gravity, is nasa's delima (they say) at the moment. they need gravity for the space travelor to keep homeostasis within their bodies. i think nasonians need to step back a moment and look at the whole picture instead of just a broken split small part of the equation. it is a very simple solution of artificial atmosphere, it can be done even with the touchy technology on board. two opposing magnets buffered by liquid and atmosphere to a minutely set point as to mimck not only pressure but gravity (gravitational pull).