BepiColombo Mercury Planetary Orbiter Research Articles

Energetic Neutral Atom (ENA) Emission Characteristics at the Moon and Mercury From 3D Regolith Simulations of Solar Wind Reflection

AbstractThe reflection of solar wind protons as energetic neutral atoms (ENAs) from the lunar surface has regularly been used to study the plasma‐surface interaction at the Moon. However, there still exists a fundamental lack of knowledge of the scattering process. ENA emission from the surface is expected to similarly occur at Mercury and will be studied by BepiColombo. Understanding this solar wind backscattering will allow studies of both Mercury's plasma environment as well as properties of the hermean surface itself. Here, we expand on previous simulation studies of the solar‐wind‐regolith interaction with 3D grains in SDTrimSP‐3D to compare the predicted scattering energies and angles to ENA measurements from the Moon by the Chandrayaan‐1 and IBEX missions. The simulations reproduce a backward emission toward the Sun, which can be connected to the geometry of the regolith grain stacking. In contrast, the ENA energy distribution and its Maxwellian shape is mostly connected to the solar wind velocity. Our simulations also correctly describe a lunar ENA albedo between 10% and 20% and support its decrease with solar wind velocity. We further expand our studies to illustrate how BepiColombo will be able to observe ENAs at Mercury using hybrid simulations of Mercury's magnetosphere as an input for the complex surface precipitation patterns. We demonstrate that the variable ion precipitation will directly influence ENA emission from the surface. The orbits of BepiColombo's Mercury Planetary Orbiter and Mercury Magnetospheric Orbiter/Mio spacecraft are shown to be suitable to observe ENA emission patterns both on a local and a global scale.

Venus's induced magnetosphere during active solar wind conditions at BepiColombo's Venus 1 flyby

Abstract. Out of the two Venus flybys that BepiColombo uses as a gravity assist manoeuvre to finally arrive at Mercury, the first took place on 15 October 2020. After passing the bow shock, the spacecraft travelled along the induced magnetotail, crossing it mainly in the YVSO direction. In this paper, the BepiColombo Mercury Planetary Orbiter Magnetometer (MPO-MAG) data are discussed, with support from three other plasma instruments: the Planetary Ion Camera (SERENA-PICAM) of the SERENA suite, the Mercury Electron Analyser (MEA), and the BepiColombo Radiation Monitor (BERM). Behind the bow shock crossing, the magnetic field showed a draping pattern consistent with field lines connected to the interplanetary magnetic field wrapping around the planet. This flyby showed a highly active magnetotail, with e.g. strong flapping motions at a period of ∼7 min. This activity was driven by solar wind conditions. Just before this flyby, Venus's induced magnetosphere was impacted by a stealth coronal mass ejection, of which the trailing side was still interacting with it during the flyby. This flyby is a unique opportunity to study the full length and structure of the induced magnetotail of Venus, indicating that the tail was most likely still present at about 48 Venus radii.

High-intensity high-temperature testing of materials for the Bepi Colombo MPO and MTM solar arrays

The Bepi Colombo spacecraft was successfully launched on 20th October 2018 and is now on its way to Mercury. The materials and coatings on the Mercury Planetary Orbiter (MPO) and Mercury Transfer Module (MTM) solar arrays will need to withstand a prolonged extreme environment of high-intensity UV radiation at high-temperature (HIHT). The solar arrays must be gradually off-pointed at close sun distances in order to avoid over-heating, up to an angle of over 70° at 0.3 AU for the MPO. This means that some of the materials and coatings, as well as the solar cells, will always be operating reasonably close to their temperature limits. Extensive on-ground testing was performed to qualify the solar arrays for the BepiColombo mission environment and during this testing some materials and coatings were pushed beyond acceptable performance limits. In this paper we give an overview of selected test programmes related to the solar array materials, and we present results of experimental investigations which provide some scientific insight into the likely causes of the materials degradation observed, and the lessons learnt in the cases where over-testing had occurred. This included testing on bare and bonded optical solar reflectors, solar cell cover glass and adhesive, and the solar array edge shields.

Orbital evolution of the BepiColombo Mercury Planetary Orbiter (MPO) in the gravity field of Mercury

We assessed the impact of the uncertainties in the gravity field harmonic coefficients of Mercury on the orbital evolution of the Mercury Planetary Orbiter (MPO), part of the European-Japanese BepiColombo mission. We used in our simulations the most recent estimation of the gravity field model of Mercury determined from radio tracking data of the NASA spacecraft MESSENGER. Thereby we propagated the orbital evolution of MPO by means of plausible gravity fields compatible with the covariance matrix measured gravity field coefficients. We considered two scenarios: 1) the optimistic case, where the formal errors were considered plausible, i.e. scaled by 1; and 2) the conservative case where the formal errors were scaled with a factor of 3. With our simulations we could also demonstrate the importance of the correlation between the errors of the gravity field coefficients for the uncertainty in the trajectory evolution.As the altitude of the spacecraft above the surface is most important in Mercury's harsh thermal environment, we mostly focused our analysis on the evolution of the periherm altitude. The results of the MPO orbit propagation show a non-linear decrease in the periherm altitude over the simulated timeframe of 1000 days. However, the decrease rate varies significantly among the generated gravity fields. Moreover, the dispersion of the periherm altitude gets larger with increasing scale factor (optimistic to conservative scenario) and over the mission time. While the values of the orbital elements of MPO are still in an acceptable range after the first year, the periherm altitude may fall below a critical value of 200 km after 2 years in orbit about Mercury.

The Capon Method for Mercury's Magnetic Field Analysis

Characterization of Mercury's internal and external magnetic field is one of the primary goals of the magnetometer experiment on board the BepiColombo MPO (Mercury Planetary Orbiter) spacecraft. A novel data analysis tool is developed to determine the Gauss coefficients in the multipole expansion using Capon's minimum variance projection method. The construction of the estimator is presented along with a test against the numerical simulation data of Mercury's magnetosphere and a comparison with the least square fitting method shows, that Capon's estimator is in better agreement with the coefficients, implemented in the simulation, than the least square fit estimator.

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.

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 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.

Solar Intensity X-ray and particle Spectrometer (SIXS)

The Solar Intensity X-ray and particle Spectrometer (SIXS) on the BepiColombo Mercury Planetary Orbiter (MPO) will investigate the direct solar X-rays, and energetic protons and electrons which pass the Spacecraft on their way to the surface of Mercury. These measurements are vitally important for understanding quantitatively the processes that make Mercury's surface glow in X-rays, since all X-rays from Mercury are due to interactions of the surface with incoming highly energetic photons and space particles. The X-ray emission of Mercury's surface will be analysed to understand its structure and composition. SIXS data will also be utilised for studies of the solar X-ray corona, flares, solar energetic particles, and the magnetosphere of Mercury, and for providing information on solar eruptions to other BepiColombo instruments. SIXS consists of two detector subsystems. The X-ray detector system includes three identical GaAs PIN detectors which measure the solar spectrum at 1–20 keV energy range, and their combined field-of-view covers ∼1/4 of the whole sky. The particle detector system consists of an assembly including a cubic central CsI(Tl) scintillator detector with five of its six surfaces covered by a thin Si detector, which together perform low-resolution particle spectroscopy with a rough angular resolution over a field-of-view covering ∼1/4 of the whole sky. The energy range of detected particle spectra is 0.1–3 MeV for electrons and 1–30 MeV for protons. A major task for the SIXS instrument is the measurement of solar X-rays on the dayside of Mercury's surface to enable modeling of X-ray fluorescence and scattering on the planet's surface. Since highly energetic particles are expected to also induce a significant amount of X-ray emission via particle-induced X-ray emission (PIXE) and bremsstrahlung when they are absorbed by the solid surface of the planet Mercury, SIXS performs measurements of fluxes and spectra of protons and electrons. SIXS performs particle measurement at all orbital phases of the MPO as the particle radiation can occur also on the night side of Mercury. The energy ranges, resolutions, and timings of X-ray and particle measurements by SIXS have been adjusted to match with the requirements for interpretation of data from Mercury's surface, to be performed by utilising the data of the Mercury Imaging X-ray Spectrometer (MIXS), which will measure X-ray emission from the surface.

The Mercury Radiometer and Thermal Infrared Spectrometer (MERTIS) for the BepiColombo mission

Scheduled for launch on board the BepiColombo Mercury Planetary Orbiter (MPO) in 2014, the Mercury Radiometer and Thermal Infrared Spectrometer (MERTIS) is an innovative instrument for studying the surface composition and mineralogy of planet Mercury. MERTIS combines an uncooled grating push broom IR-spectrometer (TIS) with a radiometer (TIR), which will operate in the wavelength region of 7–14 and 7–40 μm, respectively. The spatial resolution of the MERTIS observations will be about 500 m globally and better than 500 m for approximately 5–10% of the surface. The thermal infrared range offers unique diagnostic capabilities to study the surface composition of Mercury. In particular, feldspars can easily be detected and characterized, because they show several diagnostic spectral signatures in the 7–14 μm range: the Christiansen feature, reststrahlen bands, and the transparency feature. In addition, MERTIS will allow the identification and mapping of elemental sulfur, pyroxenes, olivines, and other complex minerals. The scientific objectives of MERTIS include: (1) characterization of Mercury's surface composition, (2) identification of rock-forming minerals, (3) mapping of the surface mineralogy, and (4) study of surface temperature variations and the thermal inertia. In preparation for the MERTIS data interpretation, we are performing spectral measurements of appropriate analogue materials in the Planetary Emissivity Laboratory (PEL) and are building a spectral library (Berlin Emissivity Database (BED)) of these materials for a variety of grain sizes.

The BepiColombo Laser Altimeter (BELA): Concept and baseline design

The BepiColombo Laser Altimeter (BELA) has been selected for flight on board the European Space Agency's BepiColombo Mercury Planetary Orbiter (MPO). The experiment is intended to be Europe's first planetary laser altimeter system. Although the proposed system has similarities to the Mercury Laser Altimeter (MLA) currently flying on board NASA's MESSENGER mission to Mercury, the specific orbit and construction of the MPO force the use of novel concepts for BELA. Furthermore, the base-lined range-finding approach is novel. In this paper, we describe the BELA system and show preliminary results from some prototype testing.

Future gravitational physics tests from ranging to the BepiColombo Mercury planetary orbiter

Milani et al. recently have published careful and fundamental studies of the accuracy with which both gravitational physics information and the solar quadrupole moment can be obtained from Earth-Mercury distance data. To complement these results, a quite different analysis method is used in the present paper. We calculate the first-order corrections to the Keplerian motion of a single planet around the Sun due to the parameterized post-Newtonian theory parameters {beta}, {gamma}, {alpha}{sub 1}, {alpha}{sub 2}, and {xi}, as well as corrections due to the solar quadrupole moment J{sub 2} and a possible secular change in GM{sub {center_dot}}. The Nordtvedt parameter {eta} that is used in tests of the strong equivalence principle also is included in this analysis. The expected accuracies are given for 1 yr, 2 yr, and 8 yr mission durations, assuming that the planet-planet and asteroid-planet perturbations are accurately known. The ''modified worst-case'' error analysis method that we use is quite different from the usual covariance analysis method based on assumed uncorrelated random errors, plus a bias that is fixed or that changes in a prescribed way. We believe this is appropriate because systematic measurement errors are likely to be the main limitation on the accuracy of themore » results. Our final estimated uncertainties are one-third of the errors that would result if a 4.5-cm rms systematic error had the most damaging possible variation with time. We discuss the resulting uncertainties for several different subsets of orbital and relativity parameters.« less

Design and performance of the payload instrumentation of the BepiColombo Mercury planetary orbiter

High integration and miniaturization of payload suites have been identified by ESA as a possible strategy for future space missions. Driving argument for this effort is the wish for the future exploration of more solar system bodies in a relatively short time period. Standardizing the payload of missions may contribute to the flexibility of the mission, whereas the reduction of mass and volume could significantly reduce the all over costs. This approach was followed throughout the re-assessment of the payload of the Mercury Planetary Orbiter (MPO) for the BepiColombo mission. The exploration of Mercury imposes several challenges on the mission due to Mercury's environment: The orbiter payload instruments have to face a rather high thermal heat load and high radiation dose levels. Mass savings were intended throughout the instrumentation by definition of the instruments design to a higher level, while meeting the science requirements. The instruments design and the iteration towards a higher maturity level was carried out together with the Science Advisory group of BepiColombo.

On the concepts of a highly integrated payload suite for use in future planetary missions: The example of the BepiColombo Mercury planetary orbiter

Future low resource payload concepts will need to be developed from the viewpoint of a standard integrated payload suite where resources are dramatically reduced through high levels of integration and resource sharing. The study of this approach, its gains together with its limitations was the key objective of this work. The highly compact integration of a specific payload suite was carried out during a reassessment of the technical realisation of all instruments required to form part of the BepiColombo planetary orbiter payload (MPO) for the exploration of Mercury. A study of the heritage of other instruments developed for other missions such as Mars Express and ROSETTA was the precursor to enable identification of typical resource drivers and related problems or technology requirements. Innovative technologies aboard SMART-1 or other technology demonstration reference missions were also taken into account for their potential in miniaturisation without sacrificing performance. In the specific example of the BepiColombo Mercury Planetary Orbiter (MPO) the resource reduction by a Highly Integrated Payload Suite (HIPS) was addressed. Here we give a review on the basic concept and a comparison to the classical approach.

The variability of Mercury's exosphere by particle and radiation induced surface release processes

Mercury's close orbit around the Sun, its weak intrinsic magnetic field and the absence of an atmosphere ( P surface<1×10 −8 Pa) results in a strong direct exposure of the surface to energetic ions, electrons and UV radiation. Thermal processes and particle-surface-collisions dominate the surface interaction processes leading to surface chemistry and physics, including the formation of an exosphere ( N⩽10 14 cm −2) in which gravity is the dominant force affecting the trajectories of exospheric atoms. NASA's Mariner 10 spacecraft observed the existence of H, He, and O in Mercury's exosphere. In addition, the volatile components Na, K, and Ca have been observed by ground based instrumentation in the exosphere. We study the efficiency of several particle surface release processes by calculating stopping cross-sections, sputter yields and exospheric source rates. Our study indicates surface sputter yields for Na between values of about 0.27 and 0.35 in an energy range from 500 eV up to 2 keV if Na + ions are the sputter agents, and about 0.037 and 0.082 at an energy range between 500 eV up to 2 keV when H + are the sputter agents and a surface binding energy of about 2 eV to 2.65 eV. The sputter yields for Ca are about 0.032 to 0.06 and for K atoms between 0.054 to 0.1 in the same energy range. We found a sputter yield for O atoms between 0.025 and 0.04 for a particle energy range between 500 eV up to 2 keV protons. By taking the average solar wind proton surface flux at the open magnetic field line area of about 4×10 8 cm −2 s −1 calculated by Massetti et al. (2003, Icarus, in press) the resulting average sputtering flux for O is about 0.8–1.0×10 7 cm −2 s −1 and for Na approximately 1.3–1.6×10 5 cm −2 s −1 depending on the assumed Na binding energies, regolith content, sputtering agents and solar activity. By using lunar regolith values for K we obtain a sputtering flux of about 1.0–1.4×10 4 cm −2 s −1. By taking an average open magnetic field line area of about 2.8×10 16 cm 2 modelled by Massetti et al. (2003, Icarus, in press) we derive an average surface sputter rate for Na of about 4.2×10 21 s −1 and for O of about 2.5×10 23 s −1. The particle sputter rate for K atoms is about 3.0×10 20 s −1 assuming lunar regolith composition for K. The sputter rates depend on the particle content in the regolith and the open magnetic field line area on Mercury's surface. Further, the surface layer could be depleted in alkali. A UV model has been developed to yield the surface UV irradiance at any time and latitude over a Mercury year. Seasonal and diurnal variations are calculated, and Photon Stimulated Desorption (PSD) fluxes along Mercury's orbit are evaluated. A solar UV hotspot is created towards perihelion, with significant average PSD particle release rates and Na fluxes of about 3.0×10 6 cm −2 s −1. The average source rates for Na particles released by PSD are about 1×10 24 s −1. By using the laboratory obtained data of Madey et al. (1998, J. Geophys. Res. 103, 5873–5887) for the calculation of the PSD flux of K atoms we get fluxes in the order of about 10 4 cm −2 s −1 along Mercury's orbit. However, these values may be to high since they are based on idealized smooth surface conditions in the laboratory and do not include the roughness and porosity of Mercury's regolith. Further, the lack of an ionosphere and Mercury's small, temporally and spatially highly variable magnetosphere can result in a large and rapid increase of exospheric particles, especially Na in Mercury's exosphere. Our study suggests that the average total source rates for the exosphere from solar particle and radiation induced surface processes during quiet solar conditions may be of the same order as particles produced by micrometeoroid vaporization. We also discuss the capability of in situ measurements of Mercury's highly variable particle environment by the proposed NPA-SERENA instrument package on board ESA's BepiColombo Mercury Planetary Orbiter (MPO).

Monte-Carlo simulation of Mercury's exosphere

Because of its proximity to the Sun and its small size, Mercury has not been able to retain its atmosphere and only a thin exosphere surrounds the planet. The exospheric pressure at the planetary surface is approximately 10 −10 mbar, set by the Mariner 10 occultation experiment. The existence of gaseous species H, He, and O has been established by Mariner 10. In addition Na, K, and Ca have been observed by ground based instrumentation. Other elements are expected to be found in Mercury's exosphere since the total pressure of the known species is almost two orders of magnitude less than the exospheric pressure. It is intended to measure these exospheric particle densities in situ with an instrument on board of ESA's BepiColombo Mercury Planetary Orbiter (MPO) spacecraft. Since the expected exospheric densities are very small we developed a Monte-Carlo computer model to investigate if such a measurement is feasible along the MPO spacecraft orbit. We model energy and ejection angle distributions of the particles at the surface, with the emission process determining the actual distribution functions. Our model follows the trajectory of each particle by numerical integration until the particle hits Mercury's surface again or escapes from the calculation domain. Using a large set of these trajectories bulk parameters of the exospheric gas are derived, e.g., particle densities for various atomic and molecular species. Our study suggests that a mass spectrometric measurement is feasible and, at least at MPO's periherm, all species that are released from the surface will be observed.