The USA wants NASA to develop a prototype model of the habitat using $55 million until 2018.
The Orion spacecraft that will drive astronauts to Mars has a diameter that’s about the length of a pickup truck but it is not enough space for six months long journey. Mars-bound astronauts will need a larger place to live, complete with private quarters and exercise equipment. A test version of the habitat is expected to be deployed on the International Space Station very soon.
At this point, NASA has pretty much a blank slate as to what the habitat would look like and how it would function and how it would shield astronauts from space radiation.