12 2.3. HiRISE observations on 2014 October 7 The uncertainties in nongravitational perturbations were too large to make sure the comet would be inside the HiRISE field of view for the close approach observations. For instance, if the pointing was centered on solution 97 then solution 95 would be outside the field of view at closest approach. To tackle this problem, HiRISE observations from the Mars Reconnais- sance Orbiter took place on 2014 October 7. The much smaller range distance between C/2013 A1 and MRO as well as the different geometry provided made on the nongravitational parameters. On the other hand, solution 97 is more aggressive, as A$_{3}$ is set to zero. The nominal value of A$_{3}$ for solution 95 is larger than expected, e.g., the reference scenario in Farnocchia et al. (2014) corresponded to a ± 2 × 10 − 9 au/d 2 3 σ interval for A$_{3}$ . We thought that such a large value of A$_{3}$ was unlikely and could be due to still unresolved issues in the astrometry. Moreover, the A$_{3}$ detection was only marginal (1.3 σ significance). Therefore, we decided to set A$_{3}$ to zero and choose solution 97 as baseline ephemeris for the 2014 October 7 HiRISE observations described in Sec. 2.3. Table 1: Estimate of nongravitational parameters (along with 1 σ formal uncertainties) as of 2014 Oct 3. Solution 95 has all three nongravitational parameters A$_{1}$ , A$_{2}$ , and A$_{3}$ as free parameters in the fit, while solution 97 is constrained to A$_{3}$ = 0. Solution 95 97 A$_{1}$ [10 − 9 au/d $^{2}$] 18 . 8 ± 4 . 8 15 . 8 ± 3 . 5 A$_{2}$ [10 − 9 au/d $^{2}$] − 11 . 2 ± 13 . 0 4 . 4 ± 3 . 5 A$_{3}$ [10 − 9 au/d $^{2}$] − 8 . 4 ± 6 . 5 0 ± 0