Question: We know that it is possible for humans to stay physically in shape during long term space missions (see for example [Valeri Polyakov](http://en.wikipedia.org/wiki/Valeri_Polyakov) who stayed on Mir for more than 14 months for one trip). However, the challenges of manned space exploration are different and would include even longer durations.

Reducing the need of the constant physical training required by the Zero-G environment could help to make manned exploration missions more viable. This could be achieved by artificial gravity.

Is artificial gravity feasible today? Would it indeed help to make manned exploration viable?

Answer: 
Assuming a sufficient budget, a spin-habitat is a highly viable option. To maintain 1G and an acceptable ≤2 RPM rotation, one needs a radius of 223m or so.

Given human needs, a torus of 5m habitat tube at 223m median radius, with a pair of 1mm steel hull shells, is a mass of about 55 cubic meters of steel or about 300 metric tons, just for a fairly thin structural torus. Adding additional support structure should at least double that mass. That's the prohibitive expense.

[SpinCalc Online Calculator](http://www.artificial-gravity.com/sw/SpinCalc/)
In manned long-term space exploration, where your objective is interstellar, and your time frame is within a single human lifetime. Lack of simulated gravity is not the problem, rather it is the limiting factor for transit time.

Assuming sufficient propulsion (i.e. [Bussard ramjet](http://en.wikipedia.org/wiki/Bussard_ramjet)) the ship would accelerate at 1 gravity until half way to the destination, then at the halfway point, turn around and decelerate for the other half of the journey. Other then a brief period of zero gee at turn over, the content acceleration would provide "artificial gravity". 

You can only accelerate so fast without squishing your ships occupants. So being limited to a single gravity for acceleration is one of the key limiting factors in getting very far in a human lifetime.