sional surveys. Then, we added observations for which we directly performed the astrometric reduction either because we directly performed the observa- tions or the original observers provided us with their astrometric images. This process allowed a more stable orbit update sequence as new data were available. 

Nongravitational accelerations often are the limiting factor in terms of comet ephemeris accuracy. To constrain the nongravitational perturbations acting on C/2013 A1 we planned observations from the Mars Reconnaissance Orbiter HiRISE camera twelve days before the Mars encounter. Thanks to the favorable geometry, these observations were decisive to secure the trajec- tory of C/2013 A1 and to provide accurate ephemerides for the encounter. In particular, the HiRISE observations clearly revealed a significant out-of- plane nongravitational acceleration thus showing how the common practice of restricting to in-plane perturbations can lead to inaccurate predictions. 

Though we only have astrometric data for a single revolution, the good observational coverage and the strong leverage provided by the HiRISE obser- vations acquired as the comet flew by Mars showed that the classical Marsden et al. (1973) model for nongravitational perturbations is not accurate enough to model the trajectory of C/2013 A1 over the full observed arc. We there- fore resorted to the Rotating Jet Model (RJM), which is a higher fidelity model that allows us to nicely fit the full observed arc thanks to the action of two jets. Moreover, the RJM provides an estimate of the spin pole of C/2013 A1. Though the reliability of this estimate strongly depends on the 

