Gaia will resolve the apparent angular sizes of a very large number of asteroids. In this way, it will produce for the first time a statistically significant and homogeneous data base of reliable and directly measured asteroid sizes. As a by-product, a new calibration of sizes obtained by means of indirect techniques, like polarimetry and radiometry, will also be obtained. We are analyzing the expected performances of Gaia in measuring asteroid sizes by means of a numerical algorithm to simulate asteroid signals produced on the Gaia astrometric focal plane. Assuming the most up-to-date instrumental parameters, we conclude that nearly all main-belt asteroids having diameters larger than 30 km will be measured with an accuracy equal to or better than 10% at least once during the mission operational lifetime.
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