spacecraft with a centrifuge will need a joint like that.

You don't NEED a rotary joint; Gemini played with a tether: http://www.transterrestrial.com/archives/006438.html
http://www.spaceimages.com/s66-63520.html

True. Spinning end over end with the habitat reeled out on a tether just makes too much sense to me. The mass of a steel cable/s is small compared to the mass of the associated equipment for a rotating section. And you can get a very large radius to eliminate some of the coriolis disorientation.

And maneuvering while the spacecraft is spinning is not impossible either. It just takes a software package to adjust to the rotating reference frame, IMO.

I typed up a long post "yesterday" before sleep but didn't post it, however realize that the idea behind a design like Pilgrim One is to combine both rotational (acceleration) gravity and linear acceleration gravity - the only two artificial gravity methods that seem practical with our current knowledge.

For most of its flight Pilgrim One won't be rotating nor will the "spokes" be extended as they (and the rest of the craft) will experience 1 g purely from the acceleration due to the main propulsion system and that 1 g will be parallel to the main axis of the spacecraft.

So if one wants to make use of tethers in combination with the Pilgrim One one will have to reel them in as the craft approaches 1 g linear acceleration. Might be worth it; if one deems 3 rpm acceptable one would only need a radius of roughly 100 meters, i.e. tethers less than 100 meters long, to achieve 1g rotational. For the more conservative 2 rpm the radius required jumps up to over 223 meters. Access to and from the spokes would be more complicated but it might be worth it to have a 1 g environment for tip of the spokes regardless of whether the vehicle is accelerating/decelerating or not. If there's a failure to reel in any of the tethers a design like Pilgrim One would be in serious trouble either losing parts of its structure or be prevented from traveling at the speed intended.

Experience so far indicates that tethers behave unexpectedly in vacuum, I hope they'll work out but it might be a lot harder than one would think and that goes even more so for long tethers. The problems are as badly understood as the reason why bearings, rotary joints, and gears have difficulties working as expected in space.

Luckily both tethers and rotating joints/seals can be avoided and even for the Pilgrim One design the permanent rotating joints/seals can be replaced with combinations of temporarily interconnecting non-rotating seals (that's where my long post started so I better stop now ^_^).

That should read "for [the] tip of the spokes" - always a mistake somewhere ;>_>

"For most of its flight Pilgrim One won't be rotating nor will the "spokes" be extended as they (and the rest of the craft) will experience 1 g purely from the acceleration due to the main propulsion system and that 1 g will be parallel to the main axis of the spacecraft."

Unless you have some magic propellantless drive, or a bussard ramjet or something, I doubt we'll be able to manage 1g acceleration for more than a minute or two on any interplanetary trip.

For manned mars missions, we're likely to do short burns with some sort of nuclear thermal or chemical engine.

I suppose if you wanted to do small amounts of thrust (VASIMR type propulsion) over a long period of time, you could have the engine ride down the cables to the center of mass and thrust there.

A 1g tether system sounds much more complicated than a carnival ride. More like an elevator. I think that spinning at Mars gravity for most of a Mars mission might be good: not too much force on the tether, not too much extra mass, a good analog to the work site and less of the coriolis disorientation.

If the spacecraft is carnival-ride shaped, the spacecraft can be the centrifuge. If the spacecraft is spinning faster than the typical barbecue roll, that can work. Get those microbes onto surfaces where they can be disinfected.

I suspect the suppressed immune systems of astronauts is unrelated to zero-G. Weeks on end of microbial monotony not challenging the immune system is my leading suspect. But artificial gravity would be useful to combat other ailments.

What about UV defiency and lack of Vitamin D production?

Does the ISS use full-spectrum lighting?