High precision comet trajectory estimates: the Mars flyby of C/2013 A1 (Siding Spring) D. Farnocchia $^{a}$, S. R. Chesley$^{a}$, M. Micheli$^{b,c,d}$, A. Delamere$^{e}$, R. S. Heyd$^{f}$, D. J. Tholen $^{g}$, J. D. Giorgini$^{a}$, W. M. Owen$^{a}$, L. K. Tampparia $^{a}$Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA $^{b}$ESA NEO Coordination Centre, 00044 Frascati (RM), Italy $^{c}$SpaceDyS s.r.l., 56023 Cascina (PI), Italy $^{d}$INAF-IAPS, 00133 Roma (RM), Italy $^{e}$Delamere Support Services, Boulder, CO 80304, USA $^{f}$Planetary Image Research Laboratory, Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA $^{g}$Institute for Astronomy, University of Hawaii, Honolulu, HI 96822, USA The Mars flyby of C/2013 A1 (Siding Spring) represented a unique opportu- nity for imaging a long-period comet and resolving its nucleus and rotation period. Because of the small encounter distance and the high relative ve- locity, the goal of successfully observing C/2013 A1 from the Mars orbiting spacecrafts posed strict accuracy requirements on the comet’s ephemerides. These requirements were hard to meet, as comets are known for being highly unpredictable: astrometric observations can be significantly biased and non- gravitational perturbations affect comet trajectories. Therefore, even prior to the encounter, we remeasured a couple of hundred astrometric images ob- tained with ground-based and Earth-orbiting telescopes. We also observed Email address: Davide.Farnocchia@jpl.nasa.gov (D. Farnocchia) Preprint arXiv:1507.01980v1 [astro-ph.EP] 7 Jul 2015 Abstract