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).