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tokens: We thank the referee for a thorough, probing report that has improved the manuscript. We also thank Holly Capelo, Alan W. Harris, and Anders Johansen for useful discussions. BTG and DV benefited by support by the European Union through ERC grant number 320964. SE would like to acknowledge the support of the NEOShield project funded by the European Union Seventh Framework Program (FP7/2007-2013) under grant agreement no. 282703 (NEOShield) as well as Paris Observatory's ESTERS (Environnement Spatial de la Terre : Recherche Surveillance) travel grants.