Search

LaRa is a radio-science instrument that is ready to fly and was part of the science payload of the surface platform of the ExoMars mission. Now that ExoMars is suspended, LaRa will need to find another mission to go to the Martian system (or elsewhere in the inner Solar System). The LaRa instrument consists of an X-band transponder accompanied by a set of three antennas and three RF cables. This instrument is designed to measure with a couple of mHz of accuracy the Doppler shift induced by the variations in Mars rotation and orientation on a direct-to-Earth round-trip radio link. The main scientific objective of the LaRa experiment is to constrain the interior structure of Mars by precisely measuring the precession and nutations of the planet’s spin axis. LaRa’s measurements can also be used to refine the rotation model of Mars, including the polar motion, and infer constraints on the dynamic of its atmosphere. The LaRa Instrument The radio-science experiment enabled by LaRa provides two-way Doppler measurements corresponding to the line-of-sight velocity variations between the lander on Mars and the Earth tracking stations. Here is a description of the LaRa instrument (see detailed description of LaRa in Dehant et al. (2020)): LaRa consists in a transponder box and 3 antennas that are interconnected via 3 coaxial cables. The electronic transponder box dimensions are 25x8x8 cm This electronic box weights 1.5 kg while the total weight of LaRa is 2.15 kg. LaRa instrument uses at most 42 W (nominal power provided by the platform) to produce a radio frequency output power of about 5 W. LaRa works at X-band (channel 24) in a two-way mode (because not equipped with an ultra-stable-oscillator). LaRa’s bandwidth for the uplink is center on 7173.871143MHz, and that of the downlink is centered on 8428.580248 MHz. The coherent transponder can lock and maintain the lock for more than one hour on an uplink signal with a level ≥ -140 dBm LaRa uses one antenna (Rx) for receiving the signal transmitted by the Earth and two antennas (to ensure redundancy of the SSPA just prior to the transmitting antennas (Tx) in the transponder output chain) for retransmitting the signal back to Earth (see detailed description of LaRa's antennas in Karki et al. (2019)). The antennas radiate conical patterns with a maximum gain of about 5 dBi (antenna gain in dB w.r.t. an isotropic radiator) with a main lobe in the [30º, 55º] range of elevation and with a right-hand circular polarization. The frequency stability ensured by the coherent detector inside the electric transponder box is characterized by an Allan deviation lower than 10-13 at 60s integration time. The resulting thermal noise of LaRa is of the order of 10-2 mm/s at 60-s for LaRa. The accuracy of LaRa’s data is expected to be better than 2.8mHz at 60-s (i.e. The LaRa instrument has been designed to operate over at least one Martian year (687 Earth days) LaRa Technological Readiness Level is TRL 9 LaRa Flight model is currently installed on the Russian surface platform located in TAS-I, but LaRa will certainly be soon disassembled from the platform and repatriated to Belgium, where it will be stored and awaiting a new host. Scientific objectives of LaRa The main objectives of LaRa are (see details in Dehant et al. (2020)): Help positioning precisely and fast the lander, right after landing on the planet (see detailed description of the method in Le Maistre 2016) Measure variations in rotation and orientation of the planet, which gives information about deep interior and atmosphere dynamics Could be used to probe planet’s atmosphere if any and interplanetary medium Designed to establish a two-way radio link between Mars and the Earth, LaRa would be perfectly suitable for missions landing on Phobos, Deimos or for any mission that would visit a body of the inner Solar System. Indeed, if 70m-dish deep space stations are always available and used to track LaRa, one could in theory use LaRa to explore asteroids of the main belt for instance. Acknowledgements This work was financially supported by the Belgian PRODEX program managed by the European Space Agency in collaboration with the Belgian Federal Science Policy Office. References [1] V. Dehant, S. Le Maistre, R.-M. Baland, Ö. Karatekin, M.-J. Péters, A. Rivoldini, T. van Hoolst, M. Yseboodt, M. Mitrovic, the LaRa team, The radioscience LaRa instrument onboard ExoMars 2020 to investigate the rotation and interior of Mars. Planetary and Space Science, 180, 104776 (2020). [2] Karki, S., Sabbadini, M., Alkhalifeh, K., Craeye, C., 2019. Metallic monopole parasitic antenna with circularly polarized conical patterns. IEEE Trans. Antennas Propag. 67 (8), 5243–5252. [3] Le Maistre S., InSight coordinates determination from direct-to-Earth radio-tracking and Mars topography model. Planetary and Space Science, 121, 1-9, 2016. [4] S. Le Maistre, M.-J. Péters, J.-C. Marty, and V. Dehant. On the impact of the operational and technical characteristics of the LaRa experiment on the determination of Mars’ nutation. Planetary and Space Science, 180:104766, 2020.

The ideal conditions for RS measurements require an optimization of the communications and tracking systems, the spacecraft trajectories and mission operations for RS investigation. However, these ideal conditions are rarely achieved because of the several trades off that should be made with many other mission and science requirements. This is particularly true for missions in the Neptune system where the identified scientific goals are broad and varied. The exploration of Triton has been always identified as a key science goal for such a mission. In fact, Neptune’s largest moon Triton is presumably one of the ocean worlds in the outer solar system. With its own atmosphere, active geology, plumes of nitrogen gas and dust entrained deposited up to 150 km downwind, Triton intrigues planetary scientists, who proclaimed it as key ‘ocean world’ for future exploration [1]. According to space agencies’ long term plans (e.g. the US decadal surveys in planetary science, the ESA Cosmic Vision 2 and Voyage 2050 programs), Triton will certainly be visited by the end of the 21st century as also recommended by the radio-science (RS) community which declared Triton’s liquid layer investigation a “key RS goal” [2]. As a result of the general enthusiasm for Triton and similar icy moons of the outer solar system, several mission concepts have recently been proposed, all within the 2029–2035 time frame (e.g. [3-8]). According to the mission designs proposed in literature, most of the time Neptune is identified as the main target, but fly-bys of Triton will provide in-situ measurements whether there will be just one as Voyager 2 did and the TRIDENT team proposed [9], or multiple fly-bys by leveraging particular Neptune centric orbit (periodic and quasi-synchronous orbits) as Cassini did in the Saturn system or the as the “Triton tour” proposed by the Neptune Odyssey mission concept [10]. We focus on gravity science, i.e. the study of gravitational field with the purpose of inferring planetary properties such as bulk mass, mass/density distribution, internal density gradients, and interior structures and composition. In fact, radio tracking observations of changes in the velocity vectors of a spacecraft as it flies near Triton can be used to define the gravitational field. We perform a sensitivity study in order to asses the expected precision of the gravity recovery of a combination of different onboard instrumentation, trajectory geometries and radio science experiment configurations. In particular, we perform a covariance analysis in order to determine how the precision in the estimated parameter depend upon the various characteristics of an RS campaign, including: Orbit geometry Measurement accuracy/data quality (Doppler noise) Nongravitational forces Arc length Mission duration As an example of the proposed discussion, in Fig.1 we investigated the impact of different orbit geometries: we perform a covariance analysis on several fly-by geometries (see Fig.2), simulating Doppler tracking data performed with an onboard coherent X-band transponder and an High Gain Antenna (accuracy of 0.1mm/s at 60s integration time) over an 8h long arc. We modeled the gravity field the as a spherical harmonic expansion characterized by the Cl,m, Sl,m coefficients [11]. Since the spherical harmonics of Triton have never been measured, we used the Kaula power law in order to have an estimation of the power spectrum of the gravity field. Given the Doppler geometry configuration and relative position between Triton and the Earth in January 2030, we see that the C2,1 and S2,1 will be better determined if the spacecraft has an highly inclined orbit, whereas the estimation of C2,2 and S2,2 has a low uncertainty with slightly inclined orbits. Similar sensitivity analyses will be discussed over a wide range of mission scenarios, that is with different values for the aforementioned RS settings. The end goal of this study is to provide not only the optimal combination of these settings, but a complete parametric study that will help the design of a future mission to Triton. Acknowledgments This work was financially supported by the French Community of Belgium within the framework of the financing of a FRIA grant. References [1] Hendrix, A.R. et al., jan 2019. doi:10.1089/ast.2018.1955. [2] Asmar, S. et al., mar 2021. doi:10.3847/25c2cfeb.9d29ef85. [3] Arridge, C. et al. ,104:122–140, dec 2014. doi:10.1016/j.pss.2014.08.009. [4] Hofstadter, M. et al., Planetary and Space Science, 177:104680, nov 2019. doi:10.1016/j.pss.2019.06.004. [5] Masters, A. et al., Planetary and Space Science, 104:108–121, dec 2014. doi:10.1016/j.pss.2014.05.008. [6] Simon, A.A., Stern, S.A. and Hofstadter, July 2018. [7] Simon, A.A. et al., feb 2020. doi:10.1007/s11214-020-0639-1. [8] Turrini, D. et al., Planetary and Space Science, 104:93–107, dec 2014. doi:10.1016/j.pss.2014.09.005. [9] Prockter, L.M. et al., In Lunar and Planetary Science Conference, Lunar and Planetary Science Conference, page 3188, Mar. 2019. [10] Rymer, A.M. et al., sep 2021. doi:10.3847/psj/abf654. [11] Kaula, W.M. Theory of satellite geodesy. Applications of satellites to geodesy. 1966.