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The NASA DART and the ESA HERA missions aim to provide an experiment in asteroid defection, though a kinetic collision. DART spacecraft will be sent to collide at high speed, approximately at 6.6 km/s, with the smaller asteroid, usually called Didymoon of the binary asteroid system Didymos. HERA spacecraft will be sent to study the effects of the impact, so that our knowledge of the energy transmission due to the collision is improved. HERA spacecraft will evaluate Didymoon orbit change, structure of the asteroid, crater size [1].
HERA spacecraft carries several payload instruments to provide these studies, namely: Cameras, Radar, Satellite-to-Satellite Doppler tracking, LIDAR, Seismometer and Gravimeter.
In this work we report the LIDAR, also known as PALT for HERA, conception, design, and manufacturing process that is currently ongoing, as well its scientific aims and contribution to spacecraft navigation.
PALT is a ToF altimeter that provides time tagged distances measurements. The instrument can be used to support near asteroid navigation and provides scientific information (e.g. asteroid 3D topography and fall velocity) and also reports the power of the received pulse being possible to calculate the target reflectivity.
PALT first version EM is based on a Laser Landing Altimeter Engineering Model developed by EFACEC and Faculdade de Ciências, Universidade de Lisboa (FCUL), in the frame of an ESA NEO-MAPP project. THE PALT comprises a compact low power consumption microchip laser that emits 1.5 µm light pulses and a low noise sensor. This laser technology enables rangefinder compact designs. The synergies between these two technologies enable the development of a compact instrument for range measurements of from 500 m to 14 km with a low power consumption and envelope of 12 cm×15 cm×10 cm. The PALT electronics was designed to endure a TID of 100 krads.
PALT has four main blocks, power supply, processing unit, electronics frontend, ToF optical front end. Optical front end is composed by emitter and receiver.
Power supply uses a traditional flyback solution, optimised for the altimeter secondary powers consumption and outputs filtering.
Processing unit is based on a FPGA since it simplifies the process of keeping precise timings, required to operate the ToF unit. FPGA is also responsible to perform all the housekeeping acquisitions, to monitor the health of the altimeter and for the interface with the spacecraft, via Universal Serial Link.
ToF is the key block of the LIDAR altimeter with respect to its accuracy and precision. This unit is responsible to time tag all the laser emitter pulses as well as all the APD receptions, with a precise timed tag that will be then managed by the processing unit FPGA to compute the distance.
Frontend Electronics is responsible for the Laser power supply and triggering, also for the Laser pulses digitalization (emitted and received).
The preferred LASER source for PALT is currently being developed at FCUL. The laser used as source is a diode pumped, passively Q-switched Yb-Er Microchip Laser targeting a 100 μJ Gaussian pulse with a FWHM of 2 ns. The backscattered radiation is a gaussian pulse shape.
The main optical specifications of the optical front end follow the receiver, emitter, and filter parameters. The receiver optical aperture diameter and obscuration are 100 mm and 30 mm, respectively. The FOV receiver has 1.5 mrad value, a transmittance of 0.91; a sensor with a 230 kV/W responsivity. Relatively to the emitter properties, it has a FOV of 1 mrad and optics transmittance of 0.94. The energy budget was calculated using (1), which allowed an estimation of the magnitude of the returned power [2]:
Er≈ ETR rs⁄π Ar⁄D2 τR OV (1)
where ETR is the emitter transmittance, rs is the asteroid reflectance, Ar is the telescope area, D is the distance, τR is the receiver transmittance and OV is the overlap.
Considering the emitted laser pulse has FWHM of 2 ns and Gaussian shape, the receiver power can be calculated.
The returned peak power Figure 1 along with saturation limits of the sensor and minimum detectable power considering solar background, sensor NEP and M=20.
Figure 1. Detected peak power (higher and lower bound represents a 0º and 20º surface inclination).
The most critical component of the optics front end is the receiver telescope. The receiver telescope has a Cassegrain design. The primary mirror is made of zerodur and has 100 mm diameter. The secondary mirror is assembled on a carbon fiber tripod structure, the telescope ray tracing (zemax design), the footprint (on sensor) for different operating distances (Figure 2).
Figure 2. (a) Telescope ray trace; (b) spot diagram for several raging distances.
The LIDAR has to withstand the launcher load and maintain integrity and performance of the optical receiver telescope (Figure 3) (a) system; (b) present a light telescope structure that withstand launch vibrations.
Figure 3. (a) Lidar system 3D CAD design; (b) Vibration simulation of telescope structure.
Acknowledgments
This paper has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement Nº. 870377 (project NEO-MAPP).
References
[1] P. Gordo, D. Seixas, B. Couto, A. Amorim, B. Livio, R. Melício, A. Marques, T. Sousa, C. Pinto, G. Tezeremes, P. Michel, M. Küppers, I. Carnelli, "HERA lidar instrument development", Proc. of the 4th Symposium: Small Satellites for Sustainable Science and Development, pp. 1–6, Herzliya, Israel, 4–8 November 2019.
[2] J.L. Bufton, "Laser altimetry measurements from aircraft and spacecraft", Proceedings of the IEEE 77(3), pp. 463–477, March 1989.
How to cite: Gordo, P., Dias, N., Couto, B., Arribas, B., Amorim, A., Livio, B., Melicio, R., Marques, A., Sousa, T., Granadeiro, V., and Michel, P. and the NEO-MAPP Team: Planetary Altimeter for HERA Development, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-302, https://doi.org/10.5194/epsc2020-302, 2020.
The AIDA (Asteroid Impact Deflection Assessment) mission involves DART (Double Asteroid Redirection Test) and Hera spacecraft. DART is led by the NASA and will impact the smaller body of Didymos, a binary asteroid. HERA [1], on the other hand, is a European project and will rendezvous Didymos in 2026 following up the impact produced in 2022 by DART.
Didymos is a near-Earth binary asteroid composed by Didymain, the main asteroid, and its moon called Didymoon. Didymain has an estimated radius of 387 m and a rotation period of 2.26 hours. Didymoon, whose approximate radius is 103 m, has a circular tidally locked orbit of 1180 m around Didymain and an orbital period of 11.92 hours.
The Austrian contribution to Phase B2 Part 1 of the HERA mission was carried out by JR, VRVis, and two science collaborators under GMV contract. It consisted mainly of the design of tools needed to help define optimized images of Didymos (Didymain and Didymoon) for tactical and strategic purposes as well as to determine the asteroid shape. For both purposes, the further development of PRoViP [2] and PRo3D [3], tools already available for the ExoMars mission, was required. To validate the 3D reconstruction chain, a simulation approach was designed and implemented, see Figure 1:
Image rendering
In order to generate synthetic images of Didymos in the most representative and faithful way, the information about the geometry of the Asteroid Frame Camera (AFC) aboard the HERA spacecraft, its planned positions and orientations along the available trajectories (e.g.: Early Characterization Phase –ECP–, Detailed Characterization Phase –DCP-1–, Detailed Characterization Phase –DCP-2– and Very Close Fly-Bys –VCFB–), the shape parameters of Didymos as well as solar illumination direction had to be considered.
For image rendering, the PRo3D Viewer [3] was used and interfaced via a .json file with these parameters for each individual image to be rendered, including information about the AFC geometry as "fieldOfView" and "resolution" entries. AFC poses, Sun, Didymain and Didymoon positions and orientations were established by means of the HERA Spice Kernels, delivered by GMV, where the barycenter of Didymain is defined as the origin of the coordinate system and its position is always fixed at (0, 0, 0).
For rendering with PRo3D, the shape and dimensions of Didymos were necessary and provided as *.txt files by GMV (Figure 2: Left Bottom). To enhance the realisms of the synthetic images JR added texture and relief to this data (Figure 2, right). For this purpose, the texture and 3D information of Earth Moon surface (as arbitrary texture to test the workflow) were used (Figure 2, Left top and Left centre). In this step, the *.txt files were converted into *.opc (Ordered Points Cloud) files [4], i.e.: the mandatory format for the PRo3D Viewer.
A subset (463/731) of the resulting rendered images of Didymain along DCP-1 trajectory can be seen in Figure 3. The HERA approach to Didymain (from 23 to 10 Km) along the trajectory can be observed.
Shape reconstruction
In order to determine the Didymain shape, the images rendered with PRo3D (e.g.: Figure 3) were supplied to ColMap [5] tool, embedded in PRoViP. ColMap performs 3D reconstruction by means of Structure from Motion (SfM) technique, which assumes that the object to be reconstructed (Didymain) or the sensor (AFC) remains motionless. As Hera AFC moves along the trajectory (e.g. DCP-1) at the same time that Didymain performs its rotation movement, this assumption is violated. In order to establish correct SfM conditions, all AFC poses and orientations calculated with the Spice kernels had to be transformed with the inverse rotations of Didymain to simulate a moving camera around a static scene. Once the rotations were computed the new transformation matrices were provided to ColMap in the “image.txt” file, with following format:
<#>
“#” denotes the frame number, “Q0-Q3” specify the rotation in (Hamilton) quaternion notation and “T0-T2” the translation for the current frame. “CID” indicates the used camera, for this case always the AFC and “filename” defines the corresponding image file, i.e.: the rendered image. In the same way, the intrinsic parameters of the AFC were given in the “cameras.txt” file, which format is as follows:
<#>
Here “#” specifies the camera id (i.e.: the AFC). “CAMERA_TYPE” defines the used camera model (i.e. Pinhole, Radial etc.), set to SIMPLE_PINHOLE for the AFC. “Width” and “Height” define the image width in pixels following the AFC specifications (1024 by 1024 pixels) and “PARAMS” contains (in the case of SIMPLE_PINHOLE cameras) the FoV in pixels and the principal point.
The reconstructed camera poses calculated with ColMap are represented in Figure 4. They are expressed in the Didymain reference frame and, as mentioned before, do not represent the real AFC positions along the DCP-1 trajectory as calculated with the kernels, but the transformed cameras’ positions required for SfM.
Several views (9) of the resulting 3D reconstructed dense Didymain model (“Didymain_DCP1_meshed-poisson.ply” file) are represented in Figure 5. The resulting statistics of the reconstruction can be seen in Table 1:
Conclusion
The Austrian contribution to Phase B2 Part 1 of the HERA project revealed by means of a simulation approach that PRo3D and ColMap are powerful and flexible tools that were further developed to fulfil requirements for 3D-reconstruction of small bodies (asteroids) in space exploration projects.
[1] – Pellacani, Graziano, Fittock, Gil, Carnelli (2019). HERA vision based GNC and autonomy. 8th European Conference for Aeronautics and Space Sciences (EUCASS).
[2] – Paar, Deen, Muller, Silva, Iles, Shaukat, Gao (2016). Vision and image processing. In Contemporary Planetary Robotics: An Approach Toward Autonomous Systems, John Wiley & Sons.
[3] – Barnes, Sanjeev, Traxler, Hesina, Ortner, Paar, et al. (2018). Geological analysis of Martian rover-derived Digital Outcrop Models using the 3D visualisation tool, Planetary Robotics 3D Viewer - Pro3D. In Planetary Mapping: Methods, Tools for Scientific Analysis and Exploratio.
[4] – Brunnhuber, May, Traxler, Hesina, Glatzl, Kontrus (2017). Using Different Data Sources for New Findings in Visualization of Highly Detailed Urban Data. REAL CORP 2017 Proceedings
[5] – ColMap tool.
How to cite: Caballo Perucha, M. P., Nowak, R., Hafner, P., Klopschitz, M., Pellacani, A., Fritz, L., Paar, G., and Traxler, C.: Didymos images´ simulation for 3D reconstruction within HERA project, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-123, https://doi.org/10.5194/epsc2020-123, 2020.
The Hera mission is the planetary defence mission approved under the ESA Space Safety Programme. Together with the NASA DART mission, it is the first Asteroid Deflection Demonstration. DART will impact Dimorphos, the natural satellite of Near Earth Asteroid Didymos, in 2022. Using the kinetic impactor technology, it will demonstrate the ability to change the trajectory of a small asteroid. Hera will launch in 2024 and arrive in the Didymos system at the end of 2026. Its primary goals are to accurately measure the mass of Dimorphos and to characterise the crater and effect of the DART impact on this small asteroid.
Hera will spend over 2 years matching orbits with Didymos, travelling through near-Earth space and the inner main-belt. This potentially allows Hera to perform an additional asteroid flyby enroute to Didymos. For such a flyby, a number of factors need to be considered for target selection. Primary factors are the accuracy of the orbit of the asteroid, and distance from the nominal trajectory. Resolution of geomorphology requires a minimum number of spatial resolution elements at the desired flyby distance. Previous observations on the form of lightcurves, colours or spectra are important for flyby science planning. Finally, a dynamical class of asteroid may be targeted for specific science priorities.
Analysis of the nominal trajectory shows approximately 100 asteroids passing the nominal position of Hera within 0.02 au. Ten of these potential targets are Near-Earth Asteroids, the rest being Mars-crossers and main-belt asteroids. We will present our current knowledge on these targets, and highlight the highest priority targets for further observation in the next 2-3 years. Although ground-based reconnaissance has started on some targets, further observations are required to support the flyby planning process.
Figure 1. Osculating orbital elements of asteroids passing close to Hera on its nominal trajectory to the Didymos system, without any additional trajectory adjustment. Red dots indicate a current flyby distance d < 0.01 au, blue circles indicate a current flyby distance 0.01 au < d < 0.02 au. The solid line indicates a perihelion distance of 1.3 au, asteroids to the left of this line are Near-Earth Asteroids.
How to cite: Fitzsimmons, A., Khan, M., Küppers, M., Michel, P., and Pravec, P.: Potential Flyby Targets for the ESA Hera Mission, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-1064, https://doi.org/10.5194/epsc2020-1064, 2020.
Introduction:
Earth is continuously impacted by space debris and small asteroids, and, while large asteroid impacts are very rare, they have the potential to cause severe damage. NASA's Double Asteroid Redirection Test (DART) aims to be the first mission to test a controlled deflection of a Near-Earth binary asteroid [1, 2], by impacting the smaller component of the 65803 Didymos asteroid system, Dimorphos. The impact will thereby alter the binary orbit period by an amount detectable from Earth [3].
ESA's Hera mission [3, 4], that will arrive at Dimorphos several years after the DART impact. It will rendezvous with the asteroid system and perform detailed characterisation of Dimorphos's volume and surface properties, as well as measure the DART impact outcome, such as change in the binary system orbit and the volume and morphology of the DART impact crater.
In high velocity impacts on an asteroid the change in momentum of the asteroid ΔP can be amplified by the momentum of crater ejecta that exceeds the escape velocity, which is often expressed in terms of the parameter β=ΔP/mU, where mU is the impactor momentum [5]. The amount by which crater ejecta enhances asteroid deflection-that is, the normalised momentum of the crater ejecta that escapes the gravitational attraction of the target body (β-1)-has been found to vary significantly depending on the target asteroid's properties and composition [6].
Previous numerical simulations [7, 8] have quantified the sensitivity of the asteroid deflection to target material properties. To allow for a large variety of material properties to be studied, these simulations employed a two-dimensional shock physics code with an axially-symmetric mesh geometry, which restricted the studies to vertical impacts only. However the DART spacecraft will impact the surface of Didymoon at an oblique angle [3]. Here we investigate the influence of impact angle on the ejecta momentum transfer with the aim of developing an empirical scaling relationship for β as a function of impact angle.
Numerical methods:
We used the iSALE3D shock physics code [9] to numerically simulate the DART impact in two and three dimensions. The DART spacecraft structure was modelled as a porous aluminium sphere, impacting a 20% porous, homogeneous basaltic regolith target at 7 km/s. The cohesive strength of the damaged material was 10 kPa.
Influence of the impact angle on the net momentum:
Consistent with previous laboratory-scale oblique impact experiments [10, 11] and DART impact models [12], our simulations show that the ejecta from oblique impacts displays higher speeds and lower ejection angles in the downrange direction, and lower speeds and higher ejection angles in the uprange direction of the impact.
Figure 1 compares the surface topography of a vertical DART impact, at a 90o angle of incidence to the target plane, and an oblique DART impact at a 45o angle. The time-frames of the oblique impact Figure 1b show a highly asymmetric ejecta distribution at early times of the cratering process <0.10 s, compared to the same times in the vertical impact (Figure 1a). The asymmetric ejecta flow becomes more symmetric as the crater grows towards its final size.
The asymmetry of the ejecta can have important implications for momentum transfer vector. The net momentum of the target after the impact is the vector sum of the impactor momentum and the crater ejecta momentum enhancement vectors. The projectile imparts an its momentum along the impact direction; that is, downrange and into the target. As most of the ejecta momentum is launched in the downrange direction, the net momentum imparted to the target, which acts in the opposite direction, acts downward and slightly uprange. The vector sum of the impactor momentum and the momentum enhancement vectors is in between the individual vectors, implying that the ejecta momentum acts to increase the vertical component of momentum transfer and reduce the horizontal downrange component.
Figure 2 shows the direction of the total momentum imparted to the asteroid, as a function of time, measured relative to the downrange horizontal direction, for four different impact angles. As the crater grows towards its final diameter (at about 1 s), the uprange direction of the ejecta momentum becomes more perpendicular to the surface. The direction of the net momentum imparted to the target also changes, from the downrange direction, towards the vertical direction. In the scenarios simulated here, for impacts into a 10 kPa target, the direction of the net momentum at the end of the crater growth is about 83o for the 60o impact, ~77o for the 45o impact and ~66o for the 30o impact.
In the simulations presented here, crater growth is halted by the target's strength before the total momentum direction becomes vertical. However, it is expected that with increasing cratering efficiency (e.g. decreasing strength), the ejecta momentum will make a larger contribution towards the total momentum vector.
Towards an ejecta scaling relationship for oblique impacts:
Ejecta scaling relationships are useful to predict the ejecta distribution and momentum transfer for vertical impacts. However, most planetary impacts are oblique and stationary point-source scaling [6] becomes inadequate. An aim of this work is to develop an ejecta scaling relationship for oblique impacts, based on numerical simulation data.
Three-dimensional simulations of the DART impact at vertical, 60o, 45o and 30o impact angles provide information about the ejecta mass-velocity distribution as a function of impact angle and azimuth. The momentum carried away by the ejecta from a vertical impact, β-1, can be found from integrating the mass, dM, within the radial distance range from n1 to n2R/a [1]. Here we apply this semi-analytical equation on a per azimuth basis and then sum each azimuthal ejecta distribution to derive β-1 values from all ejecta in oblique impact scenarios (Fig. 3).
Acknowledgements: We gratefully acknowledge the developers of iSALE (www.isale-code.de) and STFC for funding (Grant ST/N000803/1).
How to cite: Raducan, S. D., Davison, T. M., and Collins, G. S.: Momentum transfer from oblique impacts, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-837, https://doi.org/10.5194/epsc2020-837, 2020.
In order to study the mechanical properties of a small body surface, accelerometers can be used to record the acceleration profile during landing and rebounding of a surface package. However, given the low gravity environment, the behaviour of the grains can be different to under terrestrial gravity [e.g., 1].
Here we present data from experiments of low-velocity impacts of projectiles of various shapes into different types of granular material in both normal and reduced gravity. Then, to further investigate the dynamics, we employ a collisional model in order to fit the experimental data using an empirical force law involving both a hydrodynamic drag force term and a static resistance force term. The contributions of these two force terms are discussed and compared for the different configurations.
In terms of collision frequency with the Earth, the most threatening NEOs are the smaller ones (< 1 km diameter). They are also the less well known, because ground observations cannot provide adequate information at these sizes. The JAXA Hayabusa 1 and 2 missions [2, 3], and the NASA OSIRIS-REX [4] missions, have highlighted the complex histories experienced by small bodies. The unintuitive nature of the asteroid response to the Small Carry-on Impactor experiment of the Hayabusa 2 mission [5] also demonstrated the importance of interacting directly with the surface in order to constrain the mechanical properties. The upcoming ESA Hera mission (launch planned for late 2024; [6]) to the binary asteroid Didymos is going to be a great opportunity to study an even smaller asteroid of 150 m diameter.
This work is carried in the framework of the NEO-MAPP project (Near Earth Object Modelling and Payloads for Protection), which aims to provide significant advances in our understanding of the response of NEOs to external forces. In the context of the Hera mission, one goal of the NEO-MAPP project [6] is to better understand the physics of low-velocity collisions, and the mechanical properties of the regolith that composes the surface of asteroids. These points are crucial for understanding the evolution of small body surfaces, the main processes that have shaped the surfaces, and for the design and operations of space missions.
Our objective is to use experimental data to obtained in both static and low-gravity trials, to develop a theoretical framework to describe low velocity collisions into granular material under varying gravity conditions.
Following the previous work of [1], low-velocity impact experiments were performed using two different projectile shapes (spherical and cubic) and four different granular surface materials (quartz sand of 1.5 mm, 5 mm, and 10 mm glass beads). The projectiles, shown in Figure 1, weigh approximately 1 kg and are made of aluminium. The cubic projectile can be oriented to fall on a face, an edge or a corner. The experiments were performed under both Earth and reduced-gravity conditions (~0.02 – 0.1g effective gravitational acceleration), using a static laboratory set-up and an Atwood-type drop tower [7], respectively. During all trials, in-situ accelerometers were mounted inside the projectiles to measure the in-situ acceleration profile during the impact.
From the recorded data, we extracted three key parameters: the maximum acceleration, the final penetration depth, and the collision duration. Figure 2 shows the measurements for the experiments performed using the spherical projectile at several drop heights, giving a range of collision velocities. Similarly to [1], the data present a quadratic trend for the maximal acceleration. The post-collision penetration depth was found to be linearly-dependent on the collision velocity, and the collision duration was found to be independent of the collision velocity, at least for collision velocity larger than 0.4 m s-1.
We employ a collisional model [8] to fit the data, using an empirical force law involving both a hydrodynamic drag force term (h(z)) and a static resistance force term (f(z)). To fit the data, one must assume specific forms for h(z) and f(z), typically that they are constants [9]. However, the scatter of the data makes this assumption difficult to consider. Instead, we reformulate the force-law model into a linear differential equation in kinetic energy [10]. This solution provides a natural way to experimentally measure f(z) and h(z), with no assumptions about the functional form of these terms. The contributions of these two terms are discussed and compared considering the different projectiles, surface materials and gravity conditions. The behaviour of the granular medium in interaction with the probe, in a very low gravity environment, is an important opportunity for mathematical models combining the dynamics of shocks, frictions and deformations. These models can find applications in various fields of engineering.
[1] Murdoch, N., et al. “An experimental study of low-velocity impacts into granular material in reduced gravity.” Monthly Notices of the Royal Astronomical Society, 468 (2), p. 1259-1272 (2017).
[2] Fujiwara, Akira, et al. "The rubble-pile asteroid Itokawa as observed by Hayabusa." Science 312.5778 (2006): 1330-1334.
[3] Watanabe, Sei-ichiro, et al. "Hayabusa2 mission overview." Space Science Reviews 208.1-4 (2017): 3-16.
[4] Lauretta, D. S., et al. "OSIRIS-REx: sample return from asteroid (101955) Bennu." Space Science Reviews 212.1-2 (2017): 925-984.
[5] Saiki, T., Imamura, H., et al. “The Small Carry-on Impactor (SCI) and the Hayabusa2 Impact Experiment”. Space Science Reviews, 208, p. 1-22 (2016).
[6] Michel, P., Falke, A., Ulamec, S., and the NEO-MAPP Team. “The European Commission funded NEO-MAPP project in support of the ESA Hera mission: Near-Earth Object Modelling And Payload for Protection”, EPSC abstract, Vol. EPSC2020-103 (2020).
[7] Sunday, C., et al. "A novel facility for reduced-gravity testing: A setup for studying low-velocity collisions into granular surfaces." Review of Scientific Instruments 87 (8), p. 084504 (2016).
[8] Katsuragi, H. “Physics of Soft Impact and Cratering”, Lecture Notes in Physics, Springer Japan (2016).
[9] Goldman, D. I. and Umbanhowar, P. “Scaling and dynamics of sphere and disk impact into granular media”. Phys. Rev. E, 77 (2), p. 021308 (2008).
[10] Bester, C. S., and Behringer, R. P. “Collisional Model of Energy Dissipation in Three-Dimensional Granular Impact.” Physical Review E, 95 (3), p. 032906 (2017).
How to cite: Drilleau, M., Murdoch, N., Sunday, C., Nguyen, G., Thuillet, F., and Gourinat, Y.: Experimental and theoretical investigations of low-velocity collisions into granular material, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-582, https://doi.org/10.5194/epsc2020-582, 2020.
