  Forthcoming astrometric missions such as the Gaia satellite will bring to the
fore the problem of estimating the enclosed mass from a set of positions,
radial velocities and proper motions of tracer stars. Here, we show how to
construct the tracer mass estimator when the proper motion data are available
in addition to the usual line-of-sight velocity data. Notably, the mass
estimators do not require any assumption on the anisotropy, as it is naturally
incorporated through the different components of proper motions. In addition,
the separate treatment of the proper motions and the line-of-sight velocities
is desirable because they are observationally independent and thus the
propagation of the combined uncertainties is rather straightforward. The
extension to projected data is also sketched, together with a possible
application of measuring the masses of Galactic globular clusters.
