Observability of comets with Gaia

The Gaia mission is a precious opportunity to collect a large amount of interesting data about the dynamical and physical properties of the minor bodies of our Solar System. About a quarter of million of asteroids will be detected by Gaia, but likely a number of active and non-active comets will cross the fields of view of its telescopes. Gaiaʹs 20 magnitude limit for detection represents, on one hand, a critical factor for the completeness of the sample of the observed cometary bodies. On the other hand, the particular scanning mode of the sky specifically designed for Gaia will provide a good time and sky coverage reducing the effects of the observational bias. In any case it is difficult to predict a definitive statistics of the detections of comets. In this paper we discuss the features of the signal of comets we expect when they will be detected by Gaia. In particular, we show how the dust coma has a prominent role for the final output CCD signal, according to the instrument consolidated specifications, affecting the shape and extension of the instrument point spread function in a different way respect to the case of the asteroids. These conclusions should be taken into account for the development of software for the mission data processing.