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(1). 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. 1. SOLAR SYSTEM OBJECTS OBSERVATIONS Gaia is an astrometric cornerstone mission of the European Space Agency. With a launch due in late 2011, Gaia will have a much more ambitious mission than its precursor Hipparcos: obtain a "3D census" of our galaxy with astrometric, photometric and spectroscopic observations. It will pinpoint its sources with an unprecedented positional precision (at the sub-milliarcsecond level for single observation) which will allow it to observe about 500,000 asteroids (mainly main belt asteroids) brighter than V = 20. The scanning law of the satellite is very specific (Hipparcos-like) yielding observation sequences much different from the classical ground-based ones, with no observation at opposition and solar elongations symmetrically distributed around the quadratures. These elongations will range from L1 = 45 � to L2 = 135 � . There is no dedicated observation mode for these resolved and moving targets which are observed like any other stellar source, but quickly identified from their motion. Their size and motion degrades the astrometry compared to stars, but it essentially remains a function of the target brightness. The astrometric precision �� is found from simulation with GIBIS (2) and Pyxis (3) of the focal plane images for these objects. These estimates represent the precision for a single 'observation' in the along-scan direction (the astrometry in the perpendicular direction is much less precise so that the measure is essentially one dimensional).