Asteroid mass determination with the GAIA mission

The ESA astrometric mission Gaia, due for launch in late 2011, will observe a very large number of asteroids (~ 500 000 brighter than V = 20) with an unprecedented positional precision (at the sub-milliarcsecond level). This precision will play an important role for the mass determination of about hundred minor planets. Presently, due primarily to their perturbations on Mars, the uncertainty in the masses of the largest asteroids is the main limiting factor in the accuracy of the solar system ephemerides. Here we present the main features of the astrometric observations of asteroids with Gaia. The high precision astrometry will enable to considerably improve the orbits of almost all observed asteroids, yielding masses of the largest from mutual approaches. As an illustration we apply the overall procedure under development to the close approaches between Ceres and smaller targets observable with GAIA and assess the expected precision on the mass of Ceres at mission completion.