  We show that an interferometer moving at a relativistic speed relative to a
point source of light offers a sensitive probe of acceleration. Such an
accelerometer contains no moving parts, and is thus more robust than
conventional "mass-on-a-spring" accelerometers. In an interstellar mission to
Alpha-Centauri, such an accelerometer could be used to measure the masses of
planets around other stars as well as the mass distribution of the Milky Way
Galaxy.
