Ground-based observations in addition to observations by space missions can be a very efficient way to get complementary data, to go beyond the primitive objectives of this mission and actually to increase the scientific return of the mission. This strategy is used in the frame of several missions, for example when a ground-based network is required to get a large coverage of observation, for a fast verification of an event or its follow-up with instruments unavailable on board. For example, the Corot space observatory for asterosismology and detection and study of exoplanets, or the future Swom space mission for the detection and study of Gamma Ray Bursts, apply this strategy. The space-ground synergy is also an efficient strategy for the Gaia mission and in particular for the Solar System Object science (Thuillot et al., 2010). Several ground-based activities are being organized in the frame of the Gaia mission: the GBOT (Ground based Optical Tracking of Gaia, see Altmann et al. ibid) is a specific observing program of the probe itself; observing sites are also foreseen in a Science Alert Network for various science alerts mainly dedicated to astrophysical objects, photometry and spectroscopy; the Gaia-FUN-SSO program, presented here, is focused on an astrometric follow-up of specific Solar System objects.