Parallaxes give the distance, while proper-motion vectors show the fractional change with time of the cluster's angular size. The latter equals the time derivative of distance, yielding the radial velocity. An animation movie of these proper motions is available at the Hipparcos website. With the distance accurately known, one solves the problem for the radial velocity instead. A stringent application of the method, however, demands a careful consideration of the stellar dynamics in the cluster, possible cluster expansion, and other effects.
Systematic differences exist between spectroscopic radial velocities, and the true stellar motions.
Work is in progress to analyze new spectroscopic data with substantially better precision. Astrometric radial velocities will be determined not only for stars in clusters, but also for individual ones, e.g. by measuring their secular change in proper motion. The astrometry group at Lund Observatory is actively involved in the planning of such future projects.