"Absence of gravity results in the loss of many familiar kinesthetic cues of weight and friction necessary to man for object discrimination and manipulation. Man's ability to discriminate small differences in mass as opposed to small differences in weight was studied. Four weight series were used, each consisting of a standard (1000, 3000, 5000, or 7000 grams) and nine comparison stimuli. Judgments for mass differences were made with the same weights supported by compressed air on an air-bearing table. Thus, the frictionless aspect of a weightless environment was simulated. Results show that the mean difference limen, mean standard deviation, and Weber ratio (∆ S/S) for each standard are much larger for mass than for weight. Thus, to be detected under a weightless condition, mass increments must be at least twice as large as the weight increments required for discrimination in a normal weight-lifting situation. "--Abstract.