rate ephemerides (Yeomans et al., 2004): these perturbations can be hard to model and estimate due to the lack of information and yet significantly affect comet trajectories. Comet ephemeris challenges associated with the Deep Space 1 mission’s encounter with Comet 81P/Borrelly in 2001 (Rayman, 2002) led to the de- velopment of a rotating jet model for nongravitational accelerations (Chesley and Yeomans, 2005), which provided far superior comet ephemeris predic- tions, albeit ex post facto. Later, the ephemeris development efforts for subsequent NASA comet flyby missions, such as Deep Impact, Stardust, EPOXI and Stardust-NExT, went smoothly, in large part because of the im- provements in nongravitational acceleration modeling (see, e.g., Chesley and Yeomans, 2006). As we shall see, the technical approaches developed during these five comet flyby missions were invaluable in deriving an accurate ephemeris for the Siding Spring encounter of Mars. A persistent challenge in such cases revolves around understanding how astrometric bias affects the measured nucleus position due to the effects of an asymmetric coma. Complicating the situation is the possible presence of nongravitational accelerations that could be affecting the trajectory in ways that are difficult to distinguish from astrometric bias. Separating these two sources of error, nongravitational ac- celerations and astrometric bias can be helped by carefully examining fits and predictions associated with subsets of the astrometric data set and by taking special care in the astrometric reduction to measure the peak of the 4