people traveling into orbit. however, even with all of the human spaceflight experience gained over the past fifty years, no single completely effective countermeasure, or combination of countermeasures, exists against the negative effects of long-duration exposure to weightlessness. if a crew of astronauts were to embark on a six-month journey to mars today, the countermeasures currently employed would presumably leave them less operational after landing. many believe that physiological adaptation to mars gravity (0.38 g) and re-adaptation to earth gravity (1 g) would be enhanced by frequent exposure to artificial gravity on board the spacecraft en route to and from mars. this would require an on-board human-rated centrifuge or spacecraft rotation to produce a centrifugal force similar to gravity. this solution, while potentially effective, raises a number of operational, engineering, and physiological issues that will need to be addressed. the human physiological responses to long-duration exposure to anything other than zero-gravity or earth's gravity are unknown. research is needed to identify the minimum level, duration, and frequency of gravity level required to maintain normal cns functions, as well as the importance of a gravity gradient across the body. the complex functioning of the cns, even in the 1-g environment of earth, has not revealed all its secrets. the most basic space neuroscience questions must be answered to minimize risks and optimize crew performance during transit and planetary operations. the results of this research will certainly find other applications in medicine and biotechnology. our ability to understand how earth's gravitational environment has shaped the evolution of sensory and motor systems can give us a clearer understanding of the fundamental mechanisms of cns functions. knowledge of the effects of gravity on cns functions in humans, as well as elucidation of the basic mechanisms by which these effects occur, will be of direct benefit to understanding the impact of, and providing countermeasures for, long-term exposure of humans to the weightlessness of spaceflight and the partial gravity of moon and mars bases.