New astrometric observations of Phobos with the SRC on Mars Express

Abstract

Methods. The measurements have been made using a newly developed technique that involves positional measurements of surface control points and verification of camera pointing by background stars. Results. The astrometric positions are in excellent agreement with currently available Phobos orbit models. However, we find remaining systematic offsets of 1.5−2.6 km such that Phobos is ahead of its predicted position along the track. Conclusions. Our observations will be a basis for further improvements in the Phobos ephemeris. The methods that we have developed will be useful for the astrometric tracking of planetary or asteroidal targets and spacecraft optical navigation in future planetary missions.

CassiniISS astrometry of the Saturnian satellites: Tethys, Dione, Rhea, Iapetus, and Phoebe 2004–2012

“…For instance, from Voyager 2 optical images, Jacobson (1991Jacobson ( , 1992 provided astrometric positions of the Neptunian and Uranian main satellites. Mars Express observations were used to reduce the positions of the Martian satellites Phobos and Deimos (Willner et al 2008, Pasewaldt et al 2012. Cassini ISS observations have also been used for astrometric reduction; Cooper et al (2006) modelled the orbits of the Jovian moons Amalthea and Thebe, Tajeddine et al (2013) provided astrometric positions of Mimas and Enceladus, Cooper et al (2014) presented astrometry of the small inner moons of Saturn, Atlas, Prometheus, Pandora, Janus, and Epimetheus, Cooper et al (2014) reduced mutual-event observations of the mid-sized icy moons of Saturn, and Desmars et al (2013) used the astrometric positions presented in this paper to model the orbit of Phoebe.…”

Astrometric observations of Phobos with the SRC on Mars Express

“…The astrometric residuals we quoted are less a measure of quality for the used data than a quantity reflecting the consistency between our observations and the particular Phobos orbit. Hence, a possible explanation for observed discrepancies may be the contradiction between our data and MEX astrometric observations that are already included in the MAR085 ephemeris (Oberst et al 2006;Willner et al 2008). Consequently, our recommendation is to use only one of the two available orbit and/or attitude data sets to reduce all MEX astrometric observations 3 .…”

“…Therefore, we maintained procedures developed in our previous analyses (Willner et al 2008;Pasewaldt et al 2012), and verified and improved the camera pointing by using background stars with accurately known coordinates in the inertial reference frame (ICRF). We used Tycho-2 catalog data (Høg et al 2000), which were corrected for proper motion, parallax, the relativistic deflection of light in the Sun's gravitational field, and stellar aberration (Kaplan et al 1989).…”

“…comm. ), and the Super Resolution Channel (SRC) on Mars Express (MEX; Oberst et al 2006;Willner et al 2008;Pasewaldt et al 2012). …”

“…Essentially, two different techniques have been used to measure image positions of the disk-resolved Martian moons: Oberst et al (2006) observed Phobos' and Deimos' center of figures (COFs) by fitting the limbs of triaxial ellipsoids to the observed limbs. Willner et al (2008) used the Phobos control point network to determine the Phobos COF in SRC images (Kolyuka et al 1991). Pasewaldt et al (2012) also applied the limb-fit method on images of Deimos, albeit based on 3D shape models.…”