3. Post-encounter reconstruction 3.1. Astrometry during and after the encounter Besides valuable information on the physical properties of C/2013 A1, the HiRISE observations obtained during the close approach to Mars provide fur- ther constraints to the comet’s trajectory. The series of observations taken at closest approach were assembled in a similar fashion to those of the 2014 October 7 HiRISE observations. Scanning rates and integration times were varied to account for the relative motion of the comet and the spacecraft as well as the increasing brightness of the comet as the distance closed between the spacecraft and the comet. We could not obtain reliable astrometric mea- surements for a 20 hour period during closest approach as the scanning rates were too high to allow for accurate measurement of background stars. The second block of Table 2 shows the measured astrometric positions. Similarly to the 2014 October 7 observations, these data were weighted at 1 $^{′′}$, with the exception of the last October 18 observation, which was weighted at 2 ′′ because of larger RMS errors of the plate solution. On 2014 October 19 and 20, C/2013 A1 was observed by Hubble Space Telescope (HST, Li et al., 2014b). We extracted accurate astrometry from drizzle-corrected (Hack et al., 2013) HST frames tracked on the comet. Since the reference stars in the field were trailed, we used trail-fitting techniques to obtain proper astrometric solutions, referred to the PPMXL catalog (Roeser et al., 2010). In each image, the comet was clearly detected, and the central condensation was sharp and easy to identify, allowing high-precision astro- 19