How to cite: Müller, T., Conversi, L., Licandro, J., Delbo, M., Fitzsimmons, A., Muinonen, K., Popescu, M., Tanga, P., and Moissl, R.: Detection of Chelyabinsk-type objects in the thermal infrared, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-518, https://doi.org/10.5194/epsc2024-518, 2024.

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Most current and planned near-Earth objects (NEO) surveys are made with ground-based telescopes and carried out in the visible wavelength range. However, this approach has some limitations, such as (1) weather dependency, (2) only a portion of the night sky is visible from any given location on Earth, (3) NEOs are difficult to detect at low galactic latitudes and (4) visible-light surveys can only determine the motion and apparent magnitude of an object, but its physical properties (such as size) can only be inferred indirectly and therefore require additional observations for characterisation.

The European Space Agency (ESA) is studying a NEO Mission in the Infra-Red (NEOMIR) in the framework of its Space Safety Program (S2P). NEOMIR is a space-based mission working in the thermal infrared and placed at the first Sun-Earth Lagrange point (L1), aimed of detecting and characterising new NEOs and – in particular -- focusing on the cases of imminent impactors. NEOMIR can thus serve as an early NEO impact warning system. NEOMIR is designed to overcome most of the ground-based issues discussed before, by regularly scanning an area not easily accessible from ground or other space-based NEO surveys and using modern detection techniques optimized to detect fast moving objects. NEOMIR is designed to discover the smaller NEO population, which could only be observed from ground-based surveys when close to Earth

A preliminary study of NEOMIR was performed in 2022. In this initial phase-0, various mission scenarios have been analysed by ESA’s Concurrent Design Facility (CDF). The outcome consisted of a mission scenario that has been further investigated via two industrial contract(s) during 2023. Also, in 2023, ESA established a Science Advisory Group (SAG), comprising a team of experts from multiple European institutions and coordinated by the Instituto de Astrofísica de Canarias (IAC). The purpose of this group is to offer scientific consultancy to the phase-0 industrial studies.

In this work, we will present the mission requirements and spacecraft design based on the industrial studies and SAG advice, the status of the project as well as initial results on the expected detection capabilities.

How to cite: Licandro, J., Conversi, L., Delbo, M., Fitzsimmons, A., Muinonen, K., Mueller, T., and Popescu, M.: NEOMIR: a space based infrared mission for NEO detection, characterization and early warning, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-882, https://doi.org/10.5194/epsc2024-882, 2024.

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Circular aperture photometry, is a crucial method utilized in astronomical observations for measuring the flux emitted by point sources [1]. Its utility spans diverse astronomical phenomena, encompassing tasks such as determining the absolute magnitude and spin period of asteroids [2]. In conducting traditional circular aperture photometry on objects in motion, it's essential to adjust the exposure time to prevent both the moving object and the stars from appearing elongated in the image. 

When dealing with near-Earth objects (NEOs) traditional aperture photometry is challenging due to their rapid motion on the sky (from several arc-seconds per minute to several arc-degree per minutes during close fly-bys). Such rapid angular motion necessitates brief exposure times, resulting in a significant portion of observation time being consumed by CCD read-out rather than actual on-sky observations. This limitation has hindered the detection of extremely rapid rotation periods. The quickest rotation period, P = 2.99 s, was identified using a rapid read-out camera [3].

Hence, when limited to a CCD with slow read-out time, the most straightforward and dependable method for acquiring high-quality data on these fast moving objects is to conduct sidereal tracking observations, enabling the asteroid to appear as a trail in the images.

We are presenting here a novel aperture photometry method to obtain photometric measurements on trailed observations of asteroids. Our strategy capitalizes on the rapid motion of the object across the sky while keeping the stars sharp. This enables us to employ standard circular apertures for the stars, facilitating both photometric and astrometric field calibration. As for the asteroids, they are spanning hundreds of pixels on the image as we need a new approach to extract their photometric variation. 

For conducting aperture photometry on trailed asteroid observations, we employ a square or rectangular aperture oriented parallel to the direction of the NEO's trail. This method optimizes the signal-to-noise ratio of the extracted photometry within a limited portion of the trail. Subsequently, we traverse along the trail to gather photometric data over time.

We applied our new technique on three recently observed targets at the Schiaparelli observatoty and with the TRAPPIST-North telescope [4]. The first two targets, 2023 CX1, 2024 BX1, impacted the Earth shortly after our observations while 2024 EF perfomed a very close fly-by.

For 2023 CX1, minimal photometric variation was noted, suggesting either a spherical shape or minimal rotation during our observation span. A double-peaked phase curve was obtained for a spin period of 18.33 seconds. In contrast, our analysis of the 2024 BX1 observations  revealed it to be the fastest rotating asteroid to date, with a measured period of 2.5888 +- 0.0002 seconds. The 0.7 magnitude amplitude suggest an elongated shape. 

For 2024 EF, we detected a rotation period of around 3.95 minutes through standard asteroid tracked observations. However, we demonstrate that photometry extracted from a single 90-second exposure trailed image aligns accurately with the regular observations when phased. 

[1] Howell, S. B. 1989, Publications of the Astronomical Society of the Pacific, 101, 616
[2] Mommert, M. 2017, Astronomy and Computing, 18, 47
[3] Beniyama, J., Sako, S., Ohsawa, R., et al. 2022, Publications of the Astronomical Society of Japan, 74, 877
[4] Jehin, E., Gillon, M., Queloz, D., et al. 2011, The Messenger, 145, 2

TRAPPIST-North is a project funded by the University of Liège, in collaboration with the Cadi Ayyad University of Marrakech (Morocco) and the the Belgian Fund for Scientific Re- search (FNRS) under the grant PDR T.0120.21. E. Jehin is a FNRS Senior Research Associate

How to cite: Devogele, M., Buzi, L., Micheli, M., Cano, J. L., Jehin, E., Ferrais, M., Ocaña, F., Föhring, D., Drury, C., Benkhaldoun, Z., and Jenniskens, P.: New aperture photometry technique for fast moving fly-by NEOs: detection of the 2.58s rotation period of 2024 BX1., Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-719, https://doi.org/10.5194/epsc2024-719, 2024.

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Introduction: Kinetic impact deflection is one of the strategies for addressing the threat of near-Earth small body impacts. The DART mission has confirmed the effectiveness of deflection and demonstrated the generation and morphological evolution of ejecta. However, what happens to the remaining asteroid shortly after the impact remains a blind spot for in-situ detection. The propagation of residual kinetic energy within the remaining rubble-pile asteroid, and the possible internal structural damage, are crucial for understanding the subsequent evolution of the remaining asteroid. Using a proof-of-principle numerical model, we simulated the response within rubble-pile asteroids to residual impacts and found that the propagation strongly depends on the inherent internal stress chains. Such chains occur in three- and two-dimensional aggregates and we mainly study this phenomenon in 2D.

Methods: We first establish the 2D granular aggregate model and show the existence of initial stress chains based on the particle-scale stress tensor calculation^1–3. Velocity perturbations are applied in the form of pulses to particles in strong stress chains or weak stress regions, and then we observe the propagation of particle speeds, normal and shear stresses, and contact failure events. The dependences of propagation patterns on the magnitude and direction of perturbation velocity, the location of perturbed particles, and the particle size distribution of aggregates are investigated. All the numerical simulations are employed using discrete element code DEMBody^4–6.

Results: In bi-disperse granular aggregates (Fig. 1a), we observed that particle speed response preferentially propagates along the initial (pre-impact) stress chains than in the direction of perturbation velocity (Fig. 1b). The dynamic normal and shear stresses propagate faster and are higher along the inherent stress chain structure (Fig.1c). According to the probability scatter map in Fig.1d, particles under high dynamic shear stress are highly likely to have initially high stress levels, which means that they belong to initial stress chain structures. The heterogeneity of the initial stress chain structure causes anisotropy in the propagation of impact responses, as demonstrated by the D (it characterizes the mean propagating distance of particle speed response) evolution in the 8 sectors around disturbed particle I (Fig. 2).

Figure 1. (a) Initial stress distribution of the bi-disperse aggregate. Particles are colored by the trace of stress tensor Tr(σ₀) and the negative sign means the compressed state. The redder the color, the larger the stress. Yellow circles and arrows denote the disturbed particles and directions of perturbation velocities respectively. (b) The particle speed (blue-red) at  τ=6.6×10^-4 superpose on the initial stress Tr(σ₀) in grey. Particles with u≤0.01 are invisible. (c) The post-impact particle stress trace Tr(σ). (d) The bi-variate probability scatter map in the ε-ζ plane at τ=6.6×10^-4.  ε=Tr(σ₀)-Tr(σ₀)_med and ζ=σ₁₂-σ_12med with (•)_med  denoting the median value. All the quantities are non-dimensional.

Figure 2. (a) A zoom on Tr(σ₀) around the location I, with the region divided into 8 sectors. (b) The evolution of D in the 8 sectors under impact perturbation b. All the quantities are presented in dimensionless form.

We also noticed that sliding and rolling failure events frequently occur in weak stress particles that are in contact with particles in the initial stress chains (Fig. 3a and 3b). When the magnitude of the perturbation velocity is large, the preference for response propagation remains unchanged, but particles near the disturbed particle exhibit significant displacement, and the particles scattered by the impact (coordinate number = 0) are predominantly distributed in the cavities formed by the initial stress chain structure (Fig. 3c).

Figure 3. Impact perturbation is applied on particle II along the direction d, as shown in Fig. 1a. (a) Particles that undergo sliding failure (yellow) superpose on the initial stress chains. u_impact=1.0, τ=6.6×10^-3. (b) Particles that undergo rolling failure (green). The same condition and moment as panel (a). (c) Particles with coordinate number = 0 (red). u_impact=100.0, τ=6.6×10^-3.

Impact perturbation simulations conducted in bi-disperse aggregates with surface boulders and power-law-distribution aggregates exhibit the same response propagation patterns. All the quantities are presented in dimensionless form.

Conclusion and Discussion: To conclude, the response to impact perturbations propagates preferentially along the initial stress chains, which bear the majority of the load induced by the residual impact kinetic energy. It suggests the impact disturbance may travel further than expected along these invisible paths, rather than dissipated uniformly in all directions from the impact crater. Particles in the weak stress regions enclosed by the initial stress chains are more easily scattered by impacts. Downstream of this work, we will focus on the extent and patterns of structural damage in the remaining rubble-pile asteroids and their dependency on impact perturbations.

Acknowledgments: C.H. is grateful for the hospitality of Imperial College London, where this work was carried out. C.H. is supported by the international joint doctoral education fund of Beihang University. Y.Y. acknowledges the financial support provided by the National Natural Science Foundation of China Grants No. 12272018.

Reference:

1. Ball, R. C. & Blumenfeld, R. Stress Field in Granular Systems: Loop Forces and Potential Formulation. Phys. Rev. Lett. 88, 115505 (2002).

2. Blumenfeld, R. Stresses in Isostatic Granular Systems and Emergence of Force Chains. Phys. Rev. Lett. 93, 108301 (2004).

3. Nicot, F., Hadda, N., Guessasma, M., Fortin, J. & Millet, O. On the definition of the stress tensor in granular media. Int. J. Solids Struct. 50, 2508–2517 (2013).

4. Cheng, B., Yu, Y. & Baoyin, H. Collision-based understanding of the force law in granular impact dynamics. Phys. Rev. E 98, 012901 (2018).

5. Cheng, B., Yu, Y. & Baoyin, H. Numerical simulations of the controlled motion of a hopping asteroid lander on the regolith surface. Mon. Not. R. Astron. Soc. 485, 3088–3096 (2019).

6. Cheng, B. et al. Reconstructing the formation history of top-shaped asteroids from the surface boulder distribution. Nat. Astron. 5, 134–138 (2021).

How to cite: Huang, C., Yu, Y., Cheng, B., King, P., and Blumenfeld, R.: Structure-dependent impact response propagation in residual rubble-pile asteroids after ejecta, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-431, https://doi.org/10.5194/epsc2024-431, 2024.

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On 26 September 2022, the NASA/DART spacecraft performed the first test of a kinetic impactor in the history of human space exploration hitting Dimorphos, the small moon of the Near Earth and Potentially Hazardous Asteroid (65803) Didymos (Cheng et al., Nature 616, 2023; Rivkin & Cheng, Nature Communications 14, 2023). The impact effectively reduced the orbital period of the secondary by 33 minutes (Thomas et al., Nature 616, 2023).                                                                                                   

The Italian Space Agency ASI/LICIACube cubesat witnessed the event in-situ after the release from the mothercraft DART, 15 days before impact (Dotto et al.,  PSS 199, 2021; Dotto & Zinzi, Nature Communications 14, 2023; Dotto et al., Nature 627, 2024).

We used images from the instruments DRACO (Didymos Reconnaissance and Asteroid Camera for Opnav) onboard DART and LEIA (Liciacube Explorer Imaging for Asteroid) oboard LICIACube to search for photometric detections of small objects that may be present in the Didymos sphere of gravitational influence before the impact, to provide hints on the complete dynamical characterization of the system.

This portrait will be compared to the future environment the ESA/Hera mission will encounter at its arrival at Didymos in late 2026 (Michel et al., PSJ 360, 2022).

Our investigation for new light sources around the primary asteroid, performed with optimal photometric source extraction techniques, produced a negative outcome. Therefore, we determined the size limits for any potential object that may be present in the system but undetected in the instruments images, using an estimate of the instruments limiting magnitude, a Monte Carlo code to populate the asteroid Hill sphere of 100,000 virtual satellites, and the asteroid photometrical Lumme-Bowell model and its adaptation to the H-G system (Bowell et al, Asteroids II, 1989). We estimated we found no unknown objects having a diameter larger than about 50 m in the entire Didymos Hill sphere. When considering closer distances from the primary asteroid during the approach phase with increasing spatial resolution, the limiting size gets smaller, reaching a minimum diameter equal to about 2 m within a radial distance of about 2 km from Didymos. Regardless the fact that the dynamical environment of the two linked asteroids is extremely perturbed and unstable for additional bodies, there can be still a small chance that large objects could find a stable niche in the phase space of the dynamical system and survive in long time orbits. Anyway, the null result of our work is a strong indication that this niche is either non-existent or was empty at the time of DART and LICIACube approach.

Our negative result is also very important to the HERA mission science, since it provides a baseline for defining that there were no pre-existing small satellites larger than the derived limits. Thus, whatever HERA will find in orbit there is a strong indication that it should be a consequence of the material ejected in the DART impact, rather than previously acting ejection mechanisms.

This work was supported by the Italian Space Agency (ASI) within the LICIACube project (ASI-INAF agreement AC n. 2019-31-HH.0) and by the DART mission, NASA Contract 80MSFC20D0004.

How to cite: Bertini, I., Della Corte, V., Rossi, A., Barnouin, O. S., Beccarelli, J., Chabot, N. L., Cheng, A. F., Deshapriya, J. D. P., Dotto, E., Farnham, T., Hasselmann, P. H. A., Ivanovski, S. L., Lucchetti, A., Mazzotta Epifani, E., Pajola, M., Palumbo, P., Rivkin, A. S., Tusberti, F., Zinzi, A., and Amoroso, M. and the LICIACube & DART 'Satellite Search' Team: The pre-impact search for further Didymos satellites using DART and LICIACube data, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-772, https://doi.org/10.5194/epsc2024-772, 2024.

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The NASA DART mission impacted the asteroid Dimorphos, the satellite of the asteroid Didymos, at 23:14 UTC on September 26, 2022, as a planetary defense test. This marked the first hypervelocity impact experiment on an asteroid relevant to planetary defense. The Italian Space Agency's LICIACube captured crucial imagery before and after the impact, aiding in studying the ejecta's distribution in inertial space. The data returned by LUKE camera aboard LICIACube thus enabled the characterisation of the orientation and geometry of resultant ejecta [1,2,3], which are closely related to the calculation of how effective the impact has been in imparting momentum to the asteroid (parameter β) [4].  

LUKE images acquired at different vantage points with respect to the ejecta, allowed the characterisation of the global ejecta structured in the aforementioned studies, by approximating it to a hollow cone. Nevertheless, the real structure of the ejecta is intricate and contains complex structures. Given the availability of LUKE images, we attempt to recover the three-dimensional structure of the ejecta by coregistering them to a Dimorphos-centred coordinate system. We trace two-dimensional ejecta shapes [5] as projected onto LUKE images and then place these 2D shapes (scaled with distance) along LUKE boresight at different distances to create depth so that we capture all the possible theoretical planes where different parts of ejecta structure could reside (Figure 1). Once we obtain such 3D profiles for four different images obtained at various vantage points, we intersect them in an inertial J2000 space. The material common to all the intersections then provides us with a three dimensional structure that is compatible with the images used as input to trace two dimensional profiles on the LUKE images. Then, we bring the resulting structure back to LUKE image space and visualise them with the original images to see how they compare. 

Fig.1. Top panel: Left: A perspective view of the three dimensional profile of the ejecta structure recovered using 3 scaled planes (image acquired at '2022-09-26T23:17:03.004' UTC) placed at the location of LICIACube, at the centre of Dimorphos and away from Dimorphos in the same line of sight between LICIACube and Dimorphos. Right: Planes removed to highlight the ejecta structure. Bottom pane: The same for the image acquired at '2022-09-26T23:17:18.000' UTC.

In this way we’re able to validate the three dimensional model we obtained by intersecting three dimensional profiles derived from LUKE images. In order to check its accuracy, the model will be cross-checked against the solutions of Deshapriya et al., 2023 and Hirabayashi et al., 2024 which are compatible with each other. The model will later be refined to remove any resulting artifacts. The result of this work will allow us to understand the three dimensional distribution of ejecta material in inertial space and hence will help constrain models of ejecta propagation following planetary impacts.    

Acknowledgments: This research was funded by the Italian National Astrophysical Institute (INAF) - Call for Fundamental Research 2022. The LICIACube team acknowledges financial support from Agenzia Spaziale Italiana (ASI, contract No. 2019-31-HH.0 CUP F84I190012600).

How to cite: Deshapriya, J. D. P., Hasselmann, P. H., Gai, I., and Dotto, E. and the LICIACube team: Reconstruction of Ejecta Shape from LICIACube Data, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-436, https://doi.org/10.5194/epsc2024-436, 2024.

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Introduction:   The NASA Double Asteroid Redirection Test (DART) impact [1], was the first space mission that successfully demonstrated the kinetic impactor technique for planetary defense. It was at the same instant, on 26^th September 2022, when ASI/Light Italian Cubesat for Imaging of Asteroids (LICIACube) [2] was the first Cubesat to image the plume coming from Dimorphos, the smaller body of the binary asteroid (65803) Didymos. The DART impact into Dimorphos [3] caused ejecta plume propagation with high velocity and very filamentary structure, composed of dust particles from μm to cm sizes in size [4]. The large aperture and observed spikes did not prevent propagation of larger excavated material, namely, boulders up to ~2 m [5]. Far-field observations such as HST clearly showed dust tail formed from the low-speed ejecta dust due to solar radiation pressure (SRP) [4].

The scientific objectives:   The estimation of the size distribution and velocity distribution of the plume in close vicinity to Dimorphos, captured in the LICIACube images is still an unanswered question. While the long-term monitoring of the tail can reveal the size distribution up to tens of cm in size, the impact simulations can constrain the initial velocity of the excavated material. Near and mid – field simulations considering different dynamical properties at local scale can address the complex collimated but inhomogeneous distribution of the dust within the plume. Here, we discuss some of the dynamical properties of the plume using the available observational DART and LICIACube data of the plume propagation. We try to constrain the particle sizes within the collimated plume structures.

The model:   We apply the 3D+t model – LIMARDE [6,7] constrained with laboratory observations [8], impact simulations and near- and far- field observations such as the LICIACube [9] images and HST [2] dust observations, respectively. The model computes single particle trajectories, the dust rotational frequencies and velocity as well as the particle orientation at any time and distance. We compute the dust velocity distribution based on the physical properties (size, mass and shape) derived from the LICIACube observations. The results are useful to check what is the role of the fragmentation of the particles and to constrain the physical properties based on the dynamical properties of the ejected dust in the near- and mid- environment.

Fig. 1. The dust speed and rotation frequency of particles with different shapes as constitutes of the dust clumps shown in the observations of ASI/LICIACube/LUKE, 2022-09-26T23:17:04.

Discussion: The LICIACube observations suggest that we have the locations of accumulation of different particles along the collimated plume streamers. The latter may contain particles of the same density and shape but with different velocity and rotation due to the initial ejected position and form not-linear motion within the collimated filament – like structures. In Fig. 1 we show LIMARDE simulations with particles of different shapes that result with different velocities suggesting a scenario where the dusty clumps could occur at the same location due to motion of particles with different shapes. The study discusses what is the probability that these dust clumps are formed owing to fragmentation, or their location is a result of their motion history of the ejected particles.

Acknowledgements: This research was supported by the Italian Space Agency (ASI) within the LICIACube (ASI-INAF agreement AC n. 2019-31-HH.0).

References: [1] Rivkin, A.S. et al. 2021, PSJ, 2, 24pp; [2] Dotto, E. et al. 2021, PSS 199, [3] Daly, R.T. et al. (2023) Nature. [4] Li, J.-Y., et al. (2023) Nature. [5] Farnham et al. LPSC abs. [6] Ivanovski et al. 2023, u.rev.; [7] Fahnestock et al. 2022, PSJ; [8] Ormo et al. 2022, E&PSL [9] Dotto et al. 2023, Nature

How to cite: Ivanovski, S. L., Hasselmann, P. H., Zanotti, G., Bertini, I., Deshapriya, P., Ieva, S., Lucchetti, A., Pajola, M., Perna, D., Poggiali, G., Dotto, E., Della Corte, V., Herreos, M. I., Ormo, J., Ferrari, F., Li, J.-Y., Amoroso, M., Pirrotta, S., Zinzi, A., and Brucato, J. R. and the LICIACUBE and a part of the DART team: Dust Clumps Dynamics of the Dimorphos Ejecta Plume, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-1168, https://doi.org/10.5194/epsc2024-1168, 2024.

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Introduction: In 2022, the NASA DART spacecraft performed a kinetic impact on the asteroid Dimorphos. This first attempt of asteroid deflection was highly effective and demonstrated the feasibility of modifying an asteroid trajectory. The on-board camera, DRACO, captured images of the binary asteroids Didymos and Dimorphos during the approach. These images showed Didymos to have a relatively smooth equatorial region compared to the polar terrains. Linear groove-like features perpendicular to the equator can be seen.

In this work, we use the DRACO images and boulder tracks visible on the surface to apply a previously validated geotechnical approach [1] and estimate the bearing capacity of the surface of the primary asteroid Didymos. Our findings have been accepted for publication at Nature Communications [2].

Geotechnical properties of asteroids affect their geology and evolution [3] and are important parameters in numerical models of the formation and history of small bodies. Moreover, they are also important for any space mission involving surface operations or interactions [4]. The ultimate bearing capacity is the maximum pressure that a surface can withstand without experiencing shear failure [5].

Methods: Image processing solutions were used to visualize and measure the boulder tracks. We applied a Laplacian filter that increases the visibility of the contours by enhancing the zones with high intensity gradients. To quantify the influence of image resolution on our measurements, we conducted successive degradations of resolution that showed measurement errors up to ~10%. The bearing capacity estimate takes into account a possible error of one pixel (~50% error) on the track width measurements.

The values for Didymos’ size, mass, gravity field and spin rate [6] show a very small effective gravity, that combines both gravitational and centripetal accelerations. The topographic data from [7] provides evidence for a change in gravitational potential on the surface of Didymos. The surface presents a minimum of geopotential at the equator (Fig. 1) due to the fast rotation of Didymos, supporting the hypothesis of the observed lineaments being tracks formed by boulders moving on the granular soil. The coordinates in latitude and longitude of these tracks were measured using the Small Body Mapping Tool [8] to locate them on the gravitational potential map (Fig. 1). The change in gravitational potential along each track is reported in Fig. 1, and shows a lower gravitational potential at the end of all the tracks, indicating that the boulders would have moved towards the equator to minimise their geopotential.

Results: From the DRACO images, 15 possible boulder tracks are identified on the surface of Didymos, shown in Fig. 2b, among which 9 are chosen to be studied further. We measure manually the width and length of each track in the processed images. The track widths have a mean and standard deviation of 8.9 +/- 1.5 m.

Using the track width measurements, we applied the Terzaghi equation, previously applied to the lunar soil [1, 9], that relates the shear strength of the soil to the material properties and the track measurements.  We performed Monte Carlo simulations to account for the uncertainty in the key parameters (friction angle, cohesion and density) which we vary within their expected ranges found in recent studies [6, 7, 10, 11]. We find an average bearing capacity of 70 N/m² (Fig. 3).

Conclusion: The linear features observed on Didymos seem to be tracks formed by boulders moving down the gravitational slope towards the equator. From the 9 tracks identified, we find an average boulder track width of 8.9 +/- 1.5 m. Using a geotechnical approach, we estimated the mean bearing capacity of Didymos to be 70 N/m². This means that every 1 m² of Didymos’ surface can support only ~70 N of force before experiencing shear failure. This is at least 1000 times smaller than the bearing capacity of dry sand on Earth (~10⁵ N/m²). The upcoming ESA Hera mission will return on site in 2026 and provide high resolution images of Didymos. This will provide confirmation that the observed features are indeed tracks formed by boulders.

Our work will also be put into practice through an experimental project that was selected for the ESA Academy Experiment Programme, led by a student team from ISAE-SUPAERO in France. Sinkage experiments in low gravity conditions will be conducted using the ZARM drop tower, in order to measure the bearing capacity of surface materials and test the influence of cohesion. 

Acknowledgements: We acknowledge funding support from the French Space Agency (Centre National d’Etudes Spatiales; CNES), support from the French ANR Tremplin-ERC ‘GRAVITE’, and funding from the European Union (ERC, GRAVITE, 101087060).

References:

1. Bickel, V. T. et al. Analysis of Lunar Boulder Tracks: Implications for Trafficability of Pyroclastic Deposits. Geophys. Res. Planets 124, 1296–1314 (2019).

2. Bigot, J., Lombardo, P. et al. The bearing capacity of asteroid (65803) Didymos estimated from boulder tracks. Nature Communications, accepted (2024).

3. Sánchez, P. Asteroid Evolution: Role of Geotechnical Properties. Proc. Int. Astron. Union 10, 111–121 (2015).

4. Zacny, K. et al. Chapter 8 - Geotechnical Properties of Asteroids Affecting Surface Operations, Mining, and In Situ Resource Utilization Activities. in Primitive Meteorites and Asteroids (ed. Abreu, N.) 439–476 (Elsevier, 2018).

5. Terzaghi, K. Theoretical Soil Mechanics. (J. Wiley and Sons, Inc.; Chapman and Hall, Limited, New York, London, 1943).

6. Naidu, S. P. & Chesley, S. Dimorphos orbit solution, JPL Memo s542. (2023).

7. Barnouin, O. S. et al. The geology and evolution of the Near-Earth binary asteroid system (65803) Didymos. Nature Communications, accepted (2024)

8. Sargeant, H. M. et al. Using Boulder Tracks as a Tool to Understand the Bearing Capacity of Permanently Shadowed Regions of the Moon. Geophys. Res. Planets 125, (2020).

9. Ernst, C. M. et al. The Small Body Mapping Tool (SBMT) for accessing, visualizing, and analyzing spacecraft data in three dimensions. LPSC 49, abstract #1043 (2018)

10. Pajola, M. et al. Evidence for multi-fragmentation and mass shedding of boulders on rubble-pile binary asteroids. Nature Communications, submitted (2024).

11. Robin C. Q. et al. Mechanical properties of rubble pile asteroids (Dimorphos, Itokawa, Ryugu, and Bennu) through surface boulder morphological analysis. Nature Communications, accepted (2024)

How to cite: Bigot, J., Lombardo, P., Murdoch, N., Scheeres, D. J., Vivet, D., Zhang, Y., Sunshine, J., Vincent, J.-B., Barnouin, O. S., Ernst, C. M., Daly, T., Sunday, C., Michel, P., Campo-Bagatin, A., Lucchetti, A., Pajola, M., Rivkin, A. S., and Chabot, N. L.: Estimating the bearing capacity of asteroid (65803) Didymos using boulder track measurements, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-395, https://doi.org/10.5194/epsc2024-395, 2024.

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The Hera mission, which is part of the Space Safety Program of the European Space Agency (ESA), will launch on 7 October 2024 (baseline date) from Cape Canaveral with a Falcon 9 from the Space X company. The launch window runs for 3 weeks. At the time of the presentation, the spacecraft will already be at Cape Canaveral for the launch campaign taking several weeks, after leaving Europe by early September. Once launched and after a first phase of commissioning, the first milestone will be a flyby of Mars in March 2025, which will allow the in-flight calibration of the instruments onboard the spacecraft by taking  images of Mars and Deimos.

Hera will perform a rendezvous with Didymos in the fall of 2026 and investigate it over 6 months. With NASA’s DART mission, Hera will offer the first fully documented asteroid deflection test. DART successfully impacted Dimorphos, the 150 meter-sized moon of Didymos, on 26 September 2022 at approximately 6.1 km/s. The DART impact resulted in a decrease of 33 minutes from the original 11 hours 55 minute orbital period of Dimorphos around Didymos. However, many questions remain and although Hera returns to the system already visited by DART, the properties of the system will have changed in a way that may surprise us. In fact, it is clear that the Dimorphos that we know from DART’s images before impact will be very different when Hera sees it,  reflecting how the DART impact modified it.  Thanks to the great efforts of the DART team, the surface properties of the hemisphere of Dimorphos that was imaged before impact are well known, and it will be extremely interesting to see how those properties were changed by the impact. Among  the questions that Hera will answer, the most important ones are:  (1) What is the mass of Dimorphos,  telling us momentum transfer efficiency of the DART impact? (2) What are the internal properties of Dimorphos, largely influencing  the interpretation of the outcome of the DART impact? (3) What is the final state of Dimorphos, i.e., what is the size of the crater left by the DART impact or was Dimorphos globally or in large parts reshaped by the impact? (4) What is the rotational state of Dimorphos? Did the impact cause it to  tumble?

The Hera mission will provide answers to these important questions that will lead to an unbiased interpretation of the outcome of the DART impact, and the possibility to fully validate numerical impact models aimed at reproducing the impact. With its mother spacecraft, which carries five instruments including a thermal infrared imager contributed by JAXA, and its two cubesats, Juventas, devoted to geophysics and Milani, devoted to mineralogy and dust analysis, Hera will investigate Didymos’ and Dimorphos’ state after the  DART impact in great detail and provide measurements that have never been obtained for an asteroid so far. In particular, thanks to the low-frequency radar JuRa onboard the Juventas Cubesat, the first measurements of subsurface and internal properties of an asteroid will be achieved. Moreover, Hera will also perform the first landing of a Cubesat on a body as small as Dimorphos, offering an opportunity to measure the surface mechanical response of an asteroid in a very low gravity environment. Thanks to its detailed characterization of the binary system, Hera will answer key questions regarding the formation of small asteroid binaries and the geophysics of small bodies.

The working groups of the Hera Science Team are, adjusting their investigation plans to account for the new and important knowledge provided by DART and LICIAcube, to support the asteroid phase, including the possible surprises that Hera will offer during its visit to Didymos in late 2026-2027.

P.M. acknowledge support from ESA and CNES.

About half the size of a Smart car, the cubic Hera spacecraft is seen with its solar arrays folded up on each side. Hera’s ‘top deck’ is visible, hosting most of its instruments, including its Asteroid Framing Cameras – seen left top – and the dispensers for its two CubeSats in the middle. Its main antenna used for communicating with Earth is seen to the right. Credit: ESA - S. Blair

How to cite: Michel, P., Küppers, M., Martino, P., and Carnelli, I.: The ESA Hera mission to the binary asteroid (65803) Didymos: a few weeks from launch, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-56, https://doi.org/10.5194/epsc2024-56, 2024.

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As the first asteroid deflection test, NASA’s successfully hit asteroid Dimorphos (secondary of the Didymos system) with the DART kinetic impactor in September 2022. To fully characterize the physical properties of the objects, and measure precisely the effects of this impact in the context of planetary defense, ESA is launching the Hera mission in October 2024, with scheduled arrival at Didymos in 2026.

Among the core payload of the mission, the Asteroid Framing Cameras are two identical panchromatic imaging systems that will support navigation and scientific activities, by acquiring images from various distances and observing geometries during the course of the mission.

Built by Jena-Optronik (Germany), the cameras will provide data that support a wide range of investigations: hazard detection, system dynamics, mapping, shape reconstruction, morphology, albedo. These observations are critical to understand the current state of the Didymos system. By providing a detailed characterization of the asteroids surfaces, shapes, and the dynamical state of the system, the AFCs will contribute necessary data to fully assess the outcome of the DART impact, concluding this first planetary defense test.

Each instrument is equipped with a 5.5 x 5.5-degree field of view, and a pixel resolution of 93.7 microradians, and meet the requirements defined by the mission science team. We expect to deliver global mapping of the asteroids at spatial scales of 2–3 m/pixel in the Early Characterization Phase, 1–2 m/pixel in the Detailed Characterization Phase, and 0.5-2 m/pixel in the Close Operation Phase. Dedicated flybys will bring the resolution down to < 10 cm/pixel on specific areas of interest such as the DART impact site and the JUVENTAS cubesat landing site.

At EPSC, we will present the technical specifications of the camera, as well as the status of the calibration. We will then summarize the planned operations in cruise and at the asteroids. Finally, we will provide examples of the scientific investigations and products that will make use of the data returned by the cameras (e.g. shape model, morphological characterization).

How to cite: Vincent, J.-B., Kovacs, G., Nagy, B., Preusker, F., Pajola, M., Kueppers, M., and Michel, P.: The Asteroid Framing Cameras on ESA’s Hera mission, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-445, https://doi.org/10.5194/epsc2024-445, 2024.

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ASPECT is a flexible hyperspectral imager based on tunable Fabry-Perót interference filter (Näsilä and Kohout 2020). It consists of three independent Vis-NIR imaging channels, one single-point SWIR spectrometer, and a dedicated data processing unit (DPU) based on Xiphos Q7 (Table 1). ASPECT is the prime payload of Milani CubeSat carried by ESA Hera mission (Michel et al. 2022, Kohout et al. 2018) to binary asteroid Didymos-Dimorphos and is also considered for RAMSES and Satis missions to asteroid Apophis.

Table 1. ASPECT specifications

Aspect will conduct global hyperspectral mapping of the target asteroids with a resolution of 1 m/px from 5 km. The spectral domain contains key information on surface composition and maturity. Combined with high spatial resolution lateral variations in surface properties can be mapped. To extract such information from ASPECT observations we developed two machine learning convolutional neural network (CNN) codes (Korda et al. 2023ab, 2024) to extract scientific information from ASPECT observations.

The first code quantitatively detects silicate mineral composition (olivine, orthopyroxene, clinopyroxene modal abundances and their Fe, Mg, and Ca content). The second code evaluates space-weathering level based on spectral match score to the spectral type Q (fresh silicate asteroid) or to the type S (space-weathered). The codes were tested on legacy spatially resolved spectra of the asteroid (25143) Itokawa revealing local space-weathering trends in both mineralogy and space weathering indicators (Fig. 1). The apparent correlation between reduction in olivine content with increasing surface maturity is due to higher olivine susceptibility to space weathering-induced damage and related progressive removal of its diagnostic spectral absorption bands relative to these of pyroxenes.

Fig. 1. Local variations in surficial space weathering (left) and olivine content (right) on asteroid Itokawa revealed by the machine learning analysis. Spectral data are obtained by NIRS (Near Infrared Spectrometer) instrument on the Hayabusa (JAXA) spacecraft. The space-weathering level is expressed as a match score to spectral S-type with higher values (redder) being more mature.

The codes will be applied on hyperspectral observations of Didymos-Dimorphos or Apophis asteroids to produce composition and space-weathering maps, detect fresh impact events, map ejecta extent, or to detect local resurfacing. The web interface to access the codes for online user spectra classification can be found here:

https://sirrah.pythonanywhere.com/upload

References:

Michel et al. 2022 DOI 10.3847/PSJ/ac6f52

Näsilä, Kohout 2020 DOI 10.1109/AERO47225.2020.9172437

Kohout et al. 2018 DOI 10.1016/j.asr.2017.07.036

Korda, Kohout 2024, DOI 10.3847/PSJ/ad2685

Korda et al. 2023a DOI 10.1051/0004-6361/202346290

Korda et al. 2023b DOI 10.1051/0004-6361/202243886

How to cite: Kohout, T., Korda, D., Penttilä, A., Rajamäki, L., and Palamakumbure, L.: Detection of composition, space weathering, and local resurfacing using ASPECT hyperspectral imager of ESA Hera / Milani mission, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-316, https://doi.org/10.5194/epsc2024-316, 2024.

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Introduction

The cosine^* HyperScout-H (HS-H) instrument will fly aboard the ESA/Hera spacecraft. This mission [1], focused on planetary defense, aims to thoroughly characterize the (65803) Didymos near-Earth binary asteroid following the impact of the NASA/DART mission. It will launch in October 2024 and it is expected to reach the target by February 2027.

HS-H captures hyperspectral images over 657 - 949 nm wavelength range. Each macropixel, made of a 5 x 5 matrix of pixels, records the signal across 25 different narrow bands, with each band corresponding to a pixel. These measurements will represent a key element for understanding the Didymos - Dimorphos system, including its composition, the space-weathering effects and the potential presence of exogenous material. Also, they will complement the data obtained by the main camera (Asteroid Framing Camera) used for geomorphological studies, and for searching the presence of boulders near the two asteroids. Moreover, HS-H will provide high spatial resolution images of the crater resulting from the NASA DART impact. Another use of the instrument may occur in March 2025, during the cruise phase, when the spacecraft will perform a swing-by of Mars. It is expected that Phobos and Deimos, the two natural satellites of Mars, can be spectrally characterized by Hera/HS-H.

This presentation will provide an overview of the ongoing scientific activities to exploit this instrument. Its focus will be on cross-calibrating HS-H in the laboratory using meteorite samples.

Experiment setup

Two bulk meteorite samples were received from Archaeology, Environmental Changes, and Geo-Chemistry (AMGC) in Belgium. One piece, approximately 6 x 3 cm in size, originated from the El Hammami meteorite, an H5 ordinary chondrite. The other piece, measuring about 4 x 3 cm, was a fragment of the Sayh al Uhaymir (SaU) 001 meteorite, also an ordinary chondrite but of L5 subtype.

The reference spectra were obtained over the 550 - 4200 nm with a dedicated setup using the SHADOWS spectro-goniometer at Institut de Planétologie et d’Astrophysique de Grenoble (IPAG), France. The sample area measured was about 5.2 mm x 5.2 mm covered with 7 optical fibers of around 1.3 x 1.7 mm each. The optical arm of the spectro-goniometer was set at the nominal geometry of incidence 0° and emergence 30°, to simulate the most common viewing geometry of asteroids from the Earth [2].

The HS-H instrument was used to obtain hyperspectral images of the two samples, in the laboratory facilities of ESTEC (Noordwijk). The setup is shown in Fig. 1. A halogen lamp was used to illuminate both the sample and the spectralon. A number of 45 images were acquired, 15 for El Hammami, and 30 for SaU 001. Different exposure times were used, 500 ms, 1 sec, and 1.5 sec. In order to reduce the read-out time, we acquired subframes (regions of interest).

Fig. 1 The setup used to obtain the hyperspectral images of the two meteorites samples with HS-H. The left image shows a piece of El Hammami, while the right one corresponds to SaU 001.

Data analysis and results

The pre-processing of the data includes the dark subtraction and the flat field correction. Then, in order to obtain the spectral data, we applied the demosaicking algorithm that takes into account the pixel position and its corresponding wavelength (the central wavelength of each narrow band filter). The reflectance spectra are computed by dividing the spectrum of the meteorite by the one obtained for the spectralon.

The first approach was to consider a wide region of 10 x 10 macropixels over each sample (red squares in Fig. 2). The resulting curves show an excellent match between the reference spectra and the data obtained with HS-H. The comparison is shown in Fig. 2.

The second step was to consider the spectral curve resulting for each macropixel of HS-H. To analyze these measurements, we generated a taxonomic classification map (Fig. 3), using the classification system developed by [2]. We also determined the spectral slopes across the surface and an estimation of the depth of the absorption band at 1 µm. This surface mapping shows clear patterns consistent with the compositional variation across the meteorite.

Fig. 2 Comparison between the spectra obtained with HS-H (in blue), and the reference spectra obtained in the laboratory (in red). The corresponding HS-H image is shown on the left of each plot. The red square on each image marks the macropixels used to retrieve the meteorite spectrum.

Fig. 3 The taxonomic type (using the Mahlke [3] classification system) assigned to each macropixel .

References

[1] Michel, P. et al. (2022) Planetary Science Journal, 3, id.160;

[2] Potin S. et al.(2018) Applied optics, 57(28), 8279-8296, (2018);

[3] Mahlke, M. et al. (2022) A&A, 665, id.A26;

How to cite: Popescu, M., de León, J., Goldberg, H., Kovács, G., Krämer Ruggiu, L., Nagy, B., Prodan, G. P., Grieger, B., Kohout, T., Licandro, J., Karatekin, Ö., Esposito, M., and Küppers, M.: Hyperspectral imaging of meteorites using the HyperScout-H instrument, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-586, https://doi.org/10.5194/epsc2024-586, 2024.

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Introduction:

The Double Asteroid Redirection Test (DART [1]) was a NASA space mission that impacted the moonlet of the Didymos binary asteroid (Dimorphos) on 26^th of September 2022. The data collected prior to the collision by the spacecraft provided extraordinary information about Dimorphos and has been analysed by several scientists around the world, allowing the extraction of Dimorphos’ characteristic properties before impact, including physical and morphological properties, among others.

The European Hera mission [2] will continue exploring Didymos with a launch scheduled in October 2024. Hera will get the first images of Dimorphos after impact by the end of 2026. Along its trajectory, Hera will be able to deliver precious information about Mars, Deimos and Phobos.

The Austrian contribution to the Hera mission mainly consists of supporting the scientific camera instruments´ teams (TIRI, HSC, AFC, and ASPECT) by reconstructing, visualizing and analysing the data acquired with the different instruments on board. For this purpose, several tools have already been partly developed and will be improved and completed up until arrival for rendezvous with the asteroid in late 2026. For the first phase of the project (April 2023-June 2024) two uses cases were defined, namely Mars-fly-by and Dimorphos visualization and analysis with a 3D-GIS (Geographic Information System). In order to evaluate the tools in the most realistic way, a MARS MOLA shape model [3], the results of the DART mission available on its repository [4] and the Hera planned trajectories (SPICE kernels, [5]) calculated by ESAC in September 2023 were used.

PRo3D-GIS

PRo3D-GIS is the Austrian tool designated to support the instruments´ teams for the evaluation and analysis activities during the mission. The tool is based on PRo3D, an interactive 3D visualization tool to allow planetary scientists to work with high-resolution 3D reconstructions [6] [7]. It has major heritage from ExoMars and Mars 2020, but up to recently only supported single planets. To this end, we extended its capabilities to deal with celestial bodies as required for the Hera mission forming the PRo3D-GIS extension. To work with high-quality reconstructions, Pro3D-GIS uses the Ordered Point Cloud (OPC) format [8].

The visualization elements of PRo3D-GIS can be classified into three groups, namely:

• Structural layer (Digital Elevation Model – DEM)
• Texture layers
  • Primary texture layer (ortho image)
  • Secondary texture layers (derived products, e.g.: hyperspectral map)
• Annotation layers: comprises observations, annotations and measurements

After adding the 3D reconstructions (from DEM) called surfaces to PRo3D, users associate the surface with the SPICE kernel by assigning it a celestial body and reference frame. Given an observation time, PRo3D aligns all 3D data accordingly and fetches the required data. Depending on the visualization properties configured by the user, PRo3D automatically loads the corresponding layers, computes necessary derived products (such as contour lines) and provides an 3D visualization which can be explored interactively. Supported by the visualization layers (e.g. contour lines) users can author annotations and measurements and create screenshots or author videos.

DART repository data for Dimorphos multilayer OPC

Based on the PRo3D-GIS visualization elements mentioned before, following data were taken from the DART repository and converted into OPC format for visualization and analysis:

• OPC Dimorphos structural layer: “g_01960mm_spc_obj_dimo_0000n00000_v003.obj”, i.e.: DART Dimorphos shape model (Figure 1, left)
• OPC Dimorphos texture layers:
  • OPC primary texture layer: “dart_0401930042_08549_01_iof.fits”, DRACO image which was projected on DART shape model with a projective transformation (Figure 1, right)
  • OCP secondary texture layers: “g_01960mm_spc_dtm_dimo_0000n00000_v003.fits”, i.e. 17 DART Dimorphos information layers contained in the FITS file. An example is displayed in Figure 1, centre (i.e.: elevation layer)

Since DART data offers more than one texture layer, the OPC data scheme was enhanced to support them (i.e.: multilayer OPC), and the data import interfaces were adapted. Furthermore, the PRo3D-GIS tool was improved to allow multi-layer analysis and visualization. Additionally, the integration of large coordinates of the celestial bodies’ and the several coordinate systems provided in the SPICE kernels encountered in all mission phases, involved modifications to the design and implementation of PRo3D-GIS.

Visualization and analysis of the use cases

The Hera 3D-GIS has to support the two uses cases proposed for this phase of the development, namely:

• “Mars fly-by,” which main objectives are: The calibration of the instruments on board (e.g.: AFC, HyperScout and TIRI), the possibility to perform science observations of Deimos far side, and the observation of Mars for a couple of hours at good spatial resolution, and for several days with good phase angles
• “Visualization and analysis of Dimorphos” with information provided by the instruments teams, ordered in several information layers, in the validation process using the DART multilayer OPC as simulation example

Mars fly-by

Figure 2 (top) shows three bookmarks selected at different days and times in March 2025 (13^th March at 00:00, 16^th March at 14:00 and 17^th March at 01:00). At the bottom of Figure 2, the approach of Hera (observer, grey colour) to Mars (target, red colour) is visualized. The meta kernel used for this simulation in PRo3D-GIS was “hera_study_PO_EMA_2024.tm” and the reference frame was the ecliptic plane.

Visualization and analysis of Dimorphos

A simulation of the approach of Hera to Dimorphos on 24^th July 2027 can be seen in Figure 3. The selected times are shown on the top. At the bottom in the simulated views, Hera is represented as a grey small point in the centre of the image and Dimorphos multilayer in grey and brown colours, due to the combination of two information layers, namely the texture and elevation layers. The meta kernel file used for this simulation was “hera_study_PO_EMA_2024.tm”.

Additional analysis of the Dimorphos multilayer OPC with the available tools of the 3D-GIS was done. Some of the GIS applications available in PRo3D-GIS are shown in Figure 4, namely: layers superimposition visualization, annotations, measurements and scale analysis.

Summary and further work

Acknowledgement

References

[1] https://iopscience.iop.org/article/10.3847/PSJ/ac91cc/pdf

[2] https://iopscience.iop.org/article/10.3847/PSJ/ac6f52

[3] https://tharsis.gsfc.nasa.gov/MolaRcvrPaper.pdf

[4] https://dart.jhuapl.edu/SOC/DRA/

[5] https://s2e2.cosmos.esa.int/bitbucket/projects/SPICE_KERNELS/repos/hera/browse/kernels

[6] https://doi.org/10.1002/2018EA000374

[7] https://doi.org/10.1002/9781119313922.ch3

[8] https://www.researchgate.net/publication/228716045_Towards_True_Underground_Infrastructure_Surface_Documentation

[9] https://www.vrvis.at/publications/pdfs/PB-VRVis-2022-017.pdf

[10] https://www.vrvis.at/publications/pdfs/PB-VRVis-2023-002.pdf

[11] https://meetingorganizer.copernicus.org/EPSC2021/EPSC2021-73.html

[12] https://doi.org/10.5194/epsc2020-123

[13] https://az659834.vo.msecnd.net/eventsairwesteuprod/production-atpi-public/2387daf11cb14198bb064e103ee9747a

How to cite: Caballo-Perucha, P., Nowak, R., Steinlechner, H., Paar, G., Traxler, C., and Vincent, J.-B.: PRo3D-GIS tool for Hera: visualization and analysis of two use cases, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-322, https://doi.org/10.5194/epsc2024-322, 2024.

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How to cite: Tanga, P., Souami, D., Tsiganis, K., Siakas, A., Ferreira, J., Tsardaridis, S., Chesley, S., Dunham, D., Herald, D., Irwin, J., Kerr, S., Preston, S., Venable, R., and Watanabe, H. and the +100 observers: Didymos and Dimorphos: characterisation by stellar occultations, and plans before the Hera fly-by, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-1019, https://doi.org/10.5194/epsc2024-1019, 2024.

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The binary asteroid system Didymos-Dimorphos is an important planetary defence mission target. In September 2022 the NASA DART mission successfully changed the orbit of Dimorphos around Didymos through kinetic impact. In late 2026 the ESA Hera mission will arrive to study the aftermath of the impact in situ. One of the base requirements of the DART mission was that the orbital period change be measured from ground-based observations. This goal was achieved in the first weeks post-impact (Thomas et al. 2023). However, extended studies have put into question the stability of the new orbit (Scheirich et al. 2024, Naidu et al. 2024, Pravec et al. 2024).

We will conduct observations of the  Didymos-Dimorphos system in August 2024 with the European Southern Observatory’s 3.6m New Technology Telescope (NTT) in Chile. The goal of these observations is to monitor the orbit of the asteroid pair in the 'short-term', ahead of Hera’s arrival. The observations will be used to ​reveal whether the orbit is now stable​ and  constrain the rate of change if it is still evolving. This will give us insights into ongoing processes in the system: the ejecta clearing timeline, possible reshaping of Dimorphos through the impact ​and subsequent landslides, and inform models of energy dissipation in the Didymos-Dimorphos system​. We will also investigate whether there is any evidence of on-going mass loss from the system in the form of a tail, as was visible in the months immediately after the DART collision. The outputs of the observing campaign will provide critical input to Hera mission planning​. 

The main challenge of this campaign is the location of the asteroid system during the planned observations. The Didymos-Dimorphos pair will be crossing the Galactic plane. This means we expect extremely crowded star fields for observations of a quick-moving target. The unusual observing circumstances demand deployment of new difference image analysis (DIA) techniques, based on methods developed by e.g. Bramich et al. (2013), Hitchcock et al. (2021). The DIA methods need to be tailored to observations of fast moving targets with medium-sized telescopes, to allow construction of high-quality image templates within the limits of the awarded telescope time.

References:

• Thomas et al. 2023, Nature, 616, 448 
• Bramich et al. 2013, MNRAS, 428, 2275 
• Hitchcock et al. 2021, MNRAS, 504, 3561
• Scheirich et al. 2024, PSJ, 5, 17
• Naidu et al. 2024, PSJ, 5, 74
• Pravec et al. 2024 (Icarus, in review)

How to cite: Rozek, A., Snodgrass, C., Pravec, P., Knight, M., Thomas, C., Lister, T., Cannon, R. E., and Murphy, B.: Optical observations of Didymos between DART and Hera with the NTT, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-819, https://doi.org/10.5194/epsc2024-819, 2024.

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Introduction The successful DART mission has validated kinetic impact as the most effective method for deflecting or disrupting potentially hazardous asteroids [1]. The impact produced unexpectedly intense material ejection, expanding our understanding of asteroid response to hypervelocity collisions [2]. What insights might this violent fragmentation and ejecta reveal about Dimorphos's material strength and structural composition? Exploring these aspects before the upcoming Hera mission [3] can optimize mission design and interpretation.

The artificial impact experiment conducted by the Hayabusa2 mission demonstrated the influence of geological structures on cratering events [4, 5]. Similarly, understanding how the material and structural composition of an asteroid affect kinetic impact efficiency is crucial for planetary defense. To capture the detailed structure of asteroids, here we employ the Material Point Method (MPM), which excels in handling contact and interface problems, for modeling and simulation, aiming to address these critical questions.

Method The material point method (MPM) is a well-developed numerical technique in the mechanical industry, but remains underestimated potential in the field of planetary science. MPM discretizes the continuum body into Lagrangian material points and employs a rigidly attached Eulerian background grid at each time step, with the algorithmic flow illustrated in Fig. 1. By combining the strengths of both Lagrangian and Eulerian descriptions, MPM effectively eliminates numerical dissipation and expedites the search for neighbor points, resulting in an efficient, extendable, and robust shock-physics code. This feature makes MPM particularly well-suited for simulating complex geometries and contact interactions, which are common in planetary science applications. 

To accurately capture the dynamical behavior of materials under hypervelocity impact, a modified pressure-dependent and damage-dependent Lundborg smooth yield model is adopted for the strength model, coupled with a quantitative plasticity correction. The Tillotson equation of state (EOS) is utilized in conjunction with a sound speed modification. Besides, a resolution-independent Grady-Kipp damage model is employed to simulate the fracture process. In the future, we plan to further develop the p-alpha equation to characterize the pore compaction effect.

These proposed algorithms and material models have undergone extensive benchmarking and validation, spanning a wide range of parameter spaces, demonstrating their capability to describe the potential dynamic behavior of small celestial body materials [6]. This comprehensive approach, combining state-of-the-art numerical methods and material models, paves the way for a deeper understanding of the dynamic properties of small celestial bodies and their constituent materials.

Simulation and Result Constrained by computational resources, we designed simulation scenarios based on China's upcoming kinetic impact mission [7]. A cubic spacecraft (1.5 m in size, 600 kg in mass) impacts a spherical target body (35 m in diameter) at a velocity of 10 km/s. Given the unknown material and structure of the target, we investigated the dynamic responses of small bodies with various structures (monolithic or rubble pile), geometries (spherical or ellipsoidal), and material compositions (rocky or regolith, solid or porous). Except for the solid rocky material with a density of 2700 kg/m³, all other target bodies have equal mass and a bulk density of 1200 kg/m³. Classic basaltic material parameters were used for all materials, except for the regolith, which has lower strength. The ejecta, damage, and velocity distributions after 0.5 s of impact are visually presented in Fig. 2. We further analyzed the momentum enhancement factor and the mass and velocity distributions of the fragments for each case.

Discussion and Conclusion The dynamic responses to hypervelocity impacts exhibit significant sensitivity to the target's material and structure, as evidenced by the ejecta intensity, ejection angles, fragment morphology, velocity, and angular velocity. Investigating the relationship between target structure and impact response is crucial for mission design and ejecta avoidance. Moreover, it helps unravel the physical properties of the target body from observational results. This study focused on perpendicular impacts and analyzed the results during the impact fragmentation phase. Future research may delve into the orbital dynamics of the fragments, evaluate the effectiveness of impact defense in mitigating Earth-threatening hazards, and explore the potential of oblique impacts to disrupt the target body through spin-up. These findings may also extend to impacts between asteroids and shed light on phenomena such as binary formation, which contributes to our understanding of the origin and evolution of small celestial bodies in the solar system.

Acknowledgment X.Y. and J.L. acknowledge support from the National Natural Science Foundation of China under Grant 12372047. X.Y. acknowledges support from the National Natural Science Foundation of China under Grant 62227901. P.M. acknowledges support from the French space agency CNES and from the French National Centre for Scientific Research (CNRS) through the exploratory research program of the Mission for Transversal and Interdisciplinary Initiatives.

References [1] Daly, R. T. et al. (2023) Nature 616(7957), 443–447. [2] Cheng, A. F. et al. (2023) Nature 616(7957), 457–460. [3] Michel, P. et al. (2022) Planet. Sci. J. 3(7), 160. [4] Arakawa, M. et al. (2020) Science 368(6486), 1–10. [5] Jutzi, M. et al. (2022) Nat. Commun. 13(1), 7134. [6] Yan, X. et al. (2023) ACM Conference. [7] Zou, Y. et al. (2024) J. Deep Space Explor. 11(2), 1-8.

How to cite: Yan, X., Zhou, W., Michel, P., and Li, J.: Assessing the influence of asteroid composition and structure on kinetic impact deflection efficiency: an MPM approach, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-260, https://doi.org/10.5194/epsc2024-260, 2024.

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ESA is currently studying three Planetary Defence related missions in the framework of the Space Safety (S2P) Programme:

- NEOMIR (Near Earth Mission in the Infra Red): A space based IR telescope to be stationed at the Earth-Sun Lagrange point (L1), which will be scanning the inner solar system for Asteroids with solar elongations not readily accessible from groundbased observations.

- RAMSES (Rapid Apophis Mission for SEcurity and Safety): Rendezvous mission to asteroid (99942)Apophis (based on maximum re-use of components from the Hera spacecraft) with the objective to arrive at the NEO prior to its close encounter with Earth (ECA) on Friday 13th of April 2029 and to perform a full pre-ECA characterisation of the Object. This data will be highly valuable in combination with the data acquired by the NASA OSIRIS-APEx mission post-ECA.

- Satis: A lightweight Cubesat (12U-XL baseline) mission which is intended to demonstrate a low-cost asteroid inspection concept for Fast Asteroid Reconnaissance (FAR) of Near Earth Objects with a minimum payload configuration needed for the assessment of basic physical parameters necessary for Planetary Defence purposes. 

In this presentation we will provide a programmatic update on these three mission studies and present plans for upcoming mission concepts.

How to cite: Moissl, R., Conversi, L., Cano, J.-L., Walker, R., Skrypnyk, I., and Martino, P.: Update on ESAs Planetary Defence missions under study and in planning, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-789, https://doi.org/10.5194/epsc2024-789, 2024.

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The objective: 99942 Apophis is a potentially hazardous asteroid with a diameter of about 370 metres that on 13 April 2029 will approach Earth’s surface at a distance that is closer than orbiting geosynchronous satellites (Fig. 1). Significant tidal torques will be exerted on the body of Apophis, with consequences that might include alterations of its rotation state and/or internal structure, measurable seismic waves and real-time surface disturbances.

This very close Earth flyby therefore presents an unprecedented planetary defense and science opportunity. A mission to Apophis could allow transforming our understanding of the mechanical response to external natural forces of the surface, interior and rotational properties of potentially hazardous asteroid, with both planetary defense and science implications.

Figure 1: Trajectory of Apophis during its close encounter with Earth on 13 April 2029, at a minimum distance of 31860 km from Earth’s surface.

The mission: Parallel ESA-funded studies for small missions to Apophis are ongoing. The first is “Satis”, a study based on a 12U CubeSat; in parallel a second small-satellite study (“RAMSES”, Rapid Apophis Mission for SpacE Safety) is exploring an adaptation of the Hera spacecraft design, to fit to the updated mission profile, while minimizing any new developments given the short timescale until launch.

To rendezvous with the asteroid before April 2029, the RAMSES spacecraft needs to launch in 2028 with a 3-weeks launch window opening on April 20th. Following a 10-months direct transfer, RAMSES will rendezvous with Apophis no later than March 1^st, 2029, and will perform a detailed characterization campaign of the Asteroid. Initially RAMSES will run passively-safe hyperbolic arcs with pericentre at 20 km from Apophis, then will move into a pseudo-hovering position with fixed relative distance and phase angle with respect to the target asteroid. The distance will be decreasing in consecutive steps from 20 km to 10, 5 and finally 1 km, At the closest distance, global imaging at 10 cm resolution will be achieved (exploiting the Apophis rotation). This series of observations will be performed both before and after the close encounter with Earth.

In addition, during the close encounter, characterization of Apophis with high temporal resolution (1 picture per minute) will be performed in a safe hovering position at 5 km to observe in detail the abovementioned physical and dynamical alterations of the asteroid.

The Spacecraft: RAMSES is largely based on the Hera design. The key design drivers with respect to the original mission profile are: a much reduced distance from the Sun (allowing smaller Solar Arrays and reduced heater power), a closer distance from the Earth during critical operations (allowing a smaller antenna), but an increased delta-V demand (introducing the need for larger tanks and/or lower dry mass).

Payload: RAMSES will embark as a minimum two visible cameras with complementary Fields of View and two 6U-XL CubeSats which will be released in proximity of Apophis before the close encounter and will operate independently, using RAMSES as relay satellite. One of the two CubeSats could possibly land on Apophis before the close encounter, carrying relevant instruments such as, e.g., seismometers or gravimeters. Additional payloads will be accommodated either on RAMSES or on the CubeSats on the basis of available on-board resources. These might include a Thermal Infrared Imager, a Laser Altimeter, a Multispectral Camera onboard RAMSES, and/or a Low Frequency Radar, and other possible instruments of interest for planetary defense and/or science purposes.

International Cooperation: RAMSES will work in synergy with the NASA mission OSIRIS-APEX that will arrive at Apophis a few days after the closest approach of the asteroid to the Earth, allowing a comparison of the asteroid properties measured by the two missions and emphasizing the international cooperation that is at the heart of planetary defense.

Conclusion: The study of RAMSES will be instrumental to make an informed decision at ESA’s Council at Ministerial Level in 2025, both on the most effective technical solution as well as the best approach to reduce mission cost and allow fast implementation.

How to cite: Küppers, M., Martino, P., Carnelli, I., and Michel, P.: Status of ESA’s Rapid Apophis Mission for Space Safety (RAMSES) Concept , Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-199, https://doi.org/10.5194/epsc2024-199, 2024.

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Introduction:

Satis is an ESA Phase A Planetary Defense mission study targeting (99942) Apophis, a Potentially Hazardous Asteroid (PHA) with a diameter of about 350 m. The mission consists of a stand-alone 12U-XL CubeSat that aims to rendezvous with Apophis prior to its Earth Closest Approach (ECA) at a distance from Earth's surface of 31,500 km on Friday, 13th April 2029. In addition, Satis serves as pioneer to demonstrate a completely new class of small, fast-response missions for planetary defense. Here, we present the Satis science objectives and the overall mission definition.

Science Objectives:

The flyby of Apophis in April 2029 will be a unique opportunity to observe a PHA closely. Its gravitational encounter with Earth will enable the direct observation of changes in the asteroid’s rotation, possible surface changes, as well as its long-term orbit. Satis will observe these parameters to assess the effects of an ECA on the evolution of asteroids.

Apophis is a tumbling body or a non-principal axis (NPA) rotator. The gravitational forces on Apophis during the ECA will likely change Apophis’ rotational speed significantly, by more than 30%, and will cause a wobble in the rotation [1-5]. These changes in rotation will allow us to infer the ratios of moments of inertia and provide information on internal mass distribution and structure (Sci-1). The interior structure of an asteroid carries the imprint of its collisional and accretion history and is important for planetary defense mitigation attempts and Hypothetical impact assessment [6].

The tides and strong rotational forces have the potential to cause changes in the surfaces due to geophysical processes, such as landslides. Investigating this possible resurfacing caused by the ECA is important for interpreting present observations and to understand the evolution of asteroid surfaces (Sci-2). Surface strength and cohesion are the main drivers for surface geophysical processes and are also critical for planetary defense, as they dictate the response of the asteroid to kinetic impacts.

The orbital perturbation from Earth's gravity on the Apophis orbit is likely to increase and change the orbit by increasing the semi-major axis and the perihelion distance (Sci-3). The change in the orbit due to ECA is expected to be large, but exact predictions are not possible due to uncertainties in the asteroid’s initial orientation. Flyby-induced changes to Apophis’ spin state and surface will also affect the asteroid’s Yarkovsky acceleration, which is a relatively small perturbation but relevant for long-term orbit and planetary defense.

     Fig. 1: The modelled changes in Apophis’ gravity field due to ECA [1]

Mission definition:

The feasibility of the Satis mission concept was studied by an ESA CDF study [7] and since then the mission and system design further advanced [8]. The mission is designed to observe the asteroid before, during, and after the ECA in order to detect any changes induced by the effects of Earth’s gravity on this PHA, thus providing unique data for Planetary Defense purposes. The mission starts with an April 2027 launch on a dedicated micro-launcher equipped with a kick stage. The kick stage will be used to inject the CubeSat onto the required escape velocity vector. Following commissioning, the CubeSat will use a high-performance miniaturized electric propulsion system for the 2-year interplanetary transfer to achieve the rendezvous with Apophis. The mission duration will cover operations close to the asteroid two months before and at least two months after the ECA. Proximity operations will lead the spacecraft very close to Apophis’ surface to enable detailed analysis of the asteroid. Communication and navigation will be performed using a miniaturized X-band transponder interfacing with ESTRACK deep space ground stations. The payloads considered include a visual camera, thermal imager, radio science experiment as well as Hyperspectral, VIS/NIR imagers to fulfil the science objectives. The Mission Definition Review has been successfully completed and completion of Phase A of the mission study is foreseen for end of August 2024 with a Preliminary Requirements Review.

References: [1] Noiset, G. et al. (2023) 8th IAA Planetary Defence Conference, 3-7 April, 2003, Vienna. [2] Benson et al., (2023) Icarus, 390, 115324. [3] Scheeres, D. J. et al., (2006), Science, 314 (5803) [4] Souchay, J. et al (2018), Astronomy and Astrophysics, 617, 1–11. [5] Souchay, J. et al., (2014) Astronomy and Astrophysics, 563, 1–6. [6] Senel C. and Karatekin Ö.  8th IAA Planetary Defence Conference, 3-7 April, 2003, Vienna. [7] Satis CDF study (2022) ESA-TECSYE-HO-2022-003030. [8] Fogliano, V. et al. (2024) Small Satellites Systems and Services (4S) Symposium, 27-31 May, 2024, Palma de Mallorca.

How to cite: Karatekin, Ö., Ritter, B., Gundlach, B., Güttler, C., Patzek, M., Cabral, F. D. S. P., Catisanu, D. A., Fogliano, V., Holster, P., Simonetti, S., Walker, R., and Moissl, R.: Satis  as fast-response missions for Apophis during its Earth close encounter in 2029  , Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-249, https://doi.org/10.5194/epsc2024-249, 2024.

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The excited non-principal axis rotation state of asteroid Apophis can be described by two periods - rotation and precession. These periods (263 h and 27.38 h, respectively), together with other spin parameters and a convex shape model, were derived by [1] from photometric observations in 2012/13. Radar observations from the same apparition are consistent with the spin state derived from light curves and suggest that the shape of Apophis might be bifurcated [2]. During another favorable apparition in 2020/21, Apophis was observed extensively, and an updated shape and spin model was derived [3].

We carried out photometric observations of Apophis between November 2020 and May 2021 with the Danish 1.5m telescope at La Silla, ESO. This data set consists of 1280 calibrated photometric measurements in Cousins R filter distributed over 67 individual nights.

We used all available photometric data from 2012/13 and 2020/21 to determine Apophis's spin state and convex shape with the light curve inversion method [4]. Due to the large gap of eight years between observations, the rotation parameters cannot be determined uniquely. However, all acceptable models have about the same orientation in 2029 due to the same separation of eight years between 2021 and 2029. This enables us to reliably predict Apophis attitude during the close approach in 2029 and estimate the change of its spin state caused by the Earth's gravitation torque during the encounter.

Acknowledgments: This work was supported by the grant 23-04946S of the Czech Science Foundation.

References: [1] Pravec P. et al. (2014) Icarus, 233, 48. [2] Brozovic M. et al. (2018) Icarus, 300, 115. [3] Lee H.-J. et al. (2022) A&A, 661, L3. [4] Kaasalainen M. (2001) A&A, 376, 302.

How to cite: Ďurech, J., Pravec, P., Vokrouhlický, D., Hornoch, K., Kušnirák, P., Fatka, P., and Kučáková, H.: Rotation state of asteroid (99942) Apophis and its change during the 2029 flyby, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-257, https://doi.org/10.5194/epsc2024-257, 2024.

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Introduction:  The tidal encounter of asteroid Apophis with the Earth in 2029 offers the ideal scenario for the first in-situ seismic investigation of an asteroid. A seismometer can be used to monitor the seismicity of Apophis due to tidal forces [1], and other natural sources such as micro-meteoroid impacts or thermal cracks; [2-4]. In addition, a seismometer can image Apophis’ sub-surface and internal structure [2] providing information about the asteroid’s physical properties that is critical for planetary defense, and for understanding the evolutionary history of asteroids. As a consequence, there is strong international support for the first seismic experiment on an asteroid.

We have developed a Compact Seismometer that is designed specifically to fit inside a small asteroid surface package and function in the challenging environment of the asteroid surface. The instrument team has extensive experience in developing, testing, and operating seismic [5,6] and acoustic [7] planetary instrumentation.

Scientific Objectives: The main science and technological goal of the Compact Seismometer is to perform the first ever in-situ seismic study on the surface of an asteroid.  The specific compact seismometer science objectives are as follows:

Threshold science objectives:

(O1) Constrain the mechanical properties of the surface during landing

(O2) Measure the seismic background noise on the surface of Apophis

Baseline science objectives:

(O3) Quantify the seismicity of Apophis in response to tidal forces

(O4) Probe the subsurface and internal structure of Apophis

Bonus objective:

(O5) Perform the first active seismic experiment on an asteroid using the OSIRIS-APEX spacecraft – surface interactions

Compact Seismometer: The instrument consists of three commercial geophones that will each measure the ground motion along one axis, and dedicated electronics (Figs 1-2). The geophones are passive, contain no active electronics and are designed to withstand extreme environments (e.g., terrestrial boreholes).  The analog electronics have been carefully designed and developed to have a high sensitivity and low intrinsic noise. The instrument is currently at TRL level 4/5 (4 for the electronics, 5 for the commercial sensors), and the development schedule is compatible with RAMSES schedule. The current best estimates of the instrument mass, volume and power budgets are ~1 kg, 0.8 L and ~ 2W, respectively.

Figure 1. (left) Schematic diagram of the Compact Seismometer. (right) The Compact Seismometer containing three geophones.

Operational scenario: The Compact Seismometer should be deployed to the surface of Apophis prior to the asteroid’s closest encounter with the Earth. In the lead up to the perigee, periodic measurements will be made for instrument commissioning and background noise measurements. Then, from 3 hours before to 6 hours after the perigee - when tidal-induced seismic activity is expected to occur [1] - continuous monitoring should be performed. Following the close encounter further periodic measurements should be made to monitor the evolution and diurnal variations of the background noise and seismicity. If the surface package can survive long enough, the OSIRIS-APEX excavation experiment using thrusters [8] would provide a unique opportunity to perform an active seismic experiment using the Compact Seismometer.

Figure 2. Possible concept of operations for the Compact Seismometer.

Surface coupling and levelling: The Compact Seismometer must land on the asteroid and be in contact with the asteroid’s surface. However, the Compact Seismometer can operate in any orientation and does not require levelling.  The proposed solution is to not attempt to couple to the surface; any coupling attempt will only couple the instrument to the upper layer of regolith, which itself may not be coupled to the sub-surface. If any lofting occurs, these events can be used to characterize the surface properties during the subsequent touchdown. 

Performance and environment testing: The expected noise level of the Compact Seismometer is <1.0E-9 m/s/sqrt(Hz) in the 5-200 Hz bandwidth (<1.0E-7 m/s²/sqrt(Hz) at 15 Hz). Performance tests of the Compact Seismometer have characterized the electronic noise floor, distortion, cross talk, and the integrated instrument performance. The performance evaluation of seismic instrumentation is particularly complex due to the terrestrial seismic environment. However, we rely heavily on the experience and expertise gained from the SEIS (InSight) instrument and ARTEMIS-4 geophone performance testing. Thermal vacuum (TVAC) testing of the Compact Seismometer (Fig. 3) sensors demonstrated that the sensors survived the environment testing and that the noise level and the transfer function before and after TVAC remain unchanged.

Figure 3 (left) Compact Seismometer performance testbed at ISAE-SUPAERO. (right) Compact seismometer sensors during TVAC testing.

Conclusions: The Apophis 2029 encounter with the Earth offers the ideal scenario for the first in-situ seismic investigation of an asteroid. There is a strong interest within the planetary science and seismology community, and large international support for the first seismic experiment on an asteroid. The Compact Seismometer – the result of over a decade of studies and development – is designed to fit inside a small lander and function in the challenging environment of an asteroid’s surface. The team has extensive experience in developing, testing and operating seismic and acoustic planetary instrumentation. The Compact Seismometer is currently at TRL 4/5 (4 for electronics, 5 for commercial sensors), and the development timeline is compatible with the ESA RAMSES mission schedule.

Acknowledgments: The authors acknowledge funding support from CNES and from the European Commission's Horizon 2020 research and innovation programme under grant agreement No 870377 (NEO-MAPP project).

References: [1] DeMartini, J. et al. (2024), EPSC. [2] Murdoch, N. et al., (2017) Planetary and Space Science, 144, 89-105., [3] Murdoch, N. et al., (2015) Asteroids IV, University of Arizona Press Space Science Series. [4] Compaire, N., et al. (2022). Geophysical Journal International, 229(2), 776-799. [5] Lognonné, P. et al., (2019) Space Science Reviews. [6] Mimoun, D. et al. (2017) Space Science Reviews, 211. [7] Mimoun, D. et al. (2023) Space Science Reviews, 219. [8] DellaGiustina, D. et al (2023) Planet. Sci. J. 4 198.

How to cite: Murdoch, N., Garcia, R. F., Sournac, A., Cadu, A., Wilhelm, A., Drilleau, M., Stott, A., DeMartini, J., Kawamura, T., Lognonné, P., Michel, P., Bousquet, P., and Mimoun, D.: An in-situ seismic investigation of asteroid Apophis, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-495, https://doi.org/10.5194/epsc2024-495, 2024.

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On April 13, 2029, the asteroid 99942 Apophis will have a very close encounter with the Earth, transiting the GEO ring. This flyby represents a unique opportunity to observe a well-known potentially hazardous asteroid subject to strong tidal forces. The time-varying orbital and rotational environment can lead to changes in the surface slopes. Depending on the circumstances, this mechanism may drive significant property changes in the asteroid's internal structure and granular motion on its surface. In this context, characterizing the bulk density and its mass distribution within the asteroid nucleus before and after the encounter could represent a critical step towards understanding the evolution history of near-Earth asteroids.

In this work, we present the outline of a possible Radio Science Experiment (RSE) onboard the Rapid Apophis Mission for Space Safety (RAMSES) proposed by the European Space Agency, which is expected to rendezvous with the asteroid in February 2029, right before the close encounter. The objectives of this experiment will include characterizing the overall mass, density, and porosity of the nucleus with an accuracy of less than 1%, determining its spin rate and orientation to less than 1% and 5°, respectively, estimating the extended gravity field and internal structure of the nucleus, and improving its trajectory reconstruction.

To reach the outlined objectives, the radio science experiment will combine Earth-based radiometric measurements, namely Doppler, range, and ΔDOR, with optical images collected by the onboard narrow- and wide-angle cameras. Furthermore, the RSE will exploit the radiometric measurements collected through the Inter-Satellite Link with the CubeSats released by the RAMSES spacecraft, allowing us to collect high-accuracy Doppler measurements at closer orbital distances from the target and after an eventual landing. In this regard, the mission might serve as the first deep space application for a new X-band Inter-Satellite Link Transceiver (ISL-T) for CubeSats, which is being developed in the framework of the INNOVATOR project founded by the Italian Space Agency.

Building on the experience gained with the RSE onboard the HERA mission, this work shows that the proposed concept of operations, involving a radio link between RAMSES and two deployable CubeSats, is fully capable of satisfying the mission objectives of characterizing Apophis with high accuracy before and after the encounter.

Simulations for the orbit determination covariance analysis of RAMSES are conducted within JPL's Mission Analysis, Operations, and Navigation Toolkit Environment (MONTE) software, using the full-two body problem model to accurately describe the non-principal axis rotation of Apophis and its interactions with the Earth’s gravity field.

As part of this study, we identify possible synergies with opportunity payloads to be embarked on either RAMSES or the CubeSats and coordinated campaigns involving ground-based observatories and the OSIRIS-APEX mission.

How to cite: Lasagni Manghi, R., Zannoni, M., Gramigna, E., Tortora, P., Paialunga, G., Negri, A., Cucinella, G., De Rubeis, P. L., and Simone, L.: A Radio Science Experiment for the RAMSES Mission to Asteroid 99942 Apophis, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-822, https://doi.org/10.5194/epsc2024-822, 2024.

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Due to the increased performance of small satellites, their use in the exploration of asteroids is becoming increasingly more important. The DART mission with the CubeSat LICIACube or the Hera mission with the two CubeSats Milani and Juventas demonstrated this. Also, in the context of exploring the asteroid (99942) Apophis during its flyby of Earth in 2029 many concepts rely on the use of small satellites, which is illustrated i.e. by the plans for the RAMSES mission and the DROID mission.

The primary challenge for all mission concepts investigating (99942) Apophis is to secure funding and to meet the tight implementation schedule. In the context of the NEAlight project (Grant No. 50OO2413), carried out by IFEX at the University of Würzburg, these issues are addressed by examining three different mission concepts for a national small satellite mission in parallel: The concepts are designed to cover different mission complexities and cost constraints, and to allow utilization of different launch options, thus providing variation in implementation timescales.

RAMSES CubeSat Contribution

The first mission idea is the support of the European RAMSES mission by developing a CubeSat which aims to complement the science objectives of the RAMSES mission by providing valuable insights into the physical characteristics and possible surface changes in pre- and post- Earth closest approach (ECA) phase of (99942) Apophis. These are specifically done by observing the internal composition, shape, and dynamic state of the body. In addition, this mission concept aims to characterize the magnetization of (99942) Apophis and the plasma interactions as it passes through the Earth's magnetosphere. Table 1 shows an overview to said scientific contributions with specific instruments performing the respective measurements. Therefore, a suitable CubeSat bus must be capable of handling the deep-space environment, as well as operations in the close proximity of the asteroid. At this place the 6U+ SONATE-2 bus, which is developed and built inhouse at the University of Würzburg, serves as a viable foundation. This bus has flight heritage already and has been designed with future use in space exploration missions in mind, with a particular focus on mission autonomy and operating AI and advanced image processing hardware in space environment. Figure 1 shows an exemplary layout of this bus adapted for the mission.

Tab. 1 Scientific Contributions to the RAMSES mission and respective instruments on the CubeSat

Fig. 1 Top level concept of the CubeSat to accompany the RAMSES Spacecraft based on the SONATE-2 bus.

Apophis Encounter Mission

An independent German small satellite could study (99942) Apophis, should the RAMSES mission not receive funding. The satellite is aimed to be designed in such a way that it can serve as a template for further missions to near-Earth asteroids (NEA) in cislunar space and beyond. One of the main tasks is the selection of the trajectory and the propulsion system for the autonomous deceleration at (99942) Apophis if no transfer vehicle can be used. The launch is estimated to be in 2028 at the latest, as otherwise the fuel or time constraints may not be met. In contrast to the SATIS mission, however, the focus is here on a small satellite in the size class of approx. 27U. Depending on the final design of the trajectory, an observation period of up to two months before the ECA is assessed to be possible. Based on current announcements the usage of commercial transfer vehicles might be feasible.

Apophis Interceptor Mission

The near-Earth fly-by of (99942) Apophis also offers the opportunity to conduct a relatively inexpensive mission using a small satellite. From a few kilometers away a set of high-resolution images of (99942) Apophis at its closest approach can be captured, while also demonstrating a versatile deep-space bus for future NEA fly-by missions. A key advantage is the flexibility in the launch window, including a launch option in 2029. Additionally, launching together with similar mission concepts can further reduce costs, making this concept even more feasible with restricted budgets.

Because of the comparatively low costs of this small satellite missions and the trend of increased launch opportunities into cislunar space in the coming years, these missions class seems ideal for the exploration of near-Earth asteroids in general. In this way, these missions could make a key contribution to understanding the formation and evolution of the solar system, as well as to planetary defense against potentially dangerous asteroids.

Acknowledgments: This work is part of the NEAlight project funded by the Federal Ministry for Economic Affairs and Climate Action (BMWK) through the German Aerospace Center (DLR) based on a decision of the German Bundestag (Grant No. 50OO2413).

How to cite: Männel, J., Neumann, T., Kayal, H., and Riegler, C.: Concepts for a German Small Sat Mission to (99942) Apophis, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-1098, https://doi.org/10.5194/epsc2024-1098, 2024.

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Apophis will be one of the best studied of all asteroids due to the long visit of OSIRIS-APEX in 2029 and 2030. Here, we define a mission concept to leverage the capabilities OSIRIS-APEX as an observer and perform a cratering experiment at Apophis with an independent impactor spacecraft.

Apophis is very likely a rubble pile formed from reaccumulated debris following the disruption of a larger parent asteroid in the Main Belt and should be a generic representative of rubble pile formation and evolution. This is possibly a different formation and evolution than experienced by the target of the DART experiment – Dimorphos, which itself was a satellite of a larger asteroid. It probably formed from mass shedding events from its primary which may involve very gradual accretion.

Rubble pile asteroids have been targets of artificial cratering experiments, where Hayabusa2 Small Carry-on Impactor (SCI) created a large ~14m crater on asteroid Ryugu, and OSIRIS-REx created a nearly 10m crater on Bennu using its sampling mechanism and thrusters. The DART spacecraft impacted the asteroid Dimorphos with enough energy to likely significantly deform its shape.

Here, we propose a mission to impact Apophis, impart a measureable Delta-V, and make a larger and deeper crater than previous efforts. A 65kg impactor impacting Apophis at 7km/s will make a crater between 20-50m in diameter based on best knowledge of crater formation derived from the findings of SCI. This would result in an excavation of 2-8m deep – well below the depths previously explored and into the depths where crater morphology suggested increased strength at Bennu and Ryugu.

For the nominal mass of Apophis with simple momentum transfer (Beta=1), the resulting Delta-V would be 0.01mm/s. While incredibly small, it is ~2.5x larger than the formal 1-sigma tracking uncertainties for OSIRIS-REx at Bennu, owing to its long baseline of study and highly capable radio science package. The momentum enhancement expected in the cratering process should increase the Delta-V.

The combined knowledge of the mass of the target owing to OSIRIS-APEX and the measured Delta-V directly reports on the efficiency of momentum transfer during the cratering collision – the Beta parameter.

Owing to the importance of OSIRIS-APEX mapping and orbiting of Apophis for the interpretation of a cratering experiment, an impact after the completion of those studies would be ideal. Thus, an ideal impact time is after November 2030. This allows for a wide range of launch opportunities even some that are after the Apophis Earth flyby.

Notably, this experiment leverages the science instruments from OSIRIS-APEX entirely, requiring only key instrumentation for the impactor spacecraft survival and navigation. The large desired mass may also allow additional instrumentation to fly along in deep space for technology maturation purposes.

How to cite: Walsh, K. and Klein, V.: Apophis Cratering Experiment, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-697, https://doi.org/10.5194/epsc2024-697, 2024.

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