Preliminary geodesy study for MMX mission

In the framework of the MMX (Martian Moons Exploration) mission, a geodesy team from CNES has joined Geodesy Sub-Science Team to study the estimation of geodetic parameters of a natural satellite of Mars: Phobos.

The MMX mission aims to return a sample of Phobos to Earth, but during the mission, exceptional observations of this natural satellite will be made. What insight into the geodetic parameters of Phobos will be gained from these measurements? The available measurements will be: LIDAR measurements between the probe and the surface of Phobos, 2-way Doppler and range measurements between ground stations on Earth and the probe, as well as optical measurements from photos taken by the probe.

The presentation will focus on the results obtained on the restitution of gravity field parameters, Phobos ephemerides, as well as rotation and orientation parameters of the natural satellite, from synthetic measurements simulated on coherent QSO (Quasi Satellite Orbit) of the current mission analysis.

The LIDAR measurements correspond to a distance measurement between the surface of the body and the onboard instrument. This very accurate measurement (sigma = 22m @ 100 km) [R1] allows the trajectory to be constrained, but is dependent on the quality of the body shape model.

The 2-Way Doppler measurements contain information on the velocity of the probe in the line of sight. These measurements are available during spacecraft observation sessions by ground stations which can be multiple at a rate of one measurement per minute.

Optical measurements are angular landmark measurements on the surface of the body. Like LIDAR measurements, these contain information on the relative position of the probe with respect to the natural satellite. These measurements are derived from a pre-processing of the wide angle and narrow angle photos taken by the OROCHI and TENGOO instruments [R2].

The combination of these three types of measurements will be used to estimate the various geodetic parameters of Phobos throughout the mission. Indeed, the first and most distant orbits (QSO-H) should allow to estimate the ephemeris of Phobos as well as the low-degree coefficients of the gravity field and rotation parameters including amplitude of libration in longitude. As the mission progresses, the spacecraft will orbit Phobos with closer and closer trajectories (QSO-M, QSO-L) which will allow to refine the first estimates made at QSO-H as well as the coefficients of the higher degree field.

This knowledge of the field and the attitude of Phobos will thus allow a precise study of the internal structure of the body.

[R1]    Light detection and ranging (LIDAR) laser altimeter for the Martian Moons Exploration (MMX) spacecraft - Senshu et al. (2021) https://doi.org/10.1186/s40623-021-01537-7

[R2]    Design of telescopic nadir imager for geomorphology (TENGOO) and observation of surface refectance by optical chromatic imager (OROCHI) for the Martian Moons Exploration (MMX) - Kameda et al. (2021) https://doi.org/10.1186/s40623-021-01462-9