Figure 6: Predicted magnitude (solid line) and declination (dashed line) as observed from the geocenter. Grayed parts correspond to solar elongation < 60 $^{◦}$. The campaign for observing C/2013 A1 from the Mars orbiting space- crafts during the 2014 October 19 encounter was challenging in terms of computing reliable comet trajectories. The small field of view of Mars Re- connaissance Orbiter’s HiRISE camera and the close encounter distance set tight requirements on the ephemeris accuracy. One of the first problems we had to face was the generally poor quality of C/2013 A1’s astrometric observations. In fact, comet astrometry is often af- fected by tailward biases as the center of light does not generally correspond to the brightness peak. To mitigate this issue, we first applied a strict filter to the astrometric data by only selecting observations coming from profes- 4. Conclusions 30 2015 2016 2017 2018 2019 2020 12 14 16 18 20 22 24 26 Year Total magnitude 0 10 20 30 40 50 60 Declination [deg]