tokens: We thank the anonymous referee for a constructive and helpful report, E. Schunová for her feedback on earlier versions of this manuscript, S. D. Miller for comments on his results, and P. Wiegert for sharing the details of his pre-impact orbital solution. This work was partially supported by the Spanish “Comunidad de Madrid” under grant CAM S2009/ESP-1496. Some of the calculations discussed here were completed on the “Servidor Central de Cálculo” of the Universidad Complutense de Madrid. In the preparation of this paper, we made use of the NASA Astrophysics Data System, the ASTRO-PH e-print server, the MPC data server, and the NEODyS information service.
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