We investigate the potential of the upcoming LOBSTER space observatory (due circa 2009) to detect soft X-ray flashes from shock breakout in supernovae, primarily from Type II events. LOBSTER should discover many SN breakout flashes, although the number is sensitive to the uncertain distribution of extragalactic gas columns. X-ray data will constrain the radii of their progenitor stars far more tightly than can be accomplished with optical observations of the SN light curve. We anticipate the appearance of blue supergiant explosions (SN 1987A analogs), which will uncover a population of these underluminous events. We consider also how the mass, explosion energy, and absorbing column can be constrained from X-ray observables alone and with the assistance of optically-determined distances. These conclusions are drawn using known scaling relations to extrapolate, from previous numerical calculations, the LOBSTER response to explosions with a broad range of parameters. We comment on a small population of flashes with 0.2 < z < 0.8 that should exist as transient background events in XMM, Chandra, and ROSAT integrations.
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