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Full list of authors: Enya, Keigo; Kobayashi, Masanori; Kimura, Jun; Araki, Hiroshi; Namiki, Noriyuki; Noda, Hirotomo; Kashima, Shingo; Oshigami, Shoko; Ishibashi, Ko; Yamawaki, T.; Tohara, Kazuyuki; Saito, Yoshifumi; Ozaki, Masanobu; Mizuno, Takahide; Kamata, Shunichi; Matsumoto, Koji; Sasaki, Sho; Kuramoto, Kiyoshi; Sato, Yuki; Yokozawa, Takeshi; Numata, Tsutomu; Mizumoto, Satoko; Mizuno, Hiroyuki; Nagamine, Kenta; Sawamura, Akihiko; Tanimoto, Kazuo; Imai, Hisato; Nakagawa, Hiroyuki; Kirino, Okiharu; Green, David; Fujii, Masayuki; Iwamura, Satoru; Fujishiro, Naofumi; Matsumoto, Yoshiaki; Lingenauber, Kay; Kallenbach, Reinald; Althaus, Christian; Behnke, Thomas; Binger, Jan; Daurskikh, Anna; Eisenmenger, Henri; Heer, Ulrich; Hüttig, Christian; Lara, Luisa M.; Lichopoj, Alexander; Lötzke, Horst-Georg; Lüdicke, Fabian; Michaelis, Harald; Pablo Rodriguez Garcia, Juan; Rösner, Kerstin; Stark, Alexander; Steinbrügge, Gregor; Thabaut, Pascal; Thomas, Nicolas; del Togno, Simone; Wahl, Daniel; Wendler, Belinda; Wickhusen, Kai; Willner, Konrad; Hussmann, Hauke.--This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/ by-nc-nd/4.0/)., The Jupiter Icy Moons Explorer (JUICE) is a science mission led by the European Space Agency, being developed for launch in 2023. The Ganymede Laser Altimeter (GALA) is an instrument onboard JUICE, whose main scientific goals are to understand ice tectonics based on topographic data, the subsurface structure by measuring tidal response, and small-scale roughness and albedo of the surface. In addition, from the perspective of astrobiology, it is imperative to study the subsurface ocean scientifically. The development of GALA has proceeded through an international collaboration between Germany (the lead), Japan, Switzerland, and Spain. Within this framework, the Japanese team (GALA-J) is responsible for developing three receiver modules: the Backend Optics (BEO), the Focal Plane Assembly (FPA), and the Analog Electronics Module (AEM). Like the German team, GALA-J also developed software to simulate the performance of the entire GALA system (performance model). In July 2020, the Proto-Flight Models of BEO, FPA, and AEM were delivered from Japan to Germany. This paper presents an overview of JUICE/GALA and its scientific objectives and describes the instrumentation, mainly focusing on Japan's contribution. © 2021 COSPAR., NSO/Kitt Peak data (synoptic maps of He I Å intensity) used here are produced cooperatively by NSF/NOAO, NASA/GSFC, and NOAA/SEL. Sunspot number (version 2) comes from the Sunspot Index and Long-term Solar Observations at Royal Observatory of Belgium, Brussels. All used data are courtesy to be downloaded from their web sites. The authors would like to express their deep thanks to the staffs of these web sites. This work is supported by the National Natural Science Foundation of China (11973085, 11903077, 11803086, and 11633008), the Yunnan Ten-Thousand Talents Plan (the Yunling-Scholar Project), the national project for large scale scientific facilities (2019YFA0405001), and the Chinese Academy of Sciences., With funding from the Spanish government through the Severo Ochoa Centre of Excellence accreditation SEV-2017-0709.

We use global data from the Lunar Orbiter Laser Altimeter (LOLA) to retrieve the lunar tidal Love number h2 and find h2 = 0.0387 ± 0.0025. This result is in agreement with previous estimates from laser altimetry using crossover points of LOLA profiles. The Love numbers k2 and h2 are key constraints on planetary interior models. We further develop and apply a retrieval method based on a simultaneous inversion for the topography and the tidal signal benefiting from the large volume of LOLA data. By the application to the lunar tides, we also demonstrate the potential of the method for future altimetry experiments at other planetary bodies. The results of this study are very promising with respect to the determination of Mercury’s and Ganymede’s h2 from future altimeter measurements., DLR Space Administration, International Max Planck Research School on Solar System Science at the University of Göttingen, Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659

Context. The Love number h2 describes the radial tidal displacements of Mercury’s surface and allows constraints to be set on the inner core size when combined with the potential Love number k2. Knowledge of Mercury’s inner core size is fundamental to gaining insights into the planet’s thermal evolution and dynamo working principle. The BepiColombo Laser Altimeter (BELA) is currently cruising to Mercury as part of the BepiColombo mission and once it is in orbit around Mercury, it will acquire precise measurements of the planet’s surface topography, potentially including variability that is due to tidal deformation. Aims. We use synthetic measurements acquired using BELA to assess how accurately Mercury’s tidal Love number h2 can be determined by laser altimetry. Methods. We generated realistic, synthetic BELA measurements, including instrument performance, orbit determination, as well as uncertainties in spacecraft attitude and Mercury’s libration. We then retrieved Mercury’s h2 and global topography from the synthetic data through a joint inversion. Results. Our results suggest that h2 can be determined with an absolute accuracy of ± 0.012, enabling a determination of Mercury’s inner core size to ± 150 km given the inner core is sufficiently large (>800 km). We also show that the uncertainty of h2 depends strongly on the assumed scaling behavior of the topography at small scales and on the periodic misalignment of the instrument.

BELA is the first European planetary laser altimeter and shall be launched onboard of ESA's Mercury Planetary Orbiter (MPO) as part of the BepiColombo spacecraft in October 2018. The complete development of the in- strument was done in collaboration with team members in Switzerland, Germany and Spain. The transmitter is also the first space-qualified laser system for a planetary mission built in Europe. Highly important for scientific performance is the transmitter's performance which is specified with demanding values. Thermo-optical pointing stability, alignment, pulse energy, wavelength, pulse profile and length have a direct impact on the signal quality and strength of the instrument and in consequence on the quality of science data. Furthermore for in-orbit operation the detailed knowledge of the transmitter behavior is required for the interpretation of measurement data. Therefore the transmitter, an encapsulated diode pumped pulsed Nd:YAG laser, was extensively tested at facilities at DLR in Berlin-Adlershof under various environmental conditions in all possible representative configurations. This is necessary because thermo-elastic and optical behavior are difficult to predict accurately only theoretically. Furthermore there are temperature dependent effects, e.g. for laser energy, which directly affect science measurements and can only be calibrated on-ground. An optical test bench was designed and set up for this particular task. This work describes the test bench and the measurement procedures. The measure- ment results for the Engineering Qualification Model and Flight Model are presented and discussed as well as lessons learned. The outcome of the tests shows that the BELA FM transmitter performs well with margins and BELA is expected to achieve its scientific goals.