A ∼2-meter satellite telescope with a 1-square-degree optical imager, a small near-IR imager, and a three-arm near-UV-to-near-IR spectrograph can discover over 2000 Type Ia supernovae in a year at redshifts betweenz=0.1and 1.7, and follow them with high-signal-to-noise calibrated light-curves and spectra. The resulting data set can determine the cosmological parameters with precision: mass density&OHgr;Mto ±0.02, vacuum energy density&OHgr;&Lgr;to ±0.05, and curvature&OHgr;kto ±0.06. The data set can test the nature of the “dark energy” that is apparently accelerating the expansion of the universe. In particular, a cosmological constant dark energy can be differentiated from alternatives including a range of “quintessence” dynamical scalar-field models, by measuring the ratio of the dark energy’s pressure to its density to ±0.05 over a range of redshifts. The large numbers of supernovae across a wide range of redshifts are necessary but not sufficient to accomplish these goals; the controls for systematic uncertainties are primary drivers of the design of this space-based experiment. These systematic and statistical controls cannot be obtained with other ground-based and/or space-based telescopes, either currently in construction or in planning stages. ©2000 American Institute of Physics.