A&A 614, A15 (2018) https://doi.org/10.1051/0004-6361/201731644 Astronomy & © ESO 2018 Astrophysics Mutual event observations of solar system objects by SRC on Mars Express? Analysis and release of observations R. Ziese1 and K. Willner2 1 Technische Universität Berlin, Institute of Geodesy and Geoinformation Science, Str. des 17. Juni 135, 10628 Berlin, Germany e-mail: [email protected] 2 German Aerospace Center (DLR), Institute of Planetary Research, Rutherfordstr. 2, 12489 Berlin, Germany e-mail: [email protected] Received 25 July 2017 / Accepted 21 January 2018 ABSTRACT Context. Both Martian moons, Phobos and Deimos, have been observed during several imaging campaigns by the Super Resolution Channel (SRC) on the Mars Express probe. Several tens of images are obtained during mutual event observations – when the Martian moons are both observed or together with another solar system body. These observations provide new opportunities to determine the bodies’ positions in their orbits. Aims. A method was sought to automate the observation of the positions of the imaged bodies. Within one image sequence a similarly accurate localization of the objects in all images should be possible. Methods. Shape models of Phobos and Deimos are applied to simulate the appearance of the bodies in the images. Matching the illuminated simulation against the observation provides a reliable determination of the bodies’ location within the image. To enhance the matching confidence several corrections need to be applied to the simulation to closely reconstruct the observation. Results. A list of 884 relative positions between the different objects is provided through the Centre de Données astronomiques de Strasbourg (CDS). Key words. astrometry – ephemerides – planets and satellites: individual: Phobos – planets and satellites: individual: Deimos 1. Introduction 2003 and carries the High Resolution Stereo Camera (HRSC) designed for multi-spectral mapping of the Martian surface First observed in 1877 by Asaph Hall, an astronomer of the (Jaumann et al. 2007). This camera also features the Super US Naval Observatory in Washington, the two natural Mar- Resolution Channel (SRC), a 1 K by 1 K framing camera, which tian satellites, Phobos and Deimos, can only be observed from provides high-resolution details within HRSC scenes, thus Earth during Mars oppositions, that is, every two years (Morley enabling studies of small-scale geologic features (Oberst et al. 1989). The bright appearance of Mars combined with the 2008). In contrast to other missions, Mars Express is in a highly moons’ relatively small sizes, Phobos having approximately elliptical nearly polar orbit around Mars. The periapsis height 27 × 22 × 18 km (Willner et al. 2014) diameter and Deimos of the MEX orbit is optimized for HRSC observations of Mars measuring 16 × 12 × 10 km in diameter, as well as their low to ensure surface resolutions in the range of 10 to 15 m pixel−1 orbits, 6000 km and 20 000 km above the Martian surface, make and is thus at a height of 250 km above the Martian surface Earth-based observations difficult. (Jaumann et al. 2007) with a latitudinal precession to assure In the past Phobos’ and Deimos’ positions in space were global coverage. As a consequence, the apoapsis of MEX’s orbit observed separately by several spacecraft during their approach is at a distance of about 11 000 km to the Martian surface and to Mars and at times also during flyby or far distance observa- thus beyond the orbit of Phobos. The nearly equatorial orbits of tions, for example, by Mariner 9 (Duxbury & Callahan 1989), Phobos and Deimos allow observations of the moons by MEX Viking (Duxbury & Callahan 1988), Phobos 2 (Kolyuka et al. instruments from distances of less than 5000 km and 12 000 km, 1991), MGS (Bills et al. 2005; Jacobson 2010, and references respectively, at an interval of five to six months. therein), MRO (Jacobson 2010; Jacobson & Lainey 2014) and Several different techniques have been applied to gather Mars Express (Oberst et al. 2006; Willner et al. 2008; Pasewaldt astrometric information from SRC images (see Oberst et al. et al. 2012, 2015). 2006; Willner et al. 2008; Pasewaldt et al. 2012, 2015). In Here we focus on observations by the European Mars contrast to the method that is described below, Willner et al. Express (MEX) probe, which has been in orbit about Mars since (2008) and Pasewaldt et al.(2012, 2015) used stars in the background to control the camera pointing for astrometric ? Tables A.1–A.4 are only available at the CDS via anony- observations. According to Willner et al.(2008) pointing errors mous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via of MEX mainly relate to attitude uncertainties about the X and http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/614/A15 Y axes of the camera leading to translations of the observed Article published by EDP Sciences A15, page 1 of9 A&A 614, A15 (2018) bodies’ location in the image. Compared to these translational errors, rotations about the boresight axis can be neglected (Willner 2009). The HRSC planning team has identified occasions where both moons or one of the satellites together with another plan- etary object, for example, a planet, moon, star or star field, can be observed. This kind of observation is referred to as mutual event. Usually during such occasions a sequence of several tens of images is obtained. These observations provide a unique Fig. 1. Measurements showing a) Phobos and Deimos (HD090_0032), opportunity to derive information on the relative position of the b) Phobos and Jupiter (HC151_0085) and c) Deimos together with Martian moons with respect to another distant but well-known Saturn (HC921_0030). celestial object. However, no analysis of these data has been carried out to date. The advantage of mutual event observations over observa- the background are analyzed (Fig.1). Next to these mutual event tions of only one Martian moon is that the absolute camera observations, a wealth of data exist imaging Phobos or Deimos pointing is irrelevant as long as two objects are visible and the and a star or star field. A few observations containing the Earth’s relative angular separation between the two bodies can be deter- moon and the Galilean moons exist. These bodies are, due to mined. This leads to accurate position information of Phobos their size and distance, comparable to point-like light sources or Deimos with respect to one of the other, above-mentioned and appear similar to star records in SRC images. As this kind celestial bodies. Furthermore, mutual event observations can be of observation requires a different measurement approach, their obtained more frequently than direct observations during fly- analysis will be discussed in a separate publication. bys. Distance is considered less critical in theses cases due to In total there are 1024 images showing two planetary objects the independency from the spacecraft pointing and thus obser- (cf. Table1). If either Phobos or Deimos is imaged together with vations from a wide range of MEX orbit positions are possible. the Jovian system or Saturn, then the Martian moon is designated Both features make this kind of observation an ideal input to as the primary object and the other planetary body as the sec- orbit modeling algorithms. ondary object. If Phobos and Deimos are imaged, then Phobos is Improved ephemerides for Phobos and Deimos have some considered as the primary object. immediate benefits for navigational purposes. However, besides During mutual event observations the apparent viewing from being an indispensable prerequisite for spacecraft mis- geometry between camera and primary object (e.g., Phobos or sion design, orbit planning and optical navigation, accurate Deimos) changes only on a minimal scale. Hence, initially it ephemerides also serve scientific objectives. was assumed that the perspective between observer and primary The ephemerides are derived by numerically integrating the object as well as secondary object does not change signifi- satellite orbits while simultaneously fitting these to the astromet- cantly throughout one observation series. Under this assumption, ric observations (Lainey et al. 2007; Jacobson 2010; Jacobson techniques to detect the observed features automatically can & Lainey 2014). Dynamical model parameter values are also be applied assuring consistency in the quality of observations determined during the least-squares adjustment, including the throughout one image sequence. amplitude of Phobos’ libration in longitude and Phobos’ secular However, a detailed analysis revealed that the observation acceleration (Jacobson 2010). geometry changes throughout long observation series or simply Variations in the rotational motion (i.e., librations) impose due to an unfortunate imaging geometry, for example, view- constraints on the mass distribution inside Phobos on a finer ing Phobos from an angle from which small changes of the scale (Rambaux et al. 2012). Combined with a precise knowledge observers’ position changes the image significantly. In these of the secular drift of the periapsis of Phobos’ orbit, they allow cases a segmentation of the image series was applied; see conclusions on the satellites’ second-order gravity field coeffi- Sect. 3.3. cents (Borderies & Yoder 1990; Jacobson & Lainey 2014). Varia- Other than the secondary object, which is usually completely tions in the orbital motion of the Martian moons are also a means visible in all images, the primary object enters and leaves the of studying Mars itself. Temporal accelerations can be used field of view (FOV) throughout one image series. Images show- to investigate the Martian J2 gravity coefficient, in particular ing more than approximately 15% of the primary object are its seasonal changes. Furthermore, tidal accelerations allow the evaluated (see Sect. 3.2), images showing a lesser portion of the determination of the rate of tidal dissipation (Lainey et al. 2007; primary object are excluded from further analysis.