Deimos' astrometric position from HERA/AFC images

Abstract

Introduction: The planetary defense mission HERA [Michel et al., 2022] performed a Mars swing-by on March 12th 2025. During the flyby, the Asteroid Framing Camera (AFC) captured seven images of Deimos in front of the Martian surface. These images are useful for calibration purposes but likewise suited for astrometric observations of Deimos, thus supporting the improvement of ephemerides.

The evaluation of images containing only a single planetary object requires accurate information on the spacecraft pointing or some means to deal with pointing offsets or random errors. However, with a second object in view, the spacecraft pointing can be corrected applying the knowledge of the secondary object’s 3D-position, thus improving the accuracy of the overall measurement. Here we measure the position of Deimos by using craters on the Martian surface in the background to correct the spacecraft pointing.

Methods: First the image coordinates of Deimos’ center of figure (COF) are determined. This is achieved by computing a simulated image, based on ephemerides for Deimos and HERA, a shape and a rotational model for Deimos (Thomas et al., 2000, Stark et al., 2017) and information on the AFC’s nominal pointing and camera parameters [Vincent et al., 2024]. Moreover, the photometric behavior of Deimos’ surface is modeled using the parameterless version of the Akimov disk function (Shkuratov et al., 1999; Longobardo et al., 2014).

The simulation is then used to detect Deimos in the measured image with pixel accuracy. A cut-out of this image is matched to Deimos in the measured images with sub-pixel accuracy. As the shape model was put into the simulated image at the location of the center of mass (COM), the location of Deimos’ center in the measured image is easily determined if a good match between observation and simulation can be achieved [Ziese and Willner 2018]. However, Deimos’ ephemerides provide the position of its center of mass (COM), while Deimos’ shape model is given with respect to the center of figure. Our procedure assumes that the COF and the COM coincide. For Deimos no offsets between the COF and COM are known. If they exist, they are believed to be very small.

To correct the spacecraft’s pointing, several of the depicted Mars craters are identified using the MUTED web service [Heyer et al., 2018]. The 3D-coordinates of the craters’ centers are then derived from a crater data base and are used to compute the expected line/sample coordinates of the craters. To determine the image coordinates of the crater centers in the measured image, an ellipse fit using OpenCV [Bradski, 2000] and the lmfit library [Wuttke] is used as craters are observed under oblique viewing geometry and appear elliptic. The difference between the expected and measured line/sample coordinates of the craters is computed and applied to the measured line/sample position of Deimos, thus providing corrected line/sample coordinates for the moon. The line/sample coordinates thus corrected are then converted to RA/DEC of Deimos as seen from HERA with respect to the ICRF.