  The Mars flyby of C/2013 A1 (Siding Spring) represented a unique opportunity
for imaging a long-period comet and resolving its nucleus and rotation period.
Because of the small encounter distance and the high relative velocity, 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
nongravitational perturbations affect comet trajectories. Therefore, even prior
to the encounter, we remeasured a couple of hundred astrometric images obtained
with ground-based and Earth-orbiting telescopes. We also observed the comet
with the Mars Reconnaissance Orbiter's High Resolution Imaging Science
Experiment (HiRISE) camera on 2014 October 7. In particular, these HiRISE
observations were decisive in securing the trajectory and revealed that
out-of-plane nongravitational perturbations were larger than previously
assumed. Though the resulting ephemeris predictions for the Mars encounter
allowed observations of the comet from the Mars orbiting spacecrafts,
post-encounter observations show a discrepancy with the pre-encounter
trajectory. We reconcile this discrepancy by employing the Rotating Jet Model,
which is a higher fidelity model for nongravitational perturbations and
provides an estimate of C/2013 A1's spin pole.
