ESA BepiColombo Mission Research Articles

Radiometric calibration of the SIMBIO-SYS STereo imaging Channel

The STereo imaging Channel (STC) is a double wide-angle camera developed to be one of the channels of the SIMBIO-SYS instrument onboard of the ESA BepiColombo mission to Mercury. STC main goal is to map in 3D the whole Mercury surface. The geometric and radiometric responses of the STC Proto Flight model have been characterized on-ground during the calibration campaign. The derived responses will be used to calibrate the STC images that will be acquired in flight. The aim is to determine the functions linking the detected signal in digital number to the radiance of the target surface in physical units. The result of the radiometric calibration consists in the determination of well-defined quantities: (1) the dark current as a function of the integration time and of the detector temperature, settled and controlled to be stable at 268 K; (2) the read out noise, which is associated with the noise signal of the read-out electronic; and (3) the fixed pattern noise, which is generated by the different response of each pixel. Once these quantities are known, the photon response and the photo-response non-uniformity, which represents the variation of the photon responsivity of a pixel in an array, can be derived. The final result of the radiometric calibration is the relation between the radiance of an accurately known and uniform source, and the digital numbers measured by the detector.

Performance evaluation of the SIMBIO-SYS Stereo Imaging Channel on board BepiColombo/ESA spacecraft

The Stereo Imaging Channel (STC) is one of the channels of the Spectrometer and Imagers for MPO BepiColombo Integrated Observatory SYStem (SIMBIO-SYS) onboard the ESA BepiColombo mission to Mercury. STC is a double wide-angle camera designed to image each portion of the Mercury surface from two different lines of sights, whose main aim is to provide panchromatic stereo-image pairs required to generate the Digital Terrain Model (DTM) reconstruction. In addition, selected surface areas will be acquired in color. This work presents the expected STC on-ground and in-flight performance describing the preliminary evaluation of some key parameters: the optical performance, the on-ground resolution and detector response, the achievable Signal to Noise Ratio (SNR) for different integration times and observation strategies and the global coverage of panchromatic filters during the entire scientific phase. The estimation of the SNR has been made using the STC radiometric model with Hapke reflectance model for Mercury surface and the SPICE toolkit software. The SPICE toolkit software with kernel for BepiColombo mission has been used also for the estimation of the on-ground pixel dimension and the global coverage all over the mission.

Measurement of BepiColombo mission medium gain antenna parameters under realistic thermal conditions

This paper is focused on explaining the radiation test in temperature performed on the Engineering and Qualification Model of the Medium Gain Antenna Radiofrequency (MGA-RFA) Assembly of ESA's BepiColombo mission. The goal of this program is to observe and study Mercury and its surroundings in a very demanding environment in terms of temperature and radiation. The MGA is an X-band two-axis steerable horn, which provides bidirectional communications between spacecraft and Earth as backup of the High Gain Antenna and also operates as primary communication link at several mission stages or conditions. The paper presents the measurement set-up for the qualification campaign of the antenna, where it was necessary to characterize the antenna in a representative thermal environment, and the results obtained from this test. Results of test up to 150°C show how gain and radiation pattern shapes are slightly affected by thermal stress, but without jeopardizing mission requirements. In addition, by analyzing correlation of this test with RF analysis in the same thermal conditions, it becomes possible to accurately extrapolate the MGA-RFA behavior up to temperatures of more than 500°C. This fact allowed the successful space qualification of this model.

The pre-launch characterization of SIMBIO-SYS/VIHI imaging spectrometer for the BepiColombo mission to Mercury. II. Spectral calibrations.

The Visible and near Infrared Hyperspectral Imager (VIHI) is the VIS-IR spectrometer with imaging capabilities aboard the ESA BepiColombo mission to Mercury. In this second paper, we report the instrument spectral characterization derived by the calibration campaign carried out before spacecraft integration. Complementary measurements concerning radiometric and linearity responses, as well as geometric performances, are described in Paper I [G. Filacchione et al., Rev. Sci. Instrum. 88, 094502 (2017)]. We have verified the VIHI spectral range, spectral dispersion, spectral response function, and spectral uniformity along the whole slit. Instrumental defects and optical aberrations due to smiling and keystone effects have been evaluated, and they are lower than the design requirement (<1/3 pixel). The instrumental response is uniform along the whole slit, while spectral dispersion is well represented by a second order curve, rather than to be constant along the spectral dimension.

A comparative study of LaBr3(Ce(3+)) and CeBr3 based gamma-ray spectrometers for planetary remote sensing applications.

The recent availability of large volume cerium bromide crystals raises the possibility of substantially improving gamma-ray spectrometer limiting flux sensitivities over current systems based on the lanthanum tri-halides, e.g., lanthanum bromide and lanthanum chloride, especially for remote sensing, low-level counting applications or any type of measurement characterized by poor signal to noise ratios. The Russian Space Research Institute has developed and manufactured a highly sensitive gamma-ray spectrometer for remote sensing observations of the planet Mercury from the Mercury Polar Orbiter (MPO), which forms part of ESA's BepiColombo mission. The Flight Model (FM) gamma-ray spectrometer is based on a 3-in. single crystal of LaBr3(Ce(3+)) produced in a separate crystal development programme specifically for this mission. During the spectrometers development, manufacturing, and qualification phases, large crystals of CeBr3 became available in a subsequent phase of the same crystal development programme. Consequently, the Flight Spare Model (FSM) gamma-ray spectrometer was retrofitted with a 3-in. CeBr3 crystal and qualified for space. Except for the crystals, the two systems are essentially identical. In this paper, we report on a comparative assessment of the two systems, in terms of their respective spectral properties, as well as their suitability for use in planetary mission with respect to radiation tolerance and their propensity for activation. We also contrast their performance with a Ge detector representative of that flown on MESSENGER and show that: (a) both LaBr3(Ce(3+)) and CeBr3 provide superior detection systems over HPGe in the context of minimally resourced spacecraft and (b) CeBr3 is a more attractive system than LaBr3(Ce(3+)) in terms of sensitivities at lower gamma fluxes. Based on the tests, the FM has now been replaced by the FSM on the BepiColombo spacecraft. Thus, CeBr3 now forms the central gamma-ray detection element on the MPO spacecraft.

Influence of the Mercury gravity field on the orbit insertion strategy of BepiColombo

In this paper the Mercury orbit insertion (MOI) in January 2022 of the ESA mission BepiColombo consisting of 15 manoeuvres is optimised. To maximise the mass in the target orbit, the gravity losses of the 15 burns are minimised by distributing the burns as evenly as possible. However, severe Sun aspect angle constraints make the optimisation problem difficult to solve. The NLP-solver SNOPT is used for the optimisation.The uncertainty in Mercury's gravity field is still high, such that its influence on the MOI was evaluated in several simulations of Mercury's gravity field. Since the target orbital parameters are still discussed their influence on the MOI ΔV-budget is investigated. The results indicate that the reference case was selected adequately. Since changes in arrival date, scientific orbit, instruments and strategies until BepiColombo's launch in 2015 are possible, this paper covers a wide range of possible future arrival options.

Komatiites as Mercury surface analogues: Spectral measurements at PEL

The elemental composition of Mercury's surface, which has been recently measured by the NASA MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft, suggests a mineralogy dominated by magnesium-rich orthopyroxene and feldspar. The most magnesium-rich and aluminium-poor regions of Mercury's surface (which are presumably orthopyroxene-rich) have compositions, and possibly mineralogies, analogous to terrestrial boninites and basaltic komatiites. Unfortunately, little is known about the spectral properties of komatiites, especially at the high surface temperatures of Mercury. We therefore have collected three terrestrial komatiites with different compositions plus a synthetic komatiitic sample, and measured their reflectances in the visible and thermal infrared spectral ranges. Samples divided into four grain size ranges (when enough material was available) were measured fresh and after thermal processing in vacuum (10 Pa) at 500 °C, comparable to Mercury peak surface temperatures. Our measurements show that spectral changes between fresh and thermally processed samples occur in both spectral channels, but are stronger in the visible range, with reddening affecting all the samples, while darkening is more selective. It is important to note that darkening and reddening after thermally processing the samples are independent of the komatiites ferrous iron content. In fact the synthetic sample which is nearly iron-free is most strongly affected. From our study it turns out that thermally processing the samples in vacuum at Mercury surface temperature produces the removal of samples' colour centres. The results of our study show also that the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) instrument on MESSENGER orbiting Mercury currently cannot distinguish between different compositions of komatiites, while the future MErcury Radiometer and Thermal infrared Imaging Spectrometer (MERTIS) on the upcoming ESA BepiColombo mission will resolve their differences in the 7−14μm spectral range.

Determination of the rotation of Mercury from satellite gravimetry

Space missions can have as a goal the determination of the interior structure of a planet: this is the case for the ESA BepiColombo mission to Mercury. Very precise range and range-rate tracking from the Earth and onboard accelerometry will provide a huge amount of data, from which it will be possible to study the gravity field of Mercury and other parameters of interest. Gravity can be used to constrain the interior structure, but cannot uniquely determine the interior mass distribution. A much stronger constraint on the interior can be given by also determining the rotation state of the planet. If the planet is asymmetric enough, the gravity field as measured by an orbiting probe tracked from the Earth contains signatures from the rotation. Are these enough to solve for the rotation state, to the required accuracy, from tracking data alone, without measurements of the surface? In order to reach some result analytically, a simplified analytical model is developed, and the symmetry breaking, occurring when the shape of the planet deviates from spherical symmetry, is characterized by explicit formulae. Moreover, a full cycle numerical simulation of the Radio Science Experiment is performed, including the generation of simulated tracking and accelerometer data and the determination, by least-squares fit, of the Mercury-centric initial conditions of the probe, of Mercury's gravity field and its rotation state, together with other parameters affecting the dynamics. The conclusion is that there is no reason of principle prohibiting the determination of the rotation from gravimetry, and the sensitivity of the measurements and the coverage are good enough to perform the experiment at the required level of accuracy. This will be important also in ensuring independent terms of comparison for the rotation experiment performed with a high-resolution camera. The mission is currently under development and much care has to be taken in guaranteeing the scientific goals even if there is some change in scenario.

Light-time computations for the BepiColombo Radio Science Experiment

The Radio Science Experiment is one of the on board experiments of the Mercury ESA mission BepiColombo that will be launched in 2014. The goals of the experiment are to determine the gravity field of Mercury and its rotation state, to determine the orbit of Mercury, to constrain the possible theories of gravitation (for example by determining the post-Newtonian parameters), to provide the spacecraft position for geodesy experiments and to contribute to planetary ephemerides improvement. This is possible thanks to a new technology which allows to reach great accuracies in the observables range and range rate; it is well known that a similar level of accuracy requires studying a suitable model taking into account numerous relativistic effects. In this paper we deal with the modelling of the space-time coordinate transformations needed for the light-time computations and the numerical methods adopted to avoid rounding-off errors in such computations.

SIMBIO-SYS: The spectrometer and imagers integrated observatory system for the BepiColombo planetary orbiter

The limited knowledge about the majority of the Mercury surface leaves many open questions regarding its geological evolution, the anomalously high metal/silicate ratio, the magnetic field generation and exosphere evolution. An integrated suite of instruments, Spectrometer and Imagers for MPO BepiColombo-Integrated Observatory SYStem (SIMBIO-SYS), which includes a stereo imaging system (STC), a high-resolution imager (HRIC) and a visible–near-infrared imaging spectrometer (VIHI), has been selected for the ESA BepiColombo mission to Mercury. SIMBIO-SYS will scan the hermean surface with the three channels and map the physical, morphological, tectonic and compositional properties of the entire planet. The availability of high-resolution images will unveil details of specific target at an unprecedented resolution. The main scientific objectives and performances along with technical characteristics of SIMBIO-SYS are described in this paper.

MILD: a laser altimeter transmitter for a Mercury planetary orbiter

We report on the design and preliminary development of a micro laser (MILD) breadboard (BB). The laser source is designed to be used as the transmitter unit of a laser altimeter for planetary investigations. The reference scenario is the ESA BepiColombo mission to Mercury (presently scheduled to be launched in 2012) but possible applications to minor solar system bodies may be included.

Mercury radiometer and thermal infrared spectrometer--a novel thermal imaging spectrometer for the exploration of Mercury

The MERTIS instrument is a state of the art imaging spectrometer in the TIR range onboard ESA's Bepi Colombo mission to the planet Mercury. MERTIS has four science goals: the study of Mercury's surface composition, identification of rock-forming minerals, mapping of the surface mineralogy, and the study of surface temperature variations and of the thermal inertia. The instrument is designed to achieve a signal-to-noise-ratio above 100 in the 7-14 µm range with a spectral channel width of 90 nm and a nominal spatial ground resolution of 500 m within the complex thermal and radiation environment of Mercury.

The BepiColombo mission to planet Mercury and the fundamental rôle of ISA accelerometer for the Radio Science Experiments

The ESA BepiColombo mission, scheduled for launch in 2013 with a two–satellite configuration, will represent an important step in the exploration of the Solar System and will contribute to fundamental physics. Among the main scientific objectives of this mission are the Radio Science Experiments: gravimetry and rotation of Mercury and tests of Einstein's theory of general relativity, to be achieved by the MPO (Mercury Planetary Orbiter) satellite. The ISA (Italian Spring Accelerometer) instrument will give a fundamental contribution to these experiments by providing a continuous measurement of the non–gravitational accelerations acting on the MPO. After a description of the mission, the role of the ISA accelerometer will be discussed, together with the current activity for its development.

Miniature ion mass analyzer

The Swedish Institute of Space Physics is developing a miniature plasma analyzer for planetary missions (MIPA—Miniature ion precipitation analyzer). MIPA has been accepted to fly on-board both the ESA BepiColombo mission to Mercury (2014) and the Indian Chandrayaan—1 mission to the Moon (2007). The analyzer measures ions in the energy range 10 eV–15 keV and has a sufficient mass resolution to resolve the main groups of ions, namely M / q = 1 , 2 , 4 , 8 , 16 , > 30 . Field of view is 9 ∘ × 180 ∘ . The instrument consists of the sensor head and a separate electronic board whose total mass is 300 g. The sensor head envelope is roughly 53 × 85 × 30 mm 3 in volume and is designed for the extreme operation temperature range of - 100 to + 125 ∘ C . MIPA comprises an electrostatic scanner for angular resolution, a cylindrical electrostatic analyzer for energy discrimination and a time-of-flight (TOF) section for particle velocity measurement. Generic design allows using the instrument on various platforms including nano-satellite and multi-spacecraft missions. This document describes the design of the sensor part including ion optical as well as mechanical aspects.

A set of laboratory analogue materials for the MERTIS instrument on the ESA BepiColombo mission to Mercury

The MERTIS (Mercury Thermal Imaging Spectrometer) instrument on the ESA mission BepiColombo will study the surface of Mercury in the wavelength range from 7 to 14 μm. The analysis of the MERTIS data will be posing a number of significant challenges. To facilitate the development of analytic tools, support planned ground based observations and facilitate a cross calibration with other instruments on BepiColombo and MESSENGER, a list of Mercury analogue materials was compiled. This set of analogue materials is based on our current knowledge of the surface composition of Mercury and includes plagioclase and potassium feldspars, low and high Ca pyroxenes, olivine, elemental sulfur, and as an extraterrestrial analogue a lunar highland soil sample. The samples are prepared in four different grain size separates ranging from 0 to 250 μm to cover the expected grain size distribution on the surface of Mercury.

Curve‐of‐Growth Model for Sodium D2 Emission at Mercury

ABSTRACTRenewed interest in the Hermean sodium exosphere has resulted from the planned encounters of two spacecraft, MESSENGER and BepiColombo, with Mercury. Both of these spacecraft will have UV‐visible spectrometers on board that are capable of detecting the sodium D lines and many other species. The MSASI (Mercury Sodium Atmospheric Spectral Imager) instrument on board the Mercury Magnetospheric Orbiter (MMO), one of two to be launched with the ESA BepiColombo mission to Mercury, is a Fabry‐Pérot spectrometer that has been specifically designed to image the sodium exosphere of Mercury. Because the data rates will be high, a curve‐of‐growth model for the intensity is useful. We show in this paper how the intensity varies not only with column abundance, but also with the observing geometry. The curve of growth is shown to be a strong function of solar incidence angle and planetary phase angle. Thus, the simple curve‐of‐growth model based on the assumption of observation at zero phase angle is inadequate for observations of Mercury’s exosphere. The solar flux reflected from the planet’s surface contributes a small amount to the exospheric intensity. We show that the D2/D1 ratio is inadequate to determine column abundance and has a strong dependence on the surface reflectivity for lines observed over the surface.

Dayside H + circulation at Mercury and neutral particle emission

In this study we discuss the proton circulation and the neutral atom emission at Mercury. The H + distribution in space, energy and pitch angle has been simulated by means of a single-particle Monte Carlo model. The applied electric and magnetic field model has been parameterized to take into account different boundary conditions such as interplanetary magnetic field and cross-tail potential drop. Particular attention has been paid to the estimation of the surface-sputtered neutral atoms and the energetic neutral atoms generated via charge-exchange process. The peculiar configuration of the hermean magnetosphere, as it is expected after the Mariner-10 observation of a weak magnetic field, allows a significant part of the incoming solar wind to enter Mercury's environment. For the B z < − 10 nT conditions, intense ion fluxes are expected in the cusp regions, which are extremely large when compared to the Earth's ones. The solar wind particles are likely to rapidly leave the hermean magnetosphere or precipitate onto the planetary surface, thus originating neutral particle emission via ion-sputtering. Two instruments, proposed to fly on board ESA mission BepiColombo (namely: the NPA-IS SERENA suite on the MPO segment and the ENA instrument on the MMO segment) will monitor the neutral signal as well as the precipitating ion particles. The modeled distribution presented here may be considered as a reference tool for the future observations.

MEMORIS: a wide angle camera for the BepiColombo mission

MEMORIS (MErcury Moderate Resolution Imaging System) is a wide angle camera (WAC) concept for the ESA mission BepiColombo. The main scientific objectives consist of observing the whole surface of Mercury in the spectral range of 400–1000 nm, with a spatial resolution of 50 m per pixel at peri-Herm (400 km) and 190 m at apo-Herm (1500 km). It will obtain a map of Mercury in stereo mode allowing the determination of a digital elevation model with a panchromatic filter through two different channels. The camera will also perform multispectral imaging of the surface with a set of 8–12 different broad band filters. A third channel dedicated to limb observations will provide images of the atmosphere. MEMORIS will thus monitor the surface and the atmosphere during the entire mission, providing a unique opportunity to study the relationship between surface regions and the atmosphere, as suggested by ground-based observations and theory.

Testing general relativity with the BepiColombo radio science experiment

The ESA mission BepiColombo will explore the planet Mercury with equipment allowing an extremely accurate tracking. While determining its orbit around Mercury, it will be possible to indirectly observe the motion of its center of mass, with an accuracy several orders of magnitude better than what is possible by radar ranging to the planet's surface. This is an opportunity to conduct a relativity experiment which will be a modern version of the traditional tests of general relativity, based upon Mercury's perihelion advance and the relativistic light propagation near the Sun. We define the mathematical methods to be used to extract from the data of the BepiColombo mission, as presently designed, the best constraints on the main post-Newtonian parameters, especially $\ensuremath{\beta},\ensuremath{\gamma}$ and the Nordtvedt parameter $\ensuremath{\eta},$ but also the dynamic oblateness of the Sun ${J}_{2\ensuremath{\bigodot}}$ and the preferred frame parameters ${\ensuremath{\alpha}}_{1},{\ensuremath{\alpha}}_{2}.$ We have performed a full cycle simulation of the BepiColombo radio science experiments, including this relativity experiment, with the purpose of assessing in a realistic (as opposed to formal) way the accuracy achievable on each parameter of interest. For $\ensuremath{\gamma}$ the best constraint can be obtained by means of a dedicated superior conjunction experiment, with a realistic accuracy $\ensuremath{\simeq}2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}.$ For $\ensuremath{\beta}$ the main problem is the very strong correlation with ${J}_{2\ensuremath{\bigodot}};$ if the Nordtvedt relationship $\ensuremath{\eta}=4\ensuremath{\beta}\ensuremath{-}\ensuremath{\gamma}\ensuremath{-}3$ is used, as it is legitimate in the metric theories of gravitation, a realistic accuracy of $\ensuremath{\simeq}2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$ for $\ensuremath{\beta}$ and $\ensuremath{\simeq}2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}$ for ${J}_{2\ensuremath{\bigodot}}$ can be achieved, while $\ensuremath{\eta}$ itself is constrained within $\ensuremath{\simeq}{10}^{\ensuremath{-}5}.$ If the preferred frame parameters ${\ensuremath{\alpha}}_{1},{\ensuremath{\alpha}}_{2}$ are included in the analysis, they can be constrained within $\ensuremath{\simeq}8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$ and $\ensuremath{\simeq}{10}^{\ensuremath{-}6},$ respectively, at the price of some degradation in $\ensuremath{\beta},$ ${J}_{2\ensuremath{\bigodot}}$ and $\ensuremath{\eta}.$ It is also possible to test the change with time of the gravitational constant G, but the results are severely limited because of the problems of absolute calibration of the ranging transponder, to the point that the improvement as compared with other techniques (such as lunar laser ranging) is not so important.

Estimation of the dust flux near Mercury

In this paper, we consider the dust flux in the vicinity of Mercury due to interplanetary particles using Divine's five population model (J. Geophys. Res. 98(E9) (1993) 17029). The average accretion rate is 16 metric tonnes per day. The (mass weighted) mean impact speed is 26 km s −1 . Due to Mercury's non-negligible eccentricity, the accretion rate is found to vary by a factor of two along the orbit. However, detection rates of interplanetary particles on an impact ionisation detector mounted on a Mercury orbiting spacecraft can vary by more than a factor of ten while Mercury travels around the Sun. The variations in detection rates will reveal the dependence of the interplanetary dust cloud on heliocentric distance and ecliptic latitude (in the range covered by Mercury's orbit). An impact ionisation detector, which is sensitive to particles in the mass range m=10 −16– 10 −9 g and has a target area of 100 cm 2 would detect of order 10 particles per day. A momentum sensor, which is sensitive to particles m>10 −12 g would detect one particle every 2–3 days. We estimate that the number of impact light flashes which could be detected by the high-resolution camera on the Planetary Orbiter, planned within the ESA's BepiColombo mission, is between 0.4 and 660 light flashes during the entire mission. This result is dependent on the fraction of energy which is converted to light during an impact on the surface of Mercury. Since Mercury's thin exosphere cannot protect the planet from hypervelocity impacts of interplanetary meteoroids, Mercury will be surrounded by the ejecta created during these impacts. For the impact generated dust cloud we adopt the model described by Krivov and Hamilton (Icarus 128 (1997) 335). Although the uncertainties of this model are large, we conclude that the ejecta should be detectable at least statistically, if the dust sensor is mounted on a spinning spacecraft which is in an elliptical orbit around Mercury.