The concept and objectives
This proposal, entitled European Satellite PArtnership for Computing Ephemerides (E.S.Pa.C.E.), aims at strengthening the collaborative activities in the domain of the development of ephemerides and reference systems for natural satellites (SAT) and spacecraft (SPC).
Planetary satellites are central to the understanding of the Solar System and its dynamics. Planetary satellites include among others the Earth’s Moon, the Martian moons Phobos and Deimos, as well as the numerous (> 40) members of the satellite systems of Jupiter and Saturn. Orbits and dynamics of the satellites are typically complex due to resonance and strong tidal interactions with their parent planets. Satellite exploration by spacecraft has been highlighted by the discovery of the tidally driven volcanic activity of Io, the cryogenic activity of Enceladus, and the existence of a probable ocean inside Europa.
Though the exploration of planetary satellites has a long history, further exploration and analysis of all available data must be carried out to better understand physical phenomena and the dynamic processes that have been discovered. We must enhance our capacity to explore and understand their physical structures, their origins and their evolutions. Linking celestial mechanics, dynamics and astrometry will be a powerful strategy towards this goal. Modeling the dynamics of the natural satellites and analyzing the orbit of the space probes, combined with high precision observations, allow us to probe the interiors of these celestial bodies by the determining gravitational fields and tidal dissipation coefficients. Future missions towards planetary satellites (such as the Jupiter Ganymede mission) are currently under study within ESA’s Cosmic Vision program.
While studies of planetary geodesy and dynamics in the United States enjoy the coordinated support from several powerful space research facilities, such as NASA’s JPL (Jet Propulsion Laboratory) or NASA’s GSFC (Goddard Space Flight Center), the relevant activities and the expertise in Europe are scattered over ESA (European Space Agency) and several national research institutions and universities. Our project intends to bring together the expertise of seven main European research centers involved in these domains and to initiate a European expertise network in planetary dynamics which could subsequently collaborate on other planetary projects.
An important part of the project will consist of the extraction and analysis of astrometric data from observations by spacecraft not yet applied to solutions of the dynamics. The data will be combined with ground-based astrometry in order to improve constraints on the dynamics and the physics of the objects. Several data sets previously obtained will be analyzed:
- Data related to the main satellites of Jupiter and Saturn from ground-based astrometry, will be obtained by digitizing numerous photographic plates using new state-of-the-art digitizing equipment and software. This data will be fed to the already existing data base called the Natural Satellite Data Base (NSDB).
- Space imagery and radio tracking data obtained during the Viking, Phobos, Mars Global Surveyor (MGS), Mars Odyssey (ODY), Mars Express (MEX), Mars Reconnaissance Orbiter (MRO), and Phobos-Grunt missions to Mars and the Martian satellites Phobos and Deimos will be analyzed.
- Hubble Space Telescope (HST) observations will also be analysed for recovery of astrometric data. Several natural satellites have been identified by data mining in HST images acquired for various programs.
- Microwave ranging, laser ranging and Very Long Baseline Interferometry (VLBI) observations will provide high precision data from recent experiments and will be studied to improve ephemerides of space probes and satellites.
Other External experts may be added in due course to cater for the needs of the project We will use all these data to provide improved SAT and SPC ephemerides and the determination of rotational properties of selected satellites. All these results will be made available using standard formats (VO standard, SPICE kernels) for use by space agencies, planetary scientists, and the public.
Thanks to the accurate data available, we can use the related models of the dynamics to confine the physical parameters of the studied objects at the highest level, while more and more technologically challenging experiences can be invoked (flyby of moons at very low distances, orbital maneuvers, mission scenarios, etc.). In view of the already accomplished steps in the field of space exploration, the synergy between space agencies and ephemeris institutes at a much higher level will be a key point to fulfill the challenges of the coming decade.
Regarding the observational data applicable to the modeling of the dynamics, we must emphasize the increasing impact of the radio science. Radio range and Doppler measurements of the Viking landers were made between 1976 and 1983. Unfortunately most of the Viking lander Doppler data has since been lost, since it was in the extended mission and not considered a primary science data type. Viking also pre-dated the current Planetary Data System (PDS). Two years of the Viking Doppler data were recovered from navigation team tapes, and archived along with the Mars Pathfinder radio tracking data set. Though used in the construction of ephemerides, the precision of this data was to the level of the S-band with significant perturbation from the plasma effect. The situation changed significantly with the Mars Pathfinder lander in 1997.