The Gaia mission, in addition of doing astrometry of stars and a 3D census of our Milky Way, has specific objectives for solar system objects (Tanga, 2011). In that respect it follows the Hipparcos/Tycho mission that already observed a few solar system objects. The astrometric accuracy involved with Gaia is however 1 to 2 orders of magnitude higher, and the total number of targets observed is incomparably larger. Here we describe several scientific outcomes from the astrometry of asteroids and comets with particular emphasis on the Near Earth Objects (NEOs), their specificities and limitations.