Mercury Planetary Orbiter Research Articles

Development of a simulator of the SIMBIOSYS suite onboard the BepiColombo mission

ABSTRACT BepiColombo is the fifth cornerstone mission of the European Space Agency (ESA) dedicated to study the Mercury planet. The BepiColombo spacecraft comprises two science modules: the Mercury Planetary Orbiter (MPO) realized by ESA and the Mercury Magnetospheric Orbiter provided by the Japan Aerospace Exploration Agency. The MPO is composed by 11 instruments, including the ‘Spectrometer and Imagers for MPO BepiColombo Integrated Observatory System’ (SIMBIOSYS). The SIMBIOSYS suite includes three optical channels: a Stereoscopic Imaging Channel, a High Resolution Imaging Channel, and a Visible and near Infrared Hyperspectral Imager. SIMBIOSYS will characterize the hermean surface in terms of surface morphology, volcanism, global tectonics, and chemical composition. The aim of this work is to describe a tool for the radiometric response prediction of the three SIMBIOSYS channels. Given the spectral properties of the surface, the instrument characteristics, and the geometrical conditions of the observation, the realized SIMBIOSYS simulator is capable of estimating the expected signal and integration times for the entire mission lifetime. In the simulator the spectral radiance entering the instrument optical apertures has been modelled using a Hapke reflectance model implementing the parameters expected for the hermean surface. The instrument performances are simulated by means of calibrated optical and detectors responses. The simulator employs the SPICE (Spacecraft, Planet, Instrument, C-matrix, Environment) toolkit software, which allows us to know for each epoch the exact position of the MPO with respect to the planet surface and the Sun.

Optical design of the High Resolution Imaging Channel of SIMBIO-SYS.

This paper describes the optical design of the High Resolution Imaging Channel (HRIC), which is part of the spectrometers and imagers for the Mercury Planetary Orbiter BepiColombo Integrated Observatory SYStem (SIMBIO-SYS) suite, for imaging and spectroscopic investigation of Mercury. The optical design has been optimized to achieve the stringent scientific requirement of 5m ground sampling at 400km from the planet's surface in the harsh Mercury environment.

SIMBIO-SYS/STC stereo camera calibration: Geometrical distortion.

The STereo imaging Channel (STC) is the first push-frame stereo camera on board an European Space Agency (ESA) satellite, i.e., the ESA-Japan Aerospace eXploration Agency mission BepiColombo. It was launched in October 2018, and it will reach its target, Mercury, in 2025. The STC main aim is to provide the global three-dimensional reconstruction of the Mercury surface. STC, the stereo channel of spectrometer and imagers for Mercury Planetary Orbiter BepiColombo-Integrated Observatory System, is based on an original optical design that incorporates the advantages of a compact unique detector instrument and the convenience of a double direction acquisition system. In fact, STC operates in a push-frame imaging mode and its two optical sub-channels will converge the incoming light on a single focal plane assembly allowing to minimize mass and volume. The focal plane of the instrument is housing six different filters: two panchromatic filters in the range 600-800 nm and four broadband filters with central wavelengths in the range 420-920 nm. In this paper, the geometrical calibration of the instrument, including the optical setups used, will be described. The methods used to derive the focal lengths, the boresights, and the reference systems of the different filter models are presented, and the related distortion results are discussed. The STC off-axis configuration forced to develop a distortion map model based on the RFM (rational function model). In contrast to other existing models, which allow linear estimates, the RFM is not referred to specific lens geometry, but it is sufficiently general to model a variety of distortion types, as it will be demonstrated in this particular case.

BELA transmitter performance and pointing stability verification campaign at DLR-PF

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 instrument 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 measurement 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.

The Bepicolombo Mercury Planetary Orbiter (MPO) Solar Array Design, Major Developments and Qualification

The BepiColombo Mercury Planetary Orbiter (MPO) solar array has been designed to guarantee the operational requirements in a very severe environment, mainly characterized by high temperatures and high light intensity. Furthermore, the qualification of the MPO solar array requires ca. 5500 thermal cycles.The final configuration of the solar array is the result of a very complex system analysis aiming at reducing the solar array operative temperatures. The outcome of this was an extensive development and test effort on components which provide sufficiently high temperature resistance and a wing with unique design characteristics.The MPO solar generator is composed of one wing consisting of three panels and provides an average power output up to 1800W during the nominal 1 Earth year mission around Mercury. The wing design is characterised by temperature reduction measures. The flight wing has already passed the majority of the environmental test program.

Degradation of Solar Array Components in a Combined UV/VUV High Temperature Test Environment

BepiColombo is the joint mission of the European Space Agency (ESA) and the Japanese Aerospace Exploration Agency (JAXA) to explore the planet mercury. The European contributions, namely the mercury transfer module (MTM) and the mercury planetary orbiter (MPO), are both powered by deployable solar arrays. Many materials and technologies are at their limit under the harsh high-intensity, high-temperature (HIHT) conditions of the mission. Synergistic effects like photo fixation and photo enhanced contamination by ultra violet and vacuum ultra violet radiation (UV/VUV) on sunlit surfaces are considered to play an important role in the HIHT environment of the BepiColombo mission.A design verification test under UV/VUV conditions of sun exposed materials and technologies on component level is presented which forms part of the overall verification and qualification of the solar array design of the MTM and MPO. The test concentrates on the self-contamination aspects and the resulting performance losses of the solar array under high intensity and elevated temperature environment representative for the photovoltaic assembly (PVA).

Inorganic White Thermal-Control Coatings for Extreme Space Environments

White paints and clear anodizing are often used as thermal-control coatings. Inorganic white ceramic coating on AA2219 alloy prepared using novel Keronite plasma-electrolytic-oxidation technology was used in this study for the BepiColombo mission. The Keronite coating on a high copper-containing 2219 aluminum alloy was investigated in terms of its resistance to environmental exposures, such as corrosion, humidity, and high-temperature (400°C) UV/VUV radiation. The coating was characterized in terms of its microstructure, porosity, and adhesion, and their relations to the overall thermooptical performance and space suitability. Optical reflectivity, spectrophotometer color, solar absorptance, and hemispherical emittance have been measured and compared against that of currently used white silicone paint. The Keronite white coating has been shown to offer thermooptical characteristics acceptable as a space thermal-control material, while offering all the added benefits of 1) inorganic ceramic coating with extremely good adhesion, 2) environmentally friendly process with no toxic substances and heavy metals, 3) capability to coat shapes with complex geometries and non-line-of-sight areas, and 4) resistance to harsh space environment. The ceramic white coating has now been applied on Mercury Planetary Orbiter solar-array thermal shields.

The DEPFET detector-amplifier structure for spectroscopic imaging in astronomy and for experiments at free electron lasers

The DEPFET detector-amplifier structure possesses several unique properties which make it extremely useful as readout element in semiconductor detectors and in particular as building block of semiconductor pixel detectors. Variations of DEPFETs can be tuned to specific requirements as to be sensitive only in predetermined time intervals, to measure signal charge with sub-electron precision, dead-time-free readout and with signal compression. These devices have been shown to work in simulations and in prototypes. Recently the first two fully developed detector systems have been finished and installed in the MIXS (Mercury Image X-ray Spectrometer) instrument of the Mercury Planetary Orbiter scheduled to be launched in 2017. A further DEPFET detector system under development is the DSSC (Depfet Sensor with Signal Compression) that will be installed in one of the beam-lines of XFEL. The requirements of the two projects are rather different. While the MIXS sensors are supposed to measure precisely the energy and position of single photons down to very low energies but at moderate rates, the DSSC has to measure the number of photons arriving in each pixel within a time interval of 220ns. Here the challenge is the capability of detecting single X-ray photons in one pixel simultaneously with up to 10,000 photons in some other pixels. Device functioning has been verified with sensors produced in a research laboratory. Now process and design have been adapted to an industrial type production line, allowing additional improvements.

The BepiColombo MORE gravimetry and rotation experiments with the orbit14 software

TheBepiColombomission toMercury is an ESA/JAXAcornerstonemission, consisting of two spacecraft in orbit around Mercury addressing several scientific issues. One spacecraft is the Mercury Planetary Orbiter, with full instrumentation to perform radio science experiments. Very precise radio tracking from Earth, on-board accelerometer and optical measurements will provide large data sets. From these it will be possible to study the global gravity field of Mercury and its tidal variations, its rotation state and the orbit of its centre of mass. With the gravity field and rotation state, it is possible to constrain the internal structure of the planet. With the orbit of Mercury, it is possible to constrain relativistic theories of gravitation. In order to assess that all the scientific goals are achievable with the required level of accuracy, full cycle numerical simulations of the radio science experiment have been performed. Simulated tracking, accelerometer and optical camera data have been generated, and a long list of variables including the spacecraft initial conditions, the accelerometer calibrations and the gravity field coefficients have been determined by a least-squares fit. The simulation results are encouraging: the experiments are feasible at the required level of accuracy provided that some critical terms in the accelerometer error are moderated. We will show that BepiColombo will be able to provide at least an order of magnitude improvement in the knowledge of Love number k2, libration amplitudes and obliquity, along with a gravity field determination up to degree 25 with a signal-to-noise ratio of 10.

The pre-flight calibration setup of the instrument SIMBIO-SYS onboard the mission BepiColombo

BepiColombo, an European Space Agency (ESA) mission being conducted in cooperation with the Japan space agency, will explore Mercury with a set of eleven instruments onboard the spacecraft Mercury Planetary Orbiter (MPO). Among them, SIMBIO-SYS (Spectrometers and Imagers for MPO BepiColombo Integrated Observatory SYStem) is a complex instrument that will provide images and spectra in the 400–2000nm wavelength range of the entire surface of Mercury. Pre-flight calibration of the SYMBIO-SYS instrument is mandatory for reliable scientific interpretation of images and spectra returned from the planet Mercury. This paper presents the calibration device designed and implemented for the specific requirements of this instrument. It mainly consists of a thermal vacuum chamber simulating the space environment, an optical bench collecting calibration sources and optical elements that simulate the conditions of Mercury observations, mechanical interfaces used for positioning the three channels inside the vacuum chamber, thermal interfaces to explore the operating temperatures, computer interfaces that allow to communicate with both the instrument and the calibration elements and synchronize the calibrations sequences with the status of the calibration device. As the major goal is the characterization of the radiometric performances of the three channels of SIMBIO-SYS, radiometric performances of the test setup evaluated by simulations and measurements are emphasized.

Space-qualified laser system for the BepiColombo Laser Altimeter

The space-qualified design of a miniaturized laser for pulsed operation at a wavelength of 1064nm and at repetition rates up to 10Hz is presented. This laser consists of a pair of diode-laser pumped, actively q-switched Nd:YAG rod oscillators hermetically sealed and encapsulated in an environment of dry synthetic air. The system delivers at least 300 million laser pulses with 50mJ energy and 5ns pulse width (FWHM). It will be launched in 2017 aboard European Space Agency's Mercury Planetary Orbiter as part of the BepiColombo Laser Altimeter, which, after a 6-years cruise, will start recording topographic data from orbital altitudes between 400 and 1500km above Mercury's surface.

ELENA microchannel plate detector: absolute detection efficiency for low energy neutral atoms

Microchannel plate (MCP) detectors are frequently used in space instrumentation for detecting a wide range of radiation and particles. The capability to detect non-thermal, low energy, neutral species is crucial for the Emitted Low-Energy Neutral Atoms (ELENA) sensor, which is part of the Search for Exospheric Refilling and Emitted Natural Abundances (SERENA) package on board the Mercury Planetary Orbiter (MPO) spacecraft of the BepiColombo mission of European Space Agency to Mercury, which is scheduled for launch in August 2015. ELENA is a time-of-flight sensor based on a novel concept using an ultrasonic oscillating shutter (start section) and MCP detector (stop detector). The ELENA scientific objective is to monitor the emission of neutral atoms from the surface of Mercury by detecting energetic neutral atoms in the range 10 eV to 5 keV, within 76 deg FOV, perpendicular to the S/C orbital plane. The surface is scanned due to the spacecraft motion. In particular, processes of particle release from the surface will be investigated by identifying particles released via solar wind-induced ion sputtering (with energies >1 eV to <100 eV ) as well as energetic hydrogen atoms, which are back-scattered solar wind protons, at energies of hundreds of eV. MCP absolute detection efficiency, for very low energy neutral atoms (E<30 eV ), is a crucial point for this investigation. At Messkammer für Flugzeitinstrumente und time-of-flight facility of the University of Bern, measurements on three MCP, with different coatings, have been performed providing the first data of MCP detection efficiencies in the energy range 10 eV to 1 keV.

Optical performance of PHEBUS/EUV detector onboard BepiColombo

BepiColombo, a mission of ESA (European Space Agency) in cooperation with JAXA (Japan Aerospace Exploration Agency), will explore Mercury, the planet closest to the Sun. BepiColombo will launch in 2014 on a journey lasting up to six and a half years; the data gathering phase should occupy a one year nominal mission, with a possible extension of another year. The data which will be brought back from the orbiters will tell us about the Hermean surface, atmospheric composition, and magnetospheric dynamics; it will also contribute to understanding the history and formation of terrestrial planets. The PHEBUS (Probing of Hermean Exosphere by Ultraviolet Spectroscopy) instrument will be flown on MPO: Mercury Planetary Orbiter, one of the two BepiColombo orbiters. The main purpose of the instrument is to reveal the composition and the distribution of the exosphere of Mercury through EUV (Extreme Ultraviolet: 55–155nm) and FUV (Far Ultraviolet: 145–315nm) measurements. A consortium composed of four main countries has been formed to build it. Japan provides the two detectors (EUV and FUV), Russia implements the scanning system, and France and Italy take charge of the overall design, assembly, test, integration, and also provide two small NUV (Near Ultraviolet) detectors (for the light from calcium and potassium molecules). An optical prototype of the EUV detector which is identical to the flight configuration has been manufactured and evaluated. In this paper, we show the first spectra results observed by the EUV channel optical prototype. We also describe the design of PHEBUS and discuss the possibility of detecting noble gases in Mercury’s exosphere taking the experimental results so far into account.

Thermal analysis of a reflective baffle designed for space applications

The implementation and results from thermal mathematical modelling of a Stavroudis-type reflective baffle for the BepiColombo laser altimeter (BELA) are presented. BELA and other instruments on board the European Space Agency's Mercury Planetary Orbiter are exposed to a harsh environment in Mercury orbit. This environment is briefly discussed and the detailed design solution for the baffle is presented. Special attention has been paid to the implementation of the thermal model because specific approximations were required. The results of the thermal mathematical models show the temperature behaviour in orbit and the feasibility of the solution. The work has applications to future missions which will go inside the orbit of Venus (e.g. ESA's Solar Orbiter).

Estimates of in-flight calibration source activities for the SIXS X-ray detectors on board BepiColombo

The Solar Intensity X-ray and particle Spectrometer (SIXS) instrument on the BepiColombo Mercury Planetary Orbiter consists of three X-ray detectors and a particle detector system. The X-ray detectors must be calibrated repeatedly during the BepiColombo mission due to the variable conditions in orbit around Mercury by measuring the spectrum of a known calibration source between observations. The calibration sources of the SIXS X-ray detectors are placed inside each detector assembly. The placement, design and activity of the calibration sources must be carefully optimized since the sources are within the field of view at all times. The primary motive for the present study was to find the best design alternative for the on board calibration sources of the SIXS X-ray detectors.The calibration sources of the SIXS detectors consist of titanium coated 55Fe. Two different options for the source placement were considered and Ruby′s formula for a parallel-disk source and detector system was used, in addition to the analytical examination of the attenuation and fluorescence of X-rays inside the Ti-coating, to estimate the calibration signal for each option. Given the complexity of the source–detector geometry, approximations were required for the analytical estimations of the attenuation and fluorescence. Two different approximations were applied to each source–detector configuration. Monte-Carlo simulation program was also developed to estimate the SIXS calibration signal more accurately as well as to provide means to estimate the signal in more general source–detector systems. The Monte-Carlo simulation results of each source–detector configuration were consistent with the results of the analytical calculations. The optimal activity, dimensions and placement of the SIXS calibration sources as well as the optimal thickness of the Ti-coating were determined from the simulation results in co-operation with Oxford Instruments Analytical Ltd. On the basis of the results, the design with the radiation source above the aperture stop was adopted for the SIXS X-ray detectors.

The BepiColombo mission to Mercury: Reaction wheels desaturation manoeuvres and the ISA accelerometer Δ [formula omitted] measurements

The Mercury Planetary Orbiter will be a three-axis stabilized spacecraft and nadir pointing to Mercury center-of-mass. The pointing accuracy, needed for the very ambitious goals of the ESA space mission to Mercury denominated BepiColombo, is reached thanks to the onboard reaction wheels, and it is also required during the unobserved arcs. The unavoidable manoeuvres of desaturation of the reaction wheels, which are necessary to remove the accumulated angular momentum, represent a clear reduction of the accuracy of the objectives of the ESA space mission. Indeed, these manoeuvres are performed through the spacecraft thrusters and directly impact the accuracy of the propagated state-vector of the satellite at the beginning of the subsequent observed arc. Their impact is quantified by their number, position along the orbit and, especially, in the uncertainty in the linear momentum transferred to the spacecraft. The present paper is devoted to prove the feasibility of the speed variation measurements produced by the thruster thanks to the onboard accelerometer, ISA. Therefore, such measurements may be an essential ingredient in order to preserve the accuracy of the BepiColombo Radio Science Experiments and of other onboard instruments pointing accuracy, as is the case of BELA. This additional capability of ISA strengthens once more the key role of the accelerometer in the BepiColombo mission to Mercury.

DEPFET Macropixel Detectors for MIXS: First Electrical Qualification Measurements

The Mercury Imaging X-ray Spectrometer (MIXS) is one of the instruments on board the Mercury Planetary Orbiter of the fifth European Space Agency (ESA) cornerstone mission BepiColombo. This spectrometer comprises two instruments and allows imaging X-ray spectroscopy of the Mercurian surface. The focal plane arrays for the energy and spatial resolved detection of X-rays are based on depleted P-channel FET (DEPFET) macropixel detectors. We report on the first electrical qualification measurements of DEPFET macropixel flight hardware, which are done at room temperature. The measurement of the transistor properties of all DEPFET pixels allows the selection of 100% electrically defect-free devices at an early stage directly after completion of the production process. Additionally, these measurements allow to investigate topics such as the homogeneity of transistor parameters across the detector matrix, to monitor die-to-die variations for matrices from the same wafer, and to investigate the differences between dies from different wafers in order to learn about the stability and reproducibility of the production process. The test results demonstrate good homogeneity and yield for the investigated detector matrices from the first production batch.

The mercury imaging X-ray spectrometer (MIXS) on bepicolombo

The Mercury Imaging X-ray Spectrometer (MIXS) on the BepiColombo Mercury Planetary Orbiter (MPO) will measure fluorescent X-ray emission from the surface of Mercury in the energy range 0.5–7.5 keV, which is induced by incident solar X-rays and solar wind electrons and protons. These X-rays will reveal the elemental composition of the surface of Mercury and aid the determination of the planet's evolution. MIXS is a two component instrument. A collimated channel (MIXS-C) provides measurements on scales of 70–270 km, sufficient to separate the major Mercurian terrains. A second channel (MIXS-T) is the first imaging X-ray telescope for planetary remote sensing and will make measurements on spatial scales of less than 10 km for major elements during solar flares, sufficient to isolate surface landforms, such as craters and their internal structures. The spatial resolution achieved by MIXS-T is made possible by novel, low mass microchannel plate X-ray optics, in a Wolter type I optical geometry. MIXS measurements of surface elemental composition will help determine rock types, the evolution of the surface and ultimately a probable formation process for the planet. In this paper we present MIXS and its predicted performance at Mercury as well as discussing the role that MIXS measurements will play in answering the major questions about Mercury.

Italian Spring Accelerometer (ISA): A fundamental support to BepiColombo Radio Science Experiments

The Radio Science Experiments of the BepiColombo mission will enable substantial improvement of the knowledge of Mercury's orbit and rotation, and the relativistic dynamics in the solar system. A fundamental support to the spacecraft tracking data will be given by the Italian Spring Accelerometer (ISA). This is a three-axis accelerometer devoted to the measurement of the non-gravitational perturbations acting on the Mercury Planetary Orbiter (MPO), whose knowledge is important in order to fully exploit the quality of the tracking data. The intrinsic noise level of the instrument that will be onboard MPO, 10 - 9 m / s 2 / Hz in the 3 × 10 - 5 to 10 - 1 Hz frequency range, guarantees the fulfilment of the RSE requirements. The main scientific and technological features of the instrument are discussed, together with its current error budget, experimental activities and foreseen calibration strategies.

The Mercury Gamma and Neutron Spectrometer (MGNS) on board the Planetary Orbiter of the BepiColombo mission

The main goals and objectives are presented for the Mercury Gamma and Neutron Spectrometer (MGNS) to be flown on board the Mercury Planetary Orbiter of ESAs BepiColombo mission. The instrument design is discussed and from an analysis of its properties, it is shown that the MGNS is capable of measuring the elemental composition of the shallow subsurface, providing a sensitive tool to discriminate between a number of proposed models for Mercury's origin. The instrument will also test for the presence of water ice deposits on the permanently shadowed polar craters of the planet.