  Mass ratios of widely separated, long-period, resolved binary stars can be
directly estimated from the available data in major space astrometry catalogs,
such as the ESA's Hipparcos and Gaia mission results. The method is based on
the universal principle of inertial motion of the system's center of mass in
the absence of external forces, and is independent of any assumptions about the
physical parameters or stellar models. The application is limited by the
precision of input astrometric data, the orbital period and distance to the
system, and possible presence of other attractors in the vicinity, such as in
triple systems. A generalization of this technique to triples is proposed, as
well as approaches to estimation of uncertainties. The known long-period binary
HIP 473 AB is discussed as an application example, for which a
$m_2/m_1=0.996^{+0.026}_{-0.026}$ is obtained.
