... The difference is that inertia is an inherent property of matter, while weight also depends on gravity. ... They use gravity, balancing its force on the astronaut's body against a calibrated spring or against the force of gravity on some calibrated weights. ... Using scales and the local force of gravity, one can determine gravitational mass; or else, one might dispense with gravity, like the astronauts aboard Skylab, sand measure the "inertial mass," and in principle they may differ. ...