MITM19
Session assets
Introduction: NASA’s Origins, Spectral Interpretation, Resource Identification, and Security– Regolith Explorer (OSIRIS–REx) mission characterized and collected a sample from asteroid (101955) Bennu [1,2]. Twenty minutes after releasing its sample return capsule to Earth’s surface on September 24, 2023, the spacecraft diverted into an orbit around the Sun that allows for subsequent close Earth flybys. On this trajectory, the spacecraft will approach Earth alongside asteroid (99942) Apophis in 2029, enabling a follow-on mission to another asteroid with the same unique capabilities that led to OSIRIS- REx’s groundbreaking scientific results: OSIRIS– Apophis Explorer, or “APEX” [3].
Apophis will be well placed for an extensive ground-based telescopic campaign ahead of its Earth encounter on April 13, 2029, but observing conditions degrade just hours after the closest approach as the asteroid passes within 20° of the Sun. Immediate aftereffects of the tidal interaction will be impossible to examine from the ground—but will be distantly witnessed by APEX.
Figure 1. View of the APEX spacecraft, Earth, and Apophis trajectories around the Sun during the cruise phase. APEX will use a series of Earth gravity assists (EGAs) to approach Apophis. APEX will follow the right-hand small ellipse 3 times around the Sun over 2 years. EGA3 alters APEX’s orbit to the larger yellow path, which brings the intersection point with Earth’s orbit to the left side of the figure after 1.5 orbits. EGA4 shifts the orbit to the left-hand small ellipse for 3 orbits over 2 Earth years. About one hour after Apophis’ close approach to Earth, EGA5 puts APEX on a nearly identical trajectory as Apophis.
The Journey to Apophis: The Earth divert maneuver performed just after release of the OSIRIS- REx sample return capsule, and just before the mission’s second Earth gravity assist (EGA2), put the spacecraft on an eccentric orbit that passes to within 0.5 au of the Sun at perihelion (Fig. 1)—much closer to the Sun than the 0.77 au limit analyzed for the OSIRIS-REx mission. To mitigate this, the spacecraft adopts a “fig- leaf” attitude in which the solar arrays shadow critical spacecraft components when the spacecraft is within 0.65 au of the Sun (Fig. 2). As of January 2025, OSIRIS- APEX has successfully completed two of six close perihelion passages, and all instruments and spacecraft systems continue to operate nominally.
Figure 2. The APEX spacecraft in the fig-leaf attitude as viewed from the Sun. The solar panels shade critical spacecraft components.
Apophis Arrival and Proximity Operations: APEX will begin observing Apophis as a point source by April 2, 2029, at a distance of 5×106 km. Orbital mechanics prohibit an earlier rendezvous. APEX will observe Apophis from 50,000 km away during its close Earth encounter on April 13 and capture the evolution of its spin state in real time, revealing the consequences of a near-Earth object undergoing tidal disturbance by a planet (see presentation by Adam et al., this meeting) and continue observing as Apophis grows in its field of view. APEX will arrive at Apophis and begin its detailed study on June 5, 2029. Observations will uncover any signs of mass wasting that the tidal encounter triggered, revealing centimeter-scale topography via lidar and millimeter-scale morphology via images (Fig. 3),
Figure 3. The APEX spacecraft was designed to obtain global high-spatial-resolution imagery of small asteroids.
Chronicling the tidal encounter is only the beginning of APEX’s journey with Apophis. Having already challenged our fundamental understanding of carbonaceous (C-complex) asteroids during its exploration of Bennu, the spacecraft instrument suite will
provide first-of-its-kind high-resolution data of a stony (S-complex) asteroid—dramatically advancing our knowledge of this asteroid class and its connection to the meteorite collection. Global spectral mapping at meter scales and across a wide range of wavelengths (0.4–100 μm) will determine the composition of Apophis and identify any volatiles on its surface. Optical and radiometric tracking data will reveal Apophis’ mass and structure. We will also search for signatures of mass shedding, whether due to the tidal encounter or an episodic process like that observed at Bennu. After 15 months of orbital operations, APEX will perform the Spacecraft Thruster Investigation of Regolith (STIR), mobilizing surface material by means of its backaway thrusters, as demonstrated at Bennu. Observations during and after excavation will provide otherwise inaccessible insight into space weathering and the surface strength of stony asteroids.
Although scientific discovery is APEX’s prime motivator, Apophis’ bulk structure and surface strength have critical implications for planetary defense. As an S-complex object, Apophis represents the most common class of potentially hazardous asteroids, and knowledge of its properties can inform mitigation strategies. Monitoring Apophis after Earth approach provides the first opportunity to witness any change in an asteroid’s Yarkovsky force—a nongravitational effect that influences an asteroid’s likelihood of striking Earth.
Acknowledgement: The OSIRIS-APEX mission is supported by NASA Contract NNM10AA11C.
References:
[1] Lauretta, D.S., DellaGiustina, D.N. et al. (2019). Nature 568, 55–60, doi:10.1038/s41586-019-1033-6.
[2] Lauretta, D. S. et al. (2022). Science 377, 285–291, doi:10.1126/science.abm1018.
[3] DellaGiustina, D. N. et al. (2023). PSJ 4, 198, doi:10.3847/PSJ/acf75e.
How to cite: Nolan, M. C., DellaGiustina, D. N., Polit, A. T., Golish, D. R., Moreau, M. C., Simon, A. A., and Guzewich, S. D.: OSIRIS-APEX: NASA's Apophis Explorer Mission, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-187, https://doi.org/10.5194/epsc-dps2025-187, 2025.
Introduction
On 13 April 2029, asteroid (99942) Apophis will fly by Earth with a distance of approximately 38,000 km. Several authors suggest that the gravity field of Earth may lead to surface reshaping, e.g. avalanches [1,2,3]. These would be observed through planned space missions, providing insights into the granular dynamics of small rubble-piles asteroids. A key parameter for the models above is the so-called static angle of repose, which describes the angle of a slope that must be exceeded to start an avalanche [4].
To provide ground truth on the angle of repose under reduced gravity, an experimental setup was designed to determine the static angle of repose in vacuum and under reduced gravity. For this, the “Vacuum Angle of Repose Determination Experiment under Reduced Gravity” (VADER) was based on the concept of a rotating tumbler experiment. In these experiments, a granular material is slowly rotated in the discrete flow regime, which leads to the start of an avalanche, when the static angle of repose is exceeded. As a tumbler we use a volume of 20 mm diameter and 5 mm depth inside a rotating vacuum chamber with a glass blanking flange.
Laboratory Experiments
First successful investigations on the static angle of repose of WF 34 quartz sand were carried out in a laboratory environment under Earth gravity at our laboratory. Two series of laboratory experiments were performed under vacuum and ambient pressure conditions. Both series of experiments were performed at sample chamber rotation speeds between 0.2 and 6.9 rpm. For all vacuum experiments, the pressure after each experiment was always below 0.1hPa.
Results showed the static angle of repose at low tumbler rotation speeds to be similar in vacuum and under ambient pressure, despite the expected influence of humidity [5]. For laboratory experiments in vacuum, we find an angle of repose ϑS = 41.3±1.2° at the lowest rotation speed. For higher rotation speed and increasing duration of an experiment run, it was revealed that electrostatic charging of the sample affected the results. Sand grains were sticking to the glass blanking flange of the tumbler and the angle of repose increased. The experiments were therefore kept short and slow enough to avoid an electrostatic influence.
Drop Tower Experiments
During a first drop tower campaign in September 2024, nine drop experiments with the same tumbler setup were performed at the Bremen Drop Tower of the Center for Applied Space Technology and Microgravity (ZARM). The choice of material and the maximum vacuum gas pressure were identical to the laboratory experiments above. The static angle of repose was determined under different gravitational accelerations g by mounting the tumbler onto a rotating centrifuge to simulate accelerations between 0.10 and 0.31 gE.
After the release of the capsule (change of gravity direction, reorientation of granular sample) the tumbler was rotated at a reasonably high speed for up to 0.5 s. The practical purpose of this was to arrange the sample at an angle close to its angle of repose. After this phase, the tumbler speed was either reduced (procedure P1) or briefly stopped and restarted at a reduced speed (procedure P2) for the remaining time of the experiment. The rotation speed for P1 was small enough to remain in the discrete flow regime. In contrary, the rotation speed in P2 was allowed to be higher, because the intention was only to observe the start of an avalanche from a static state. The latter allowed experiments down to a smaller gravitational acceleration. Due to the short experiment time in the Bremen Drop Tower, there were no signs nor expected influence of electrostatic charging (sticking grains).
Experimental results under varying gravity conditions (g/gE) are shown as red and cyan squares in Fig. 1. Approximating the data with a trend line ϑS = -4.02 x log10(g/gE) + 41.26 (Eq. 1), a slight formal trend of an increased static angle of repose ϑS under reduced gravity was observed. Thus, these experiments expand on similar trends that were observed in other experiments (other data in Fig. 1). Consistent with early experiments [4,6], all experiments with irregular grains show static angles of repose that are larger than assumed in numerical models predicting avalanches on Apophis [1,2,3].
Figure 1: The static angle of repose of experimental data in the literature in comparison to our new data (red and cyan squares).
Perspective
The goal for future investigations lies in the expansion of the number of experiments and therefore datapoints. The current experimental results described above are only of small number and must be increased to get a better extrapolation to Apophis gravity and the 2029 Apophis-Earth encounter. We aim to increase the relative effect of cohesive forces (Bond number, [9]). Performing experiments at lower gravitational accelerations than what is presented above is anticipated to be achieved by procedure P2.
Acknowledgements
We acknowledge financial support by the DLR Agency (50WM2544).
References
[1] G. Noiset et al., 2022. EPSC 2022-1159. https://doi.org/10.5194/epsc2022-1159
[2] Ballouz et al., 2024. The Planetary Science Journal, 5 (11), 251. https://doi.org/10.3847/PSJ/ad84f2
[3] Kim et al., 2023. MNRAS, 520 (3), 3405-3415. https://doi.org/10.1093/mnras/stad351
[4] M.G. Kleinhans et al., 2011. J. Geophys. Res. 116, E11. https://doi.org/10.1029/2011JE003865
[5] Fraysse et al., 1999. Eur. Phys. J. B, 11 (4), 615–619. https://doi.org/10.1007/s100510051189
[6] M. Hofmann et al., 2017. MNRAS 469. https://doi.org/10.1093/mnras/stx1190
[7] A. Brucks et al., 2008. Earth & Space 2008. https://doi.org/10.1061/40988(323)9
[8] P.G. Hofmeister et al., 2009. AIP Conf. Proc. 1145. https://doi.org/10.1063/1.3180028
[9] Elekes and Parteli (2021). PNAS, 118 (38), e2107965118. https://doi.org/10.1073/pnas.2107965118
How to cite: Bannemann, L., Kost, P.-M., Güttler, C., and Gundlach, B.: Static angle of repose on asteroid (99942) Apophis: first results of a microgravity vacuum experiment, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-928, https://doi.org/10.5194/epsc-dps2025-928, 2025.
Coordinated use of modeling and in-situ radar data can advance our understanding of the internal structure of rubble-pile asteroids, providing insight into their formation and evolution mechanisms. Asteroid (99942) Apophis will fly by Earth within a distance of ~6 Earth radii on April 13, 2029. This event will be a rare opportunity to observe the reaction of a small, suspected rubble-pile body to planetary tidal forces. Caltech is leading a first-of-its-kind mission to rendezvous with Apophis before its Earth Closest Approach (ECA) and escort it through the approach. The mission consists of a spacecraft constellation comprised of a mothership and two CubeSats equipped with radar. The asteroid’s response to Earth’s gravity will be observed, and low-frequency (60 MHz) radar will be used to map its interior. The mission will perform both monostatic and bistatic radar (Fig. 1), mapping the asteroid’s internal structure at tens-of-meter scales and producing 3D backscatter and dielectric constant maps (Fig 2) to reveal Apophis's shape, density, internal block and void distribution, and spin state changes. These observations will offer groundbreaking insights into rubble-pile interiors, though methods for interpreting such data remain an open challenge.
To effectively simulate potential radar observations, realistic asteroid models are necessary. Previous work using the Discrete Element Method (DEM) has modeled Apophis as a lattice arrangement of uniform-sized spheres [1] or a collection of large aggregates of spheres [2]. We have developed an improved DEM model of Apophis using level sets [3] to represent realistic block shapes (Fig 3) and with size-frequency distributions of blocks ranging from meters to tens of meters in diameter, similar to those observed on the surfaces of analogous asteroids (e.g. Itokawa [4]) by previous missions. Simulated scenarios explore several internal configurations, such as uniform block spatial distributions, larger blocks near the core or surface, and contact binaries. These models are currently being used to predict Apophis’s response to its Earth flyby. They can also be used to generate simulated radar images to help define radar specifications and data volume requirements needed to determine whether the interior is homogeneous or heterogeneous at large scales and constrain the size-frequency distribution and spatial arrangement of interior boulders in greater detail.
This work underscores the synergy between modeling and radar tomography. We discuss how modeling can help define mission requirements and refine data interpretation methods, ensuring high scientific return from radar missions to Apophis or similar asteroids. In turn, radar tomography data can validate and improve models used to investigate the formation mechanisms and evolution processes of rubble-pile asteroids.
Fig 1. Radar modes implemented by the Caltech Mission to Apophis.
Fig 2. Single-frequency inverse scattering reconstructions of a 2D small body dielectric model (top) under four different sampling geometries (bottom) [5].
Fig 3. Example rubble pile model of Apophis with irregular particle shapes modeled by Level Set Discrete Element Method (LS-DEM).
References:
[1] DeMartini, Joseph V., et al. "Using a discrete element method to investigate seismic response and spin change of 99942 Apophis during its 2029 tidal encounter with Earth." Icarus 328 (2019): 93-103.
[2] Liu, P. Y., et al. "Tidal Effects on the Shape and Structure of Apophis during the Earth Flyby in 2029.", NEO-MAPP 2023.
[3] Kawamoto, Reid, et al. "All you need is shape: Predicting shear banding in sand with LS-DEM." Journal of the Mechanics and Physics of Solids 111 (2018): 375-392.
[4] Michikami, Tatsuhiro, et al. "Size-frequency statistics of boulders on global surface of asteroid 25143 Itokawa." Earth, planets and space 60 (2008): 13-20.
[5] Haynes, Mark, et al. "Small body radar inverse scattering in monostatic and bistatic geometries." Lunar and Planetary Science Conference. Vol. 2548. 2021.
How to cite: Daca, A., Andrade, J., DeMartini, J., Fesq, L., Haynes, M., Herique, A., and Raymond, C.: Synergy between Numerical Models and Radar Tomography Data from the Caltech Mission to Apophis, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1686, https://doi.org/10.5194/epsc-dps2025-1686, 2025.
The close encounter between asteroid Apophis and Earth in 2029 presents an unparalleled opportunity for the first in-situ seismic study of an asteroid. The Seismic Instrument for Asteroids (SIA) will be deployed to the surface of (99942) Apophis by the ESA RAMSES mission and used to measure the seismicity of the asteroid due to tidal forces [1,2], in addition to monitoring other natural sources such as micro-meteoroid impacts or thermal cracks; [3-5]. SIA is under development at ISAE-SUPAERO with the support of the French Space Agency (CNES). The instrument is the culmination of over a decade of research and development, incorporating advancements from projects such as BASiX, CNES Apophis 2029 studies, ESA COPINS, and the NEO-MAPP European Commission Horizon 2020 project.
The main science and technological goal of SIA is to perform the first ever in-situ seismic study on the surface of an asteroid. SIA will constrain the mechanical properties of the surface during landing, measure the seismic background noise on the surface of Apophis, quantify the seismicity of Apophis in response to tidal forces, and probe the subsurface and internal structure of Apophis. Such information about the asteroid’s physical properties is essential for planetary defense (for assessing potential impact damage and developing mitigation strategies), as well as for our understanding of the evolutionary history of asteroids.
The SIA instrument (Fig. 1) is a compact seismometer that incorporates three commercially available geophone sensors (built to endure extreme environments) and custom-built electronics that are carefully designed to ensure a high sensitivity and low intrinsic noise. The SIA sensor is expected to have a noise level lower than 1.0E-9 m/s/√Hz in the 5-200 Hz bandwidth. The instrument performance is evaluated at ISAE-SUPAERO, relying heavily on the experience and expertise gained from the SEIS (InSight) instrument testing [6,7]. Initial thermal vacuum (TVAC) and vibration testing demonstrated that the sensors survived the launch and the operational environments, and that the noise level and the transfer function remain unchanged.
Figure 1: Expanded CAD view of SIA. The instrument contains three geophones, in addition to the data and power board and the acquisition electronics. The current best estimate of the dimensions is 190 x 98 x 62 mm.
Based on Discrete Element Method (DEM) simulations it has been predicted that the peak seismic activity (largest occurrence of quakes) occurs as tides are dissipating in the hours after closest approach [1] (Fig. 2). This is likely due to changing centripetal forces (due to rotation rate changes during encounters [1,2]), and the relaxation following the tidal deformation. However, the largest amplitude quake(s) make occur prior to the perigee [2]. As such, SIA should be deployed on the surface of Apophis prior to the closest approach to Earth (ideally 10s of hours prior). In the lead-up to perigee, periodic measurements will be conducted for instrument commissioning and background noise assessment. Then, during the critical window from three hours before to six hours after perigee—when seismic activity induced by tidal forces is expected [1,2]—continuous monitoring will be essential. Following the close encounter, additional periodic measurements should be taken to track the evolution of background noise and seismic activity, including diurnal variations. Ideally, these post-encounter observations would continue for several tens of hours. The measurements before and/or after the close encounter are important to help identify which seismic signals originate due to the tidal forces, and which are due to the asteroid surface environment.
After deployment, SIA can function in any orientation without requiring leveling, but the instrument must be in direct contact with the asteroid's surface to make the seismic measurements. Instead of anchoring the instrument to the surface, our approach relies on simple surface contact. Attempting to secure the instrument would likely result in attachment only to the loose regolith, which may not be well connected to the underlying material. As long as the lander remains in contact with the surface, ground motion will be transmitted to the seismometer. If seismic amplitudes happen to exceed the asteroid’s local surface gravity, brief lofting may occur, with the subsequent landing offering an additional opportunity to study the asteroid’s mechanical properties.
This presentation will discuss the science objectives and operational scenario for SIA in addition to providing an update on the SIA instrument design and development.
Figure 2: The Apophis encounter trajectory indicating the period of peak seismicity (the period of largest occurrence of quakes) as predicted by DEM simulations [1].
References:
1] DeMartini, J. et al. (2024) Apophis T-5.
[2] Ballouz et al., (2025) Apophis T-4.
[3] Murdoch, N. et al., (2017) Planetary and Space Science, 144, 89-105.,
[4] Murdoch, N. et al., (2015) Asteroids IV, University of Arizona Press Space Science Series. `
[5] Compaire, N., et al. (2022). Geophysical Journal International, 229(2), 776-799.
[6] Lognonné, P. et al., (2019) Space Science Reviews, 215.
[7] Mimoun, D. et al. (2017) Space Science Reviews, 211.
Acknowledgements:
The authors acknowledge funding support from the European Commission's Horizon 2020 research and innovation programme under grant agreement No 870377 (NEO-MAPP project), and from the French Space Agency (CNES).
How to cite: Murdoch, N., Cadu, A., Garcia, R. F., Sournac, A., Wilhelm, A., Carpi, P., André, T., DeMartini, J., Mimoun, D., Logonné, P., Kawamura, T., Michel, P., Piou, V., and Bousquet, P.: Seismic Instrument for Asteroids (SIA): the RAMSES seismometer, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-417, https://doi.org/10.5194/epsc-dps2025-417, 2025.