MITM7

With the gravimeter for small solar system objects (GRASS), absolute surface accelerations in the order of nano-g can be measured. It is an innovative and extremely compact sensor that will fly as part of ESA’s Hera mission onboard the Juventas CubeSat to the binary asteroid system Didymos. In 2027, Juventas will land on the secondary body of the system, Dimorphos, and GRASS will hence measure the local gravity vector and its temporal variations at the landing site. Apart from the direct mass determination, these measurements will help in synergy with other instruments to constrain the geological substructure as well as the surface geophysical environment.

The instrument is currently under development at the Royal Observatory of Belgium, funded by the Belgian PRODEX office and in cooperation with EMXYS in Spain.

The average gravitational force expected on Dimorphos’ surface is around 4.5 x 10^-5 m s^-2 (or 4.5 mGal, Figure 1). Apart from the self-gravitation of the body, centrifugal forces and the acceleration due to the main body of the system contribute to the surface acceleration. The temporal variation of the signal is driven by the dynamical state of Didymoon with respect to Didymain and the related librations.

Figure1: Modeled surface gravity in the Didymos system (left) and on Dimorphos (right). Note that only a shape model for the primary body exists.

The table below lists the science objectives of the instrument for Dimorphos.

The gravimeter measurement system consists of monitoring the displacement and deflection of a flat spring due to a gravitational field by a capacitive transducer. Modulation of the measured g-vector by rotation allows the rejection of the zero-g bias. In addition, no levelling is required. The gravimeter will be calibrated in-situ by using electrostatic force to compensate acceleration forces.

Two orthogonally aligned gravimeter axes, each with a rotating sensor head, enable finally the reconstruction of the full 3D gravity vector. Figure 2 shows a CAD drawing with dimensions (left) and the vibration test model of the gravimeter (right).

Figure 2: CAD drawing of the two-axes gravimeter (left) and a picture of the assembles GRASS vibration test model (right)

We will present the scientific background and application of the GRASS instrument on Dimorphos, the current instrument status, its implementation and first test results and give an outlook on future application of the instrument for other small planetary bodies.

How to cite: Ritter, B., Karatekin, Ö., Carrasco, J. A., Tasev, E., Alavés Mañogil, H., Noeker, M., Van Ransbeek, E., Noiset, G., and Berk Senel, C.: Measuring gravity with the GRASS instrument on the Hera mission, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-1115, https://doi.org/10.5194/epsc2022-1115, 2022.

Introduction: The Solar System dust is the smallest Solar System building block, and therefore its study allows understanding the early stages of the Solar System formation. The comprehension of the Solar System formation and evolution processes is also constrained by the inventory and spatial distribution of volatiles in our System. In particular, the delivery process of volatiles of astrobiological interest (water and organics) to the Earth is a key issue to understand the life formation in our planet. Asteroids and comets may have been the source of these volatiles to the terrestrial planets’ atmospheres and to the Earth’s oceans. The boundary between asteroids and comets is not well defined, and many main-belt asteroids are probably volatile-rich and would become cometary if they were moved to the inner Solar System.

Piezoelectric Crystal Microbalances (PCMs) are widely used sensors to monitor dust and particles deposition processes in space and to characterize material outgassing in vacuum. This kind of sensors converts mass changes into fundamental resonance frequency variations, according to Sauerbrey equation [1].

In this work we present VISTA (Volatile in-Situ Thermogravimeter Analyser), one of the two scientific payloads on board MILANI Cubesat, as part of the ESA Hera Program. The MILANI Cubesat development is led by Tyvak International, prime contractor of a consortium composed by entities and institutions from Italy, Czech Republic and Finland. The main MILANI objective is executing a scientific mission aiming at studying the binary asteroid system Didymos-Dimorphos, characterizing the asteroid with a dust sensor, i.e. VISTA, and a spectrometer, i.e. ASPECT. VISTA is developed by an Italian Consortium composed by three Research Institutes: INAF-IAPS (National Institute of Astrophysics - Institute for Space Astrophysics and Planetology), CNR-IIA (National Council of Research – Institute of Atmospheric Pollution) and Politecnico di Milano.

HERA Mission goals: Hera is a planetary defence mission under development at the European Space Agency (ESA), launching in October 2024. Hera is the European contribution to the international Asteroid Impact Deflection Assessment (AIDA) cooperation, the first planetary defence mission, in collaboration with NASA, who is responsible for DART (Double Asteroid Redirection Test) kinetic impactor spacecraft. Hera will travel to a binary asteroid system, the Didymos-Dimorphos pair of near-Earth asteroids, and will study and characterize the asteroid system after the DART impact in September 2022. In the framework of Hera mission, VISTA will accomplish the following scientific goals: 1) detect the presence of dust particles smaller than 10 µm (residual dust particles from the impact and suspended dust in the binary system or coming from dust levitation process); 2) characterization of volatiles (e.g. water) and light organics (e.g. low carbon chain compounds) by using Thermo-Gravimetric Analysis (TGA) cycles (the desorption rates ad specific temperatures are used to characterize e volatiles and organics desorbed from the sensor  surface); 3) molecular contamination monitoring in support to other Cubesat instruments and ASPECT spectrometer, coming from outgassing processes on-board the spacecraft, that usually happen in the first days or weeks in orbit.

VISTA Heritage: The Consortium has a considerable heritage in the design, manufacturing and testing of PCM-based instrumentation both for laboratory and space applications coming from different ITT-Emits ESA Projects: 1) CAM (Contamination Assessment Microbalance), developed for “Evaluation of an in-situ Molecular Contamination Sensor for space use” (2014-2016) (Figure 1); 2) CAMLAB (Contamination Assessment Microbalance for LABoratory) developed for “Development of a European Quart Crystal Microbalance” (2017-2019) (Figure 2).

Figure 1. PCM Engineering Model developed during CAM-ESA Project for space applications.

Figure 2. PCM breadboard developed during CAMLAB-ESA Project for laboratory applications.

Working principle: PCMs exploit the piezoelectric properties of quartz crystals, as mass deposition on the sensing area of the instrument induces variations of the crystal resonance frequency. The core of VISTA is composed of: 1) two quartz crystals mounted in a sandwich-like configuration (one sensing crystal and the other reference crystal); 2) a Thermal Control System (TCS), composed by two integrated heaters and a Thermoelectric Cooler (TEC); 3) a Proximity Electronics (PE). The instrument is also capable of performing Thermo-Gravimetric Analysis, a technique used to monitor thermal processes involving volatile compounds, e.g absorption/desorption and deposition/sublimation, by means of the built-in heaters. The TEC is used to cool down the sensor and enhance the condensation of particles on the microbalance. VISTA is capable of monitoring particles lower than 5-10 µm and sub-µm particles.

VISTA technical characteristics are shown in Table 1.

Table 1. VISTA technical characteristics.

The sublimated compounds can be characterized by calculating the enthalpy of sublimation ΔH_sub, which can be retrieved with two methods: 1) by considering the deposition rates and using Van’t Hoff relation [3]; 2) by using Langmuir relation [4].

Conclusions and future works: In this work, the VISTA instrument working principle and heritage were presented. Qualification activities are currently ongoing on the VISTA Engineering Qualification Model (EQM), shown in Figure 3, in order to qualify the sensor within the expected mechanical and thermal environment and evaluate the instrument performances in a representative environment, prior integration on the MILANI Cubesat. The next phase of the research activities will be the manufacturing, integration and testing of VISTA Flight Model (FM), that will launch in 2024.

Figure 3. VISTA EQM developed for MILANI cubesat/HERA Space Mission.

References: [1] G. Sauerbrey 1959, Z. Phys., 155, 206-222; [2] Palomba E. (2016), OLEB, 46 (2-3); [3] S.W. Benson et al. 1968; [4] I. Langmuir, 1913.

How to cite: Palomba, E., Dirri, F., Longobardo, A., Gisellu, C., Biondi, D., Angrisani, M., Zampetti, E., Scaccabarozzi, D., and Saggin, B.: Volatile in-Situ Thermogravimeter Analyser (VISTA) payload developed for MILANI cubesat for HERA Space Mission, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-927, https://doi.org/10.5194/epsc2022-927, 2022.

Introduction: Quartz Crystals Microbalances (QCMs) are widely used sensors for monitoring and characterizing dust and particles deposition processes in different planetary environments and measuring material contamination coming from outgassing sources in space, in support to other instruments (e.g. spectrometers). Furthermore, they are capable of detecting and measuring the presence of volatile com-pounds of astrobiological interest such as water and organics. These measurements can be particularly relevant when performed on primitive asteroids or comets, or on targets of potential astrobiological interests, e.g. Mars [1]. These sensors convert mass changes into fundamental resonance frequency variations, according to Sauerbrey equation [2].

The VISTA (Volatile In Situ Thermogravimetry Analyser) instrument is a QCM-based device able to perform measurements of abundance of volatiles and dust particles in planetary and asteroidal environments. The instrument can characterize the planetary regolith from 5-10 µm to sub-µm particles and monitor the contamination processes on board satellites (cubesat, etc.) caused by molecular outgassing. VISTA is one of the two scientific payloads of MILANI CubeSat, developed by Tyvak International that leads a consortium composed by entities and institutions from Italy, Czech Republic and Finland, in the framework of the Hera program of the European Space Agency (ESA). Hera, due to launch in 2024, is the ESA part of the Asteroid Impact & Deflection Assessment (AIDA) international collaboration with NASA, who is responsible for the Double Asteroid Redirection Test (DART) kinetic impactor spacecraft. The main objective of MILANI is the study of the binary asteroid system Didymos-Dimorphos, characterizing the asteroid with a dust sensor (VISTA) and a spectrometer (ASPECT).

In this work, the calibration operations and the performance tests, to assess VISTA capability of characterizing volatile compounds and simulant contaminations in vacuum chamber at low temperatures. The sensor has been developed by an Italian Consortium composed by three Research Institutes: INAF-IAPS (National Institute of Astrophysics - Institute for Space Astro-physics and Planetology), CNR-IIA (National Council of Research – Institute of Atmospheric Pollution) and Politecnico di Milano and led by INAF-IAPS.

Figure 1. VISTA EQM developed for ESA Hera space mission.

Working Principle: The instrument core is a QCM whose frequency variations directly depends on the deposited sample mass on the crystal surface during sublimation, condensation and absorption/desorption processes. The instrument consists of: 1) two quartz crystals mounted in a sandwich-like configuration; 2) a Thermal Control System (TCS), composed by two integrated heaters and a Thermoelectric Cooler (TEC); 3) a Proximity Electronics (PE). VISTA is also capable of performing Thermo-Gravimetric Analysis, which is a technique used to monitor thermal processes involving volatile compounds, e.g. deposition/sublimation and absorption/desorption. It can also monitor particles lower than 5-10 µm and sub-µm particles [1].

Calibration: The QCM frequency can change not only due to the mass deposition/release, but also due to the variation of environmental parameters, such as temperature and pressure. In order to disentangle frequency variations due to mass deposition and environmental parameters, the sensor is calibrated by measuring the frequency as a function of temperature. According to literature [3], the frequency-temperature curve follows a third-degree polynomial (Figure 2).

Figure 2. QCM frequency as a function of temperature in vacuum.

Performance tests: VISTA capability to detect contaminant depositions and to monitor the accumulation and desorption processes is verified by placing an effusion cell containing an organic compound, used as a contamination source, placed in the Field Of View (FOV) of the sensing crystal and heated up to 100°C. The experimental setup is shown in Figure 3.

Figure 3. VISTA EQM experimental setup for contamination simulation.

The QCM is connected with four screws with a copper U shape and in contact with a cold sink set to -10°C to help the molecules condensation on the crystal surface. The frequency is monitored during the tests and the deposited flux can be retrieved (Figure 4) at each temperature set point.

Figure 4. Deposition test from +50°C to +100°C.

Two methods can be used to retrieve the enthalpy of sublimation ΔH_sub by using the deposition rates, i.e. the Van’t Hoff relation [4] or Langmuir relation [5]. Thus, by measuring two different deposition rates, k1 and k2, at two different close temperatures T1 and T2, it is possible to obtain the compound ΔH_sub by means of Van’t Hoff relation or throughout the temperature range by using Langmuir relation. 

TGA cycles are then performed by heating the crystals by means of the built-in heaters. After the heating cycles, the frequency returns to its initial value, thus indicating that all the deposited mass desorbed during the test (Figure 5).

Figure 5. TGA cycles from +15°C to +50°C and from +40°C to +70°C.

The desorption rates from crystals surface can be used as well to calculate the ΔH_sub and compare it with the ΔH_sub results obtained during the depositions/contamination processes.

References: [1] E. Palomba et al (2016), OLEB, 46(2-3; [2] G. Sauerbrey (1959), Z. Phys., 155, 206-222; [3] D. Salt (1987); [4] S.W. Benson et al., 1968; [5] I. Langmuir, 1913.

How to cite: Gisellu, C., Dirri, F., Palomba, E., Longobardo, A., Biondi, D., Angrisani, M., Zampetti, E., Scaccabarozzi, D., and Saggin, B.: Calibration and performance tests of VISTA, a microbalance for asteroid dust characterization and contamination for space mission applications, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-790, https://doi.org/10.5194/epsc2022-790, 2022.

Thermal Infrared Imager TIRI onboard Hera is now being developed for the investigation of the binary asteroid Didymos and Dimorphos in the European Space Agency Hera mission. This instrument uses a heritage from the Thermal Infrared Imager TIR on Hayabusa2, which observed the C-type asteroid 162173 Ryugu. In the Hera mission, its updated version of instrument is based on Lynred PICO 1024 Gen2 bolometer array (1024 x 768 effective pixels) with almost 4 times higher spatial resolution than TIR. The wavelength range shows 8-14　µm for the thermal average images, as well as 7.8, 8.6, 9.6, 10.6, 13.0 deg for multi-bands). Its FOV covers 13.3 x 10.0 deg with the spatial resolution of 0.013 deg/pixel.

TIRI is now in the test process to check its full functions and performances as well. The calibration for TIRI is to check its focus, to conduct radiometric and geometrical corrections, identification of materials (like terrestrial rocks and meteorites). A set of apparatus were prepared by the TIRI PI team using the collimator (Aperture: 200mm dia., temperature range of -20 to 150℃, Focal position is 60 deg or higher), the flat cavity blackbody ( 175 x 175　mm, -20 to 125 ℃ for fact change of temperature), and a cold plate plus the cryocooler for the target sample (room temperature to -123 deg). There are some rocks and meteorites to be set as the multiband test samples at the target material, With these information and the presentation, details of the TIRI preparation and development are shown. The test results for TIRI will be presented with some data set and physical properties using the sample.

How to cite: Okada, T., Tanaka, S., Sakatani, N., Shimaki, Y., Arai, T., Senshu, H., Demura, H., Sekiguchi, T., Kouyama, T., Kanamaru, M., and Ishizaki, T.: Calibration of the Thermal Infrared Imager TIRI onboard Hera, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-1191, https://doi.org/10.5194/epsc2022-1191, 2022.

Introduction: The dust ejected by cometary nuclei encodes valuable information on the formation and evolution of the early Solar System. Several short-period comets have already been studied in situ[1], but their pristine condition was modified by multiple perihelion passages. Dynamically new comets (DNCs) remain pristine bodies since they never visited the inner Solar System, stationing more than 2000A.U. far away from the Sun in the Oort cloud.

Comet Interceptor (CI) is the first F-class space mission selected by the European Space Agency to study a DNC or an interstellar object entering the inner Solar System for the first time[2]. The Dust Impact Sensor and Counter (DISC) is an instrument included in the Dust Field and Plasma (DFP) suite, part of the CI payload, dedicated to characterizing the dust encountered by the spacecraft (S/C) during its flyby in the coma of the target DNC. DISC will measure hypervelocity impacts (HVIs), in the range 10–70km/s, with cometary dust particles of 1–400μm diameter. It aims to characterize the mass distribution of dust particles in the range 10^-15–10^-8kg, and retrieve information on dust structural properties from impacts duration[3].

DISC design: DISC is a 121×115.5×46mm³ aluminum box containing both the detection system and the electronics (Fig.1). The former consists in a 100×100×0.5mm³ aluminum plate with three piezoelectric traducers (PZTs) at its corners. HVIs induce shockwaves in the sensing plate. Far from the impacted area, such waves become acoustic Lamb waves that propagate up to the PZTs, which start to vibrate at their resonant frequency. A couple of electronic boards at the bottom of the unit allows to retrieve the particles momentum and kinetic energy from PZTs vibration signal.

Fig.1: DISC sensing element and dust shield design.

DISC detection system is derived from the GIADA Impact Sensor measurement subsystem, that was designed to measure impacts of slow particles[4]. During CI flyby, some hypervelocity dust particles might perforate DISC outer sensing diaphragm and represent a serious hazard for the instrumentation. A dedicated mechanical element preliminarily designed as made of four 1cm-thick aerogel blocks and a 1mm-thick aluminum frame was integrated into DISC design to shield the entire S/C from such dangerous impacts.

Two key aspects need to be verified to ensure that the instrument is suitable for CI aims:

• DISC capability to survive the expected coma dust environment;
• DISC capability to measure the momentum/energy of impacting particles in the aforementioned size and mass ranges.

Dust shield assessment: We verified DISC dust shield performance using a two-stage Light-Gas Gun (LGG) (Open University, Milton Keynes) to shoot mm-sized particles of various materials at speeds around 5km/s[5,6]. This facility allowed to test the instrument resistance to momenta in the range 10^-2–10^-1kg·m/s and to energies of the order of 10²J. The dust shield showed good resistance up to energies of about 200J, released by a 3mm nylon bead at 5.5km/s. DISC resistance to higher-energy particles can be improved by increasing the aerogel thickness, without any further modifications to the general design.

These experiments proved that DISC is compatible with the foreseen coma dust environment. Integrating a thicker aerogel layer in the design will reduce the risk of failure due to higher-energy particles to low enough values even for the S/C more exposed to the dust flux. The S/C beneath DISC unit is further protected by DISC lower layers.

DISC performance: DISC will measure momenta in the range 10^-11–10^-3kg·m/s[7]. The LGG facility allows to reach high momentum values by shooting heavy particles, but their collision dynamics is very different from what expected for cometary dust. A different strategy to simulate the foreseen impact momentum range is needed.

A Van der Graaf (VdG) gun can shoot μm-sized dust particles up to 20km/s, reproducing momenta of 10^-9–10^-7kg·m/s[8].

The tested impact parameters range can be extended by simulating HVIs effects with a high-power pulsed laser beam. Laser intensity, beam dimension, and pulse duration can be regulated to respectively match impact pressure, section, and shock duration of the corresponding particle[9]. Laser intensities of 10⁹–10¹⁰W/cm² can generate surface pressures from kbar to Mbar, typical of cometary dust particles colliding at 3–6km/s. Using our Nd:YAG laser (λ=1064nm), which emits τ=6ns pulses with pulse energy of E_pulse=1.2J, we can cover a momentum range of 10^-10–10^-5kg·m/s. Since laser simulated and VdG real impacts share part of the released momentum range, laser shots can be calibrated and their representativity verified with real collisions.

The energy range expected for dust impacts measured during CI flyby is 10^-7–10²J. Laser simulated impacts cannot reach the higher energy values. However, the energy/pulse duration range is pretty vast and with some attenuators and pulse reducers the central/left part of the parameters space (around mJ energy and ns pulse time) could be reasonably covered.

Fig.2. shows the optical setup: a polarizer attenuator splits the beam and allows to regulate its power; a couple of mirrors prevents backwards reflections to get to the laser output aperture; a beam expander enlarges the beam, which enters a vacuum chamber and is focused by a plano-convex lens on the DISC breadboard mounted on a 3-axis translational stage. The vacuum chamber is fundamental to prevent plasma generation in air around the focus.

Fig.2: Optical setup for high-power pulsed laser simulated HVIs.

By properly tuning the laser parameters, this strategy allows to achieve representative simulations of cometary dust HVIs. In addition to assess DISC performances, simulating the same impact many times provides large statistics to calibrate DISC detection system and momentum/kinetic energy retrieval methodology with great accuracy.

References: [1] Keller H. U. and Kührt E. (2020) Space Sci. Rev., 216(1), 1–26. [2] Snodgrass C. and Jones G. H. (2019) Nat. Commun., 10(1), 1–4. [3] Della Corte V. et al. (2021) LPSC LII, Abstract #2332. [4] Esposito F. et al. (2002) Adv. Space Res., 29(8), 1159–1163. [5] McDonnell J. A. M. (2006) Int. J. Impact Eng., 33(1–12), 410–418. [6] Hibbert R. et al. (2017) Procedia Eng., 204, 208–214. [7] Di Paolo F. et al. (2021) LPSC LII, Abstract #1238. [8] Friichtenicht J. F. (1962) Rev. Sci. Instrum., 33(2), 209–212. [9] Pirri A. N. (1977) Phys. Fluids, 20(2), 221–228.

How to cite: Ferretti, S., Della Corte, V., Piccirillo, A. M., Rothkaehl, H., Sylvest, M., Patel, M., Ertel, H., Millinger, M., Bertini, I., Fiscale, S., Longobardo, A., Inno, L., Rotundi, A., Ammannito, E., and Sindoni, G.: Analysis of dust shield and detection system response to hypervelocity impacts for Comet Interceptor Dust Impact Sensor and Counter, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-1163, https://doi.org/10.5194/epsc2022-1163, 2022.

Introduction:

MiniPINS is an ESA study to develop and prototype miniaturised surface sensor packages (SSPs) for Mars and the Moon. The study aims at miniaturising the scientific sensors and subsystems, as well as identifying and utilizing commonalities between Mars and Moon SSPs, allowing to optimise the design, cut costs and reduce the development time.

Mars In-Situ Sensors (MINS) is a penetrator with approx. 25 kg mass, piggy-backed by another Mars mission spacecraft to Mars. In total 4 identical penetrators are deployed to different landing sites either from the approach orbit or Mars orbit. The design of MINS has significant heritage from FMI’s MetNet mission design[1]. The entry, descent and landing sequence of MINS is completely autonomous and controlled by its on-board computer. In the Martian atmosphere the penetrators undergo aerodynamic braking with inflatable breaking units until they reach the target velocity of 60-80 m/s for entering the Martian surface. The nominal mission duration is one Martian year.

Lunar In-Situ Sensors (LINS) is a miniature 7 kg station deployed on the Moon surface by a rover. LINS mission consist of 4 surface stations deployed to different sites within the rover’s traveling perimeter. The LINS scientific package consists of several scientific instruments to study the Moon for 2 years. 

Mars SSP sensors:

MINS will study the Martian atmosphere, seismology and chemistry. The MINS payload consists of a camera, a visual spectrometer, a meteorological package, an impact accelerometer, soil thermoprobes, a chemistry probe, a seismometer and a radiation monitor. Many of the instruments have Mars heritage but additional qualification is required due to landing shock. 

The meteorological package consists of air temperature sensors, pressure sensor, relative humidity sensor, wins sensors, dust sensor and solar irradiance sensor. Pressure and humidity sensors are provided by FMI and the sensors have heritage from multiple Mars missions like the M2020 Perseverance rover[2]. Air temperature sensors are provided by CAB and they have heritage from MSL[3], InSight[4] and M2020[2]. Solar irradiance sensor (SIS)[5] is provided by INTA and the technology has been used in several Mars missions[6]. The selected design for MiniPINS is the SIS for ExoMars ‘22. Also based on ExoMars ‘22 METEO is the dust sensor by UC3M. 

The chemistry probe is a new design developed by CAB. It is composed of several miniature sensing needles that share an acquisition electronics to measure pH, salinity, water content, conductivity and temperature of the Martian regolith. The current TRL is 6. The thermoprobe is also a new design developed by UPC for characterizing the thermophysical properties of the regolith.

The visual spectrometer selected for MiniPINS is based on tunable Fabry-Pérot interferometer (FPI) technology by VTT Technical Research Centre of Finland[7]. The piezo-actuated FPI technology has previously flown on board nanosatellites Aalto-1, Reaktor Hello World and PICASSO demonstrating operation in different wave lengths. Due to its maturity and flight-proven performance, the near-IR concept is the baseline for the MINS spectrometer.

Moon SSP sensors:

LINS proposal is focused on two principal science objectives: understanding the structure and composition of the lunar interior and the characterization of the lunar surface environment in view of human exploration and resource extraction. The placement of a network of short period seismic stations will allow determining the thickness of the lunar crust (upper and lower) and its lateral variability. Due to smaller size the LINS has fever scientific sensors and many are common with the MINS packages. Magnetometer selected for LINS is based on the triaxial magnetometer developed by INTA for MetNet Mars mission. The sensor will require some modifications and adaptation of the ejection mechanism as well as temperature qualification for Lunar conditions.

Shared sensors:

The cameras of both MINS and LINS are based on the Athermalized Panchromatic Imaging System (APIS) low-resolution refractive camera used by INTA as a CubeSat payload which flew on-board the OPTOS satellite. It is based on a 1.3 MP CMOS image sensor from Cypress Semiconductor Corporation but the optics will be redesigned taking into account the applicable requirements for each mission. 

Miniature wind sensors (Mars) and thermoprobes are based on the same sensor structure developed by UPC. The new type of sensor is a spherical shell divided into four sectors. To sense temperature and provide heating power, a customized 3 x 3 mm silicon die, including a platinum resistor, is attached to the inner side of each sector. In MINS the wind sensors are located in the deployable mast and the soil sensor is in a compartment below the surface in contact with the regolith. For LINS a dedicated deployment mechanism must be developed for the soil probe. The wind sensors and soil probes are currently in TRL 6.

The short period seismometer is based on a miniature MEMS resonator by Imperial College, previously used in InSight[8]. For LINS a variant of the sensor, called the Silicon Seismic Package, (SSP)[9] is proposed.

The impact accelerometer of MINS will be a new development. To miniaturise the accelerometer, a commercial part from PCB Piezotronics has been selected as the basis of the sensor and the complete sensor assembly will be developed and qualified by INTA. Lunar SSP also includes an accelerometer which is used to determine the attitude of LINS. It is based on an Analog Devices component ADXL327BCPZ, already qualified by INTA for Mars, but LINS missions requires even larger temperature range so additional qualification is foreseen.

The radiation monitors included in both missions are based in the Metal-Oxide-Semiconductor Field-Effect-Transistor (MOSFET) dosimeters. Proposed provider of MOSFET dosimeters is Tyndall National Institute.

Project status:

The MiniPINS study, consisting of project phases A and B1, is nearly completed and a final review was held in autumn 2021. As a follow-up, the European Space Agency is facilitating an industrial development of European inflatable deceleration system.

How to cite: Hieta, M., Genzer, M., Haukka, H., Kestilä, A., Arruego Rodríguez, I., Apéstigue Palacio, V., Reina Aranda, M., Gonzalo Melchor, A., Martínez Oter, J., González-Guerrero Bartolomé, M., Ortega, C., Camañes, C., Dominguez-Pumar, M., Espejo Meana, S., Guerrero, H., and Rodríguez-Manfredi, J. A.: State-of-the-art miniaturised science instruments of the MiniPINS landers, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-820, https://doi.org/10.5194/epsc2022-820, 2022.

The Visual Monitoring Camera (VMC) is a small camera onboard Mars Express, initially intended to provide visual confirmation of the separation of the Beagle 2 lander. In 2007, a few years after the end of its original mission, VMC was turned on again to obtain full-disk images of Mars for outreach purposes (Ormston et al., 2011). As VMC obtained more images, the scientific capabilities of the camera became evident (Sánchez-Lavega et al., 2018), and finally the small camera was upgraded to be a new scientific instrument, with an agreement between the European Space Agency (ESA) and the University of the Basque Country (Spain; UPV/EHU). In this work we describe the calibration and technical efforts that are allowing us to maximize the scientific output from this small camera.

Figure 1. Image of VMC before launch (left) and scheme from the Flight User Manual (right)

VMC is also called the Mars webcam, as it is similar to a typical webcam of the 2000s. The sensor has a a 640x480 pixel array, and a Field of View (FOV) of 30ºx40º. This wide FOV, together with the elliptical orbit of Mars Express, enables full-disk observations from apocenters, which are the most common product of VMC. It is also possible to use this wide FOV to image large sections of the limb, and therefore monitor the occurrence of high altitude aerosols, as shown by Sánchez-Lavega et al. (2018).

Figure 2. Full disk of Mars as seen by VMC.

Operations

Since 2018 VMC operations follow a similar routine as those used for other science instruments. The Science Ground Segment takes care of the Medium Term Planning (MTP) following the inputs from the science team. The science team performs the Short Term Planning (STP). Fig. 3 shows a typical VMC observation, which consists of a default image, followed by one to several loops of 6 images that use a set of predefined exposures. The exposure times are set to maximize the dynamic range of the final science products obtained by combining the individual images.

Figure 3. Scheme of a typical VMC observation.

Calibration of images

The images are calibrated following the standard scheme of subtracting a dark current and dividing by a flatfield image. The flat field correction is much more relevant than the dark correction in the quality of the final images after calibration. No onground calibration is known for VMC, therefore the dark current and flat-field corrections used are based only on in-flight observations. The dark was obtained by pointing VMC to the sky, specifically the area of Eridanus, where few bright stars are present.

The flat-field was created using dark-corrected images of flat portions of Mars that were well and uniformly illuminated, as free as possible from large structures, and as flat as possible.

Calibrated images are routinely archived at ESA’s Planetary Science Archive (PSA), as described by Ravanis et al. (2020)

Figure 4. VMC dark (left) and flat (right).

Geometry

The original documents indicate the design parameters for the orientation of VMC in the reference frame of MEX, and for the pixel resolution (iFOV). However, the accurate parameters once VMC was mounted on MEX were never measured on the ground. In addition to this, we find that the timestamp of images suffers a random shift of a few seconds. As a result, we have 5 free parameters: 3 Euler angles for the attitude relative to the MEX reference frame; the pixel resolution (iFOV); and the shift in time from the actual timestamp to the labeled timestamp.

In order to determine the attitude and iFOV of VMC relative to Mars Express, we used images showing stars. Many of these observations covered the stars of the constellation of Orion, because several suitable stars are present in that region of the sky. During these observations the spacecraft maintains a fixed attitude, therefore, the time related uncertainty is not present and only 4 free parameters remain: 3 Euler angles, and the pixel size. These parameters are shown in table 1.

Table 1. VMC geometric parameters as given by the Flight User Manual (FUM) and calibrated values.

The shift in time was estimated from observations showing Phobos. The relative speed of Phobos as seen from Mars Express is high, and therefore it is possible to use its position as an accurate clock. We find that our images are usually obtained between 6 and 13 seconds before the labeled time, but we find random variations. Subtracting 10 seconds is considered a good strategy in most cases, but this uncertainty remains as a limitation.

Figure 5. VMC image showing the stars of Orion (left), and Phobos in front of Mars (right). Red circles represent the expected position before calibration. Green circles are the expected positions according to the new calibration.

Conclusions

Within the expectable limitations, the performance of this new instrument is very good and VMC is enabling novel science results and techniques (e.g. Hernández-Bernal et al. 2021). This is in part because VMC provides some capabilities that are not common among instruments in orbital planetary missions. Even with no on-ground calibration available, it has been possible to calibrate the camera, both photometrically and geometrically. Some hardware limitations remain, and others have been partially overcome with specially developed operational strategies.

References

Hernández‐Bernal et al. (2021). A Long‐Term Study of Mars Mesospheric Clouds Seen at Twilight Based on Mars Express VMC Images.

Ormston et al. (2011) An ordinary camera in an extraordinary location: Outreach with the Mars Webcam.

Ravanis et al. (2020). From engineering to science: Mars Express Visual Monitoring Camera's first science data release.

Sánchez-Lavega et al. (2018). Limb clouds and dust on Mars from images obtained by the Visual Monitoring Camera (VMC) onboard Mars Express.

How to cite: Hernández Bernal, J., Cardesín Moinelo, A., Hueso Alonso, R., Ravanis, E., Burgos Sierra, A., Wood, S., Marín Yaseli de la Parra, J., Merrit, D., costa Sitja, M., Escalante, A., Grotheer, E., Esquej, P., Dias Almeida, M., Martin, P., Titov, D., Wilson, C., del Río Gaztelurrutia, T., Sánchez Lavega, A., and Sierra, M.: The Visual Monitoring Camera on Mars Express: calibrating a new science instrument made from an old webcam orbiting Mars, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-684, https://doi.org/10.5194/epsc2022-684, 2022.

The composition of the Martian atmosphere and its dust content is a key factor for understanding the climate of the Red Planet, which is of vital importance to enable future human exploration. The use of atmospheric LIDARs to characterize densities and sizes of aerosol with a height profile, is commonly used on Earth. However, Earth LIDARs are heavy and very power-demanding, which make them not easily on-boardable for planetary exploration.

We propose the development of a compact LIDAR aimed at providing the most precise characterization of the suspended dust and clouds of the atmosphere of Mars to-date, while maintaining very reduced power, mass and volume envelops to facilitate its accommodation in a wide variety of landed assets.

Rationale and heritage

In the past years, INTA has developed four miniature radiometers for different missions: MetSIS for MetNet Lander [1], DREAMS-SIS for Schiaparelli [2], SIS’20 for Kazachok [3] and RDS for Perseverance [4].  All of them, at different levels depending on their complexity, allow the estimation of the atmospheric optical depth with high time resolution, detection of clouds and estimation of the dust concentration vertical profile. A natural step forward to complement our capabilities, especially for the dust profile and characterization of clouds, is a LIDAR.

There is only one LIDAR that has already operated on Mars, on board the Phoenix mission. With a 2-wavelengths configuration at 532 and 1064 nm but without depolarization, it operated in the North Pole of Mars for 90 sols [5]. The laser pulses had a power of 30-40 kW and duration of 10 ns. Total mass was 6 kg and the power consumption 30W.

A '2B+1d' LIDAR configuration is proposed here to obtain the Particle Backscatter Coefficient (PBC) at two wavelengths (2B), and the depolarization ratio at one of them (1d). They both are used to obtain three LIDAR parameters relevant for dust and cloud characterization: the particle linear depolarization ratio (PLDR), the lidar ratio (LR), and the colour ratio (CR). The intended range is set around 20 km, depending on the scenario, and depolarization measurement capability shall be incorporated.

Key technologies for miniaturization

The target mass and power envelop are <4 kg and <15W, respectively. The instrument should be capable of operating at atmospheric temperatures below -100°C to minimize the requirements for its accommodation. Achieving those goals will be a technological challenge, and it will be necessary to use technologies not employed before in this scenario. Several key aspects are identified:

• Pulsed semiconductor laser diodes are proposed for the transmitter, instead of usual (and bulkier) diode-pumped, Q-switched solid-state lasers. This reduces the emitted energy and in turn requires higher repetition rates to improve the Signal-to-Noise ratio.
• Silicon Photomultipliers, also known as Multi Pixel Photon Counters, will be used as detectors. They allow lower biasing voltages than other high-gain technologies, while they show some trade-offs such as their so-called "cross talk" noise. Its performance under the applicable thermal and radiation environments must be addressed.
• The use of new synthesis methods to obtain very low thermal expansion coefficient (CTE) materials (crystalline β- eucryptite) to reduce the thermal requirements related to structure and alignment.
• To reduce the size of the optics and to mitigate the complexity of the collimation of a semiconductor laser (with large spots and emission apertures), 'free form' optics will be considered in the optical design.
• From the signal processing point of view, the aim is to design a receiver that autonomously adapts the measurement mode (analogue or pulse counting) and parameters (e.g., variable time intervals for pulse-counting) in real time, providing the best signal quality vs. vertical resolution compromise for each altitude.

Feasibility

To preliminarily assess the feasibility of this LIDAR to reach a scientifically meaningful range, we have performed several simulations of the expected signal and signal-to-noise ratio using extinction and backscattering profiles representative of different dust conditions on Mars, together with technical data extracted from the datasheets of real pre-selected parts and opto-mechanical parameters values that we consider feasible for a compact instrument as the intended one. Emitted pulses have a power of 600W and a duration of 150ns. We have simulated three different background illumination scenarios: night, twilight, and midmorning, as this is critical from the point of view of the “offset” signal it generates (that must be filtered out) and the noise associated with it. We employed worst-case (high) diffuse radiance values on Mars. Extinction and backscattering coefficients have been extracted from Phoenix LIDAR measurements [6], and background illumination values from RDS measurements in Perseverance.

The conclusion is that, with adequate signal processing, we should be capable of reaching 4 km during the day, 6-7 at twilight and 15 at night, for the worst-case scenario. 20 km are feasible in better scenarios.

On-going activity and next steps

A breadboard prototype of the system is being built at INTA for only one channel (one wavelength; no polarization) based on pre-selected pulsed laser diode and silicon photomultiplier, with standard off-the-shelf optical parts and filters. Preliminary laboratory and outdoor measurements will be done to validate the simulations and confirm the feasibility of reaching a meaningful range with the strict miniaturization constraints.

Low-temperature characterization of the emitters and detectors, as well as Displacement Damage tests to assess their robustness to the radiation environment will be done by the end of the year. The electronic design has a large heritage from previous Martian sensors as the ones mentioned above, all of them designed to operate down to -130ºC with no heating. Next steps will include thermal cycling testing of the optics and the support materials and feasibility analysis of the optomechanical frame to face such conditions.

[1] Ari-Matti Harri et al., Geosci. Instrum. Method. Data Syst., 6, 103–124, 2017.

[2] I. Arruego et al., Advances in Space Research 60 (2017) 103–120.

[3] I. Arruego, Proc. IPPW 2018.

[4] V. Apéstigue et al., Sensors 2022, 22, 2907

[5] J. A. Whiteway et al., Journal Geophys. Res., Vol. 113, E00A08.

[6] J. A. Whiteway, et al., Science 325, 68 (2009).

How to cite: Arruego, I., Jiménez, J. J., Martín-Ortega, A., García, E., Rivas, J., Carrasco, I., Vázquez, G., Córdoba-Jabonero, C., Gómez, L., Toledo, D., Belenguer, T., González, L. M., Moya, A., Whiteway, J. A., Daly, M. G., Scaccabarozzi, D., Saggin, B., Fernández-Valdés, A., Heckl, A., and Fuchs, U.: Miniature LIDAR for Mars Exploration, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-835, https://doi.org/10.5194/epsc2022-835, 2022.