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: 
"Artificial" gravity is the name given to techniques to create acceleration that mimics gravitational force. There are two major ways to do this -- both of which are very feasible:

1. Rotation -- in this case, the acceleration is created by [centripetal force](https://en.wikipedia.org/wiki/Centripetal_force). The rotating structure accelerates the crew by forcing them to follow a curved (usually circular) path. This is usually depicted as a rotating torus, but is probably easier to do with a crew compartment tethered to a counterweight spinning about their combined center of gravity.
2. Continuous acceleration -- in this case, the vehicle is undergoing constant thrust, which accelerates the crew and gives the impression of gravity. This is probably only practical for very long (interstellar?) missions and in fact is probably a beneficial side effect of the propulsion required.

Both of these methods probably improve the long term environment for the crew, but do add significant complexities to the spacecraft.
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.