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The possible role of ground-based support for a better determination of asteroid physical parameters based on Gaia data

The determination of bulk physical properties for the largest possible number of asteroids which will be observed by Gaia is an important task which can be made easier by performing some dedicated groundbased observing campaigns. The proposed actions, consisting of applications of different observing techniques, primarily photometry and polarimetry, are not expected to be triggered by Gaia alarms, nor do they require in principle prompt reaction times triggered by special circumstances. We deal here, conversely, with the problem of improving the amount and quality of data at our disposal concerning some properties of a suitable sample of asteroids, to be used for the purposes of a better calibration of some fundamental relations which are commonly used in asteroid science. We remind that the excellent astrometric, photometric and spectrophotometric performances of Gaia are expected to make it possible to obtain direct size measurements of main belt asteroids larger than about 25 km, rotation properties and overall shapes for a number of the order of 10 objects, and reflectance spectra and taxonomic classification for a number of asteroids which could be 10 times larger. In addition to this, we will have also a formidable improvement in the knowledge of asteroid orbital parameters, and the derivation of mass estimates for a number expected to be of the order of 100 individual asteroids, without taking into account the masses of binary systems derived by determination of their mutual orbital periods. For a small number of near-Earth asteroids, moreover, it is expected that a measurement of the drift in orbital semi-major axis due to the Yarkovsky effect might also be measurable. Two basic parameters, however, namely the albedo and absolute magnitude, will be difficult to determine based on Gaia data, due to reasons essentially related to the fact that Gaia will never observe asteroids close to solar opposition. The situation, however, is not totally hopeless, and there are possible ways to determine, at least, the albedo. The albedo quantifies the surface reflectance at visible wavelengths, and is a very important physical parameter, being determined by surface composition, roughness and texture, and by the overall thermal history of the body. In this paper we explain why some dedicated groundbased observing campaigns are strongly recommended in order to improve our capability to derive from Gaia data reasonable estimates of the albedo, if not also of the absolute magnitude, for many objects.