tokens: We thank F. Mignard and Ch. Ordenovich (OCA, Nice) for putting at our disposal the use of the Gaia simulator of Solar system observations. The work of TSR was carried out through the Gaia Research for European Astronomy Training (GREAT-ITN) network. He received funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement no. 264895. The work of AC was partly funded by ASI contract I/058/10/0.
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