Oral presentations and abstracts

NASA’s Double Asteroid Redirection Test (DART) is designed to be the first demonstration of a kinetic impactor for planetary defense against a small body impact hazard. The target is the smaller component of the Didymos-Dimorphos binary asteroid system. The DART impact will abruptly change the relative velocity of the secondary (Dimorphos), increasing the binary eccentricity and exciting librations in the secondary. The observed change in the binary orbit period will be used to infer the “beta factor”, or the momentum transfer efficiency, an important parameter used in planetary defense. The post-impact spin and librational dynamics are expected to be highly dependent on the momentum transferred to the target (i.e., beta) and the shape of the secondary, which is still unconstrained.

In this work, we explore the possible post-impact spin state of Dimorphos, as a function of its shape and beta, assuming it has an ellipsoidal shape and that both bodies have a uniform density. We have conducted attitude dynamics simulations with a modified 3-D spin-orbit model, accounting for the secondary’s shape and the primary’s oblateness, to understand the underlying dynamical structure of the system. In addition, we have used the radar-derived polyhedral shape model of Didymos in high-fidelity Full Rigid Two-Body Problem (FR2BP) simulations to capture the fully three-dimensional nature of the problem. We consider the outcomes from a simplified planar impact, where the DART momentum is transferred within the binary orbit plane, opposite the motion of Dimorphos, in addition to a more realistic case that accounts for the full DART velocity vector (which contains out-of-plane components).

With both simulation tools, we produce the expected signatures of the 1:1 and 2:1 secondary resonances between the free and forced libration periods, corresponding to axial ratios of a/b = 1.414 and a/b = 1.087, respectively. For moderate values of beta (~3), we find that the libration amplitude can exceed ~40 degrees in most cases. For some possible axial ratios, it is even possible to achieve a libration amplitude exceeding 40 degrees with beta values as low as 1. In addition, both codes reveal that the secondary may be attitude unstable in many cases, and can enter a chaotic tumbling state for larger values of beta (~5). In some cases, Dimorphos is able to break from its assumed 1:1 spin-orbit resonance.

In the case with a more realistic impact geometry (where some momentum is transferred out-of-plane), the results are relatively similar. The most noticeable difference is in the cases that result in a chaotic tumbling state. In those cases, the characteristic timescale for entering the chaotic tumbling state is much shorter – typically only several orbit periods are required. We also discuss the feasibility of detecting the post-impact spin state of Dimorphos with ground-based observations.

This study was supported in part by the DART mission, NASA Contract # NNN06AA01C to JHU/APL. The work of K.T. and I.G. is supported by the EC Horizon 2020 research and innovation programme, under grant agreement No. 870377 (project "NEO-MAPP"). Some of the simulations herein were carried out on The University of Maryland Astronomy Department’s YORP cluster, administered by the Center for Theory and Computation.

How to cite: Agrusa, H., Tsiganis, K., Gkolias, I., Richardson, D., Davis, A., Fahnestock, E., and Hirabayashi, M.: On the post-impact spin state of the secondary component of the Didymos-Dimorphos binary asteroid system, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-377, https://doi.org/10.5194/epsc2020-377, 2020.

AIDA (Asteroid Impact & Deflection Assessment) is an international collaboration between NASA and ESA which involves both DART (Double Asteroid Redirection Test, NASA) and Hera (ESA) missions. The target is an asteroid of approximately 160 m in size, namely the secondary of the binary Near-Earth Asteroid (65803) Didymos. Little is known about the shape of the satellite, with a moderately elongated shape (b/a<1.2) compatible with available ground-based estimations. In this work we investigate the possible reaction of the target to the DART collision to be performed in 2022, under the assumption that it is a gravitational aggregate produced in the formation of the binary system. The very structure of the target is unknown, therefore we model it by (1) mono- and multi-dispersed distributions of spherical basic elements and by (2) considering irregular components. We perform numerical simulations of the collision event by using a discrete-element N-body numerical code (PKDGRAV-SSDEM). We do not perform simulations of the shattering phase, we instead concentrate on the effect of the collision on the target, after the shattering phase implying material damage (melting, vaporization, heating and deformation), is over. Therefore, our synthetic projectile carries the same nominal momentum as the DART mission does, but it delivers to the target only the kinetic energy expected to survive once the shattering (non-elastic) phase has dissipated most of the impact kinetic energy. We account for different centre- and off-centre- possible impact geometry compatible with DART nominal impact angle with respect to the target orbital plane.

Here we report on results obtained so far on the effects of the DART impact on the structure of the Didymos satellite, including changes in its spin period and direction of the direction of the spin axis, as well as change of shape.

Moreover, we look at the velocity field of surface particles to infer if any motion is expected away from the impact point and regolith particles can be ejected from locations far from it.

Such predictions may be of interest in the study of the post-impact dynamics of the system –that will be determined by the Hera mission measurements. This, in turn will help in the interpretation of the results of the outcome of the DART impact mission, including the determination of the momentum multiplication (beta) factor.

How to cite: Benavidez, P., Campo Bagatin, A., Perez-Molina, M., Richardson, D. C., Santana-Ros, A., and Álvarez-Candal, Á.: Effects of the DART (NASA) mission collision on the structure and spin state of the secondary of the Near-Earth asteroid (65803) Didymos binary system., Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-411, https://doi.org/10.5194/epsc2020-411, 2020.

The Hera mission contributes to the international effort towards the validation of the kinetic impactor asteroid deflection technique by retrieving all the physical and dynamical properties of Dimorphos in order to validate numerical impact codes. In particular, Hera’s core objectives from the point of view of the deflection demonstration are the following:

• (i) Measuring the mass of Dimorphos to determine the momentum transfer efficiency from DART to the asteroid;
• (ii) Investigating the crater in detail to improve our understanding of the cratering process and the mechanisms by which the crater formation drives the momentum transfer efficiency;
• (iii) Observing subtle dynamic effects (e.g. libration imposed by the impact, orbital and spin excitation of the secondary) that are difficult to detect for remote observers;
• (iv) Characterising the physical properties of Dimorphos (including size, shape, volume, density, porosity, size distribution of surface material) to allow scaling of the momentum transfer efficiency to different asteroids.

In addition, the Hera mission will allow the demonstration of two key technologies for future deep-space missions:

• (v) The use of CubeSats for multipoint investigations operated via an inter-satellite network link via Hera;
• (vi) Autonomous visual-based navigation for very low altitude flybys over the surface of Dimorphos.

The Hera spacecraft will launch in October 2024 onboard an Ariane 6 launcher with an 18-days launch window. The trajectory foresees a Mars swing-by in mid-March 2025 and the rendezvous phase with Didymos starting end-December 2026. Following the operationally safe capture sequence, the asteroid close-proximity operation phase will start from a gate position of about 30 km. Operations will continue for about 6 months and allow for detailed investigations of the Didymos surface down to few kilometres or less from the surface of Dimorphos depending on the performance of the onboard feature-tracking navigation system.

The mission is currently in phase B2 with OHB System as prime contractor together with GMV, QinetiQ, Spacebel and OHB-I as core team members. The preliminary design review is schedules in October 2020. The paper will provide an overview of the mission together with its latest system and payload configuration.

How to cite: Carnelli, I., Martino, P., Escorial, D., rugina, A., Gil, J., Greus, H., Valverde, A., Zuccaro, A., Bonnafous, B., Muñoz Moya, C., Honvault, C., Perez Lissi, F., Tzeremes, G., Khan, M., Küppers, M., Tourloukis, M., Muñoz, P., Concari, P., Moissl, R., and Accomazzo, A.: ESA’s Hera mission to asteroid Dimorphos, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-1119, https://doi.org/10.5194/epsc2020-1119, 2020.

The NASA DART and the ESA HERA missions aim to provide an experiment in asteroid defection, though a kinetic collision. DART spacecraft will be sent to collide at high speed, approximately at 6.6 km/s, with the smaller asteroid, usually called Didymoon of the binary asteroid system Didymos. HERA spacecraft will be sent to study the effects of the impact, so that our knowledge of the energy transmission due to the collision is improved. HERA spacecraft will evaluate Didymoon orbit change, structure of the asteroid, crater size [1].

HERA spacecraft carries several payload instruments to provide these studies, namely: Cameras, Radar, Satellite-to-Satellite Doppler tracking, LIDAR, Seismometer and Gravimeter.

In this work we report the LIDAR, also known as PALT for HERA, conception, design, and manufacturing process that is currently ongoing, as well its scientific aims and contribution to spacecraft navigation.

PALT is a ToF altimeter that provides time tagged distances measurements. The instrument can be used to support near asteroid navigation and provides scientific information (e.g. asteroid 3D topography and fall velocity) and also reports the power of the received pulse being possible to calculate the target reflectivity.

PALT first version EM is based on a Laser Landing Altimeter Engineering Model developed by EFACEC and Faculdade de Ciências, Universidade de Lisboa (FCUL), in the frame of an ESA NEO-MAPP project. THE PALT comprises a compact low power consumption microchip laser that emits 1.5 µm light pulses and a low noise sensor. This laser technology enables rangefinder compact designs. The synergies between these two technologies enable the development of a compact instrument for range measurements of from 500 m to 14 km with a low power consumption and envelope of 12 cm×15 cm×10 cm. The PALT electronics was designed to endure a TID of 100 krads.

PALT has four main blocks, power supply, processing unit, electronics frontend, ToF optical front end. Optical front end is composed by emitter and receiver.

Power supply uses a traditional flyback solution, optimised for the altimeter secondary powers consumption and outputs filtering.

Processing unit is based on a FPGA since it simplifies the process of keeping precise timings, required to operate the ToF unit. FPGA is also responsible to perform all the housekeeping acquisitions, to monitor the health of the altimeter and for the interface with the spacecraft, via Universal Serial Link.

ToF is the key block of the LIDAR altimeter with respect to its accuracy and precision. This unit is responsible to time tag all the laser emitter pulses as well as all the APD receptions, with a precise timed tag that will be then managed by the processing unit FPGA to compute the distance.

Frontend Electronics is responsible for the Laser power supply and triggering, also for the Laser pulses digitalization (emitted and received).

The preferred LASER source for PALT is currently being developed at FCUL. The laser used as source is a diode pumped, passively Q-switched Yb-Er Microchip Laser targeting a 100 μJ Gaussian pulse with a FWHM of 2 ns. The backscattered radiation is a gaussian pulse shape.

The main optical specifications of the optical front end follow the receiver, emitter, and filter parameters. The receiver optical aperture diameter and obscuration are 100 mm and 30 mm, respectively. The FOV receiver has 1.5 mrad value, a transmittance of 0.91; a sensor with a 230 kV/W responsivity. Relatively to the emitter properties, it has a FOV of 1 mrad and optics transmittance of 0.94. The energy budget was calculated using (1), which allowed an estimation of the magnitude of the returned power [2]:

E_r≈ E_TR r_s⁄π A_r⁄D² τ_R OV          (1)

where E_TR is the emitter transmittance, r_s is the asteroid reflectance, A_r is the telescope area, D is the distance, τ_R is the receiver transmittance and OV is the overlap.

Considering the emitted laser pulse has FWHM of 2 ns and Gaussian shape, the receiver power can be calculated.

The returned peak power Figure 1 along with saturation limits of the sensor and minimum detectable power considering solar background, sensor NEP and M=20.

Figure 1. Detected peak power (higher and lower bound represents a 0º and 20º surface inclination).

The most critical component of the optics front end is the receiver telescope. The receiver telescope has a Cassegrain design. The primary mirror is made of zerodur and has 100 mm diameter. The secondary mirror is assembled on a carbon fiber tripod structure, the telescope ray tracing (zemax design), the footprint (on sensor) for different operating distances (Figure 2).

Figure 2. (a) Telescope ray trace; (b) spot diagram for several raging distances.

The LIDAR has to withstand the launcher load and maintain integrity and performance of the optical receiver telescope (Figure 3) (a) system; (b) present a light telescope structure that withstand launch vibrations.

Figure 3. (a) Lidar system 3D CAD design; (b) Vibration simulation of telescope structure.

Acknowledgments

This paper has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement Nº. 870377 (project NEO-MAPP).

References

[1] P. Gordo, D. Seixas, B. Couto, A. Amorim, B. Livio, R. Melício, A. Marques, T. Sousa, C. Pinto, G. Tezeremes, P. Michel, M. Küppers, I. Carnelli, "HERA lidar instrument development", Proc. of the 4th Symposium: Small Satellites for Sustainable Science and Development, pp. 1–6, Herzliya, Israel, 4–8 November 2019.

[2] J.L. Bufton, "Laser altimetry measurements from aircraft and spacecraft", Proceedings of the IEEE 77(3), pp. 463–477, March 1989.

How to cite: Gordo, P., Dias, N., Couto, B., Arribas, B., Amorim, A., Livio, B., Melicio, R., Marques, A., Sousa, T., Granadeiro, V., and Michel, P. and the NEO-MAPP Team: Planetary Altimeter for HERA Development, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-302, https://doi.org/10.5194/epsc2020-302, 2020.

Abstract
The Asteroid Impact and Deflection Assessment (AIDA) is an international collaboration supported by ESA and NASA to assess the feasibility of the kinetic impactor technique to deflect an asteroid, combining data obtained from NASA’s DART and ESA’s Hera missions [1, 2]. In 2022, DART will perform a kinetic impact on the secondary of the binary near-Earth asteroid (65,803) Didymos, recently named Dimorphos. After 2 years, Hera will follow-up with a detailed post-impact survey of Didymos, to fully characterize this planetary defense technique. Additionally, Hera will deploy two CubeSats around Didymos once the Early Characterization Phase has completed, to complement the observations of the mother spacecraft and increase the scientific return of the mission. The first Cubesat, called Juventas, will complete a low-frequency radar survey of the secondary, to unveil its interior, while the second one has not yet been selected.
One of the main objectives of Hera is to characterize the mass and mass distribution of both Didymos primary and secondary by means of radio science investigations. This paper describes the concept of the gravity science investigations to be jointly carried out by the three mission elements, i.e. Hera, Juventas and CubeSat-2. The experiment will combine classical ground-based radiometric measurements, spacecraft-based optical images of Didymos, and Satellite-to-Satellite radiometric tracking between Hera and the Cubesats. Finally, our results and achievable accuracy for the estimation of the mass and gravity field of Didymos and Dimorphos are presented.

1. Introduction
Most of the information about the formation processes of an asteroid lies in its interior structure. One of the very few constraints of the internal mass distribution of a celestial body is given by its gravity field, even if the inversion process is not unique. First, the bulk density can be inferred by measuring the mass of the body, combined to the volume estimated from optical images. In addition, the higher degrees of the gravity field provide information about the internal distribution of mass, such as the moments of inertia.
The main scientific goals of the Hera radio science investigations are:

• Determine the mass and gravity field of Didymos and Dimorphos;
• Reconstruct the motion of Dimorphos around Didymos;
• Contribute to the characterization of the energy transfer between DART and Dimorphos.

Such objectives are a valuable contribution to the Hera mission objectives, leading to a better understanding of the formation and evolution processes of the Didymos system.

2. Technique
The determination of the gravity field of a celestial body is an application of the orbit determination process of deep space spacecraft. In particular, the gravity of Didymos can be estimated precisely reconstructing the trajectory of Hera during a selected number of close encounters (about 10 km at closest approach). The classical observables used in the orbit determination are obtained from the X-band radio link between the spacecraft and the Earth. The microwave signal is sent to spacecraft from a ground antenna and coherently retransmitted back to Earth, where Doppler and range measurements are obtained. A previous study performed for the AIM proposed mission [4] demonstrated that gravity science at Didymos is feasible using radio tracking data only, under realistic assumptions on the technological capabilities of the space and ground segment. Shorter pericenter distances increase the attainable accuracy. However, a significant improvement can be obtained even at relatively large distances processing also optical images of Didymos and Dimorphos taken by the spacecraft.
In addition, Hera may track Juventas and CubeSat-2 by means of a space-to-space inter-satellite link (ISL), capable of determining the relative distance (ranging) and the relative line-of-sight velocity (Doppler) between the two bodies. In particular, the latter is expected to represent a very nice add-on to the gravity investigation carried out by the Hera mission, as the Doppler shift that affects the inter-satellite link contains the information on the dynamics of the system, i.e. masses and gravity field of Didymos and Dimorphos.
The expected accuracy in the estimation of Didymos gravity fields were obtained through numerical simulations of the orbit determination of Hera and the two Cubesats. Conservative assumptions were made in terms of both radiometric and optical measurement noises, and large a-priori uncertainties for the estimated parameters were used.  

3. Results
As a result of the numerical simulations, the masses of Didymos and Dimorphos are expected to be estimated with relative uncertainties less than 10^-4 and 10^-3, respectively. The addition of the ISL measurements improves the achievable accuracies but it is not required to estimate the masses. However, given the relatively large distance of Hera from the system, the higher degree gravity of Didymos and Dimorphos can be estimated only adding the ISL Doppler measurements between the Cubesats and the mother spacecraft. In this case, the gravity field of Didymos can be estimated to at least degree 3, depending on the assumptions about the ISL operations and performance. Similarly, ISL Doppler measurements allows to estimate the extended gravity field of Dimorphos up to degree 2, with an uncertainty of about 10%.

Acknowledgements

This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 870377 (project NEO-MAPP). MZ, IG, ML, EG, RLM, and PT wish to acknowledge Caltech and the Jet Propulsion Laboratory for granting the University of Bologna a license to an executable version of MONTE Project Edition S/W.

References
[1] Cheng A. F., et al., “AIDA DART asteroid deflection test: Planetary defense and science objectives,” Planet. Space Sci., vol. 157, no. February, pp. 104–115, 2018.
[2] Michel, P., et al. “European component of the AIDA mission to a binary asteroid: Characterization and interpretation of the impact of the DART mission”. Adv. Space Res. (2018), Volume 62, Issue 8, pp. 2261-2272.
[3] Lasagni Manghi, R., Modenini, D., Zannoni, M., Tortora, P., “Preliminary orbital analysis for a CubeSat mission to the Didymos binary asteroid system”, Adv. Space Res. (2018), Volume 62, Issue 8, pp 2290-2305
[4] M. Zannoni, G. Tommei, D. Modenini, P. Tortora, R. Mackenzie, M. Scoubeau, U. Herfort, I. Carnelli, “Radio science investigations with the Asteroid impact mission”, Adv. Space Res. (2018), Vol. 62, Issue 8, pp. 2273-2289.

How to cite: Zannoni, M., Gai, I., Lombardo, M., Gramigna, E., Lasagni Manghi, R., Tortora, P., Karatekin, O., Goldberg, H., Martino, P., Kueppers, M., Michel, P., and Carnelli, I.: Didymos Gravity Science Investigations with the Hera mission, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-691, https://doi.org/10.5194/epsc2020-691, 2020.

Large planetary science missions carry a suite of instruments that must negotiate observations and priorities to fulfill their scientific objectives.  A new paradigm of mission brings use of deployable nano-spacecraft as independent operating observers to provide added science.  As in the case of the Hera mission, the Hera mothercraft will carry through the cruise phase two small CubeSats and deploy them once in the vicinity of the Didymos asteroid system.  These small CubeSats are able to navigate relative to the observing planetary body and conduct meaningful science through 1-2 miniaturized instruments.  

The Juventas CubeSat for Hera will be discussed along with presentation of its low frequency radar, JuRa.  Its scientific objectives and contribution to the Hera and AIDA objectives will be presented.

How to cite: Goldberg, H., Van wal, S., Herique, A., Rogez, Y., Karatekin, O., and Villa, V. M. M.: Nano to Mini satellite and dedicated instruments: a new opportunity for planetary exploration, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-176, https://doi.org/10.5194/epsc2020-176, 2020.

The ESA Hera mission approved by the last ESA council Sapec19+ will be launched in 2024 to deeply investigate the Didymos binary system and especially its moonlet. Onboard the Juventas small platform, the JuRa radar will fathom Didymoon and provide the first direct observation of an asteroid deep interior. The question of asteroids’ internal structure is crucial for science, planetary defense and exploration. After several rendezvous missions, our knowledge relies entirely on inferences from remote sensing observations of the surface and theoretical modeling [1].

Didymos binary asteroid system is an S-type asteroid system orbiting the sun with a semi-major axis of 1.64 AU. The primary body has a diameter of 800 m and Didymoon, the secondary body with a diameter of 160 meters is orbiting around the main at a distance of 1.2 km [2]. In 2022, DART, the NASA contribution to the AIDA program, will impact the moonlet to quantify the mechanical response of the body, mainly from ground-based observation [3]. Five years later, Hera is a unique opportunity to observe in detail the bodies, the crater and the ejecta in order to better constraints mechanical models [2], [4]. Hera will provide a global characterization of the binary system: shape, density, dynamic properties, thermal properties, composition, …

JuRa, aboard the small platform Cubsat Juventas, will probe the moonlet. The JuRa instrument is a monostaic radar, BPSK coded at 60MHz carrier frequency and 20MHz bandwidth, inherited from CONSERT/Rosetta [5], [6] and redesigned in the frame of the AIDA/AIM phase A/B [2], [7]. JuRa maps the backscatter coefficient (sigma zero - s0) of the surface or subsurface, which quantifies the returned power per surface or volume unit. It is related to the degree of heterogeneity at the scale of the wavelength and to the dielectric contrast of heterogeneities, giving access to both, the sub-meter texture of the constituent material and larger scale structure.

• The first objective of JuRA is to characterize the moonlet’s interior, to identify internal geological structure such as layers, voids, sub-aggregate, to bring out the aggregate structure and to characterize its constituent blocks in terms of size distribution and heterogeneity at different scales (from submetric to global).
• The second objective is to estimate the average permittivity and to monitor its spatial variation in order to retrieve information on its composition and porosity, especially in the area of the impact crater. Radar bypasses the near surface alteration by space-weathering and thermal-cycling as observed with optical remote sensing.
• The same characterization applied to the main asteroid of the binary system is among the secondary objectives, to detect differences in texture and composition and to support the modeling of the binary system’s formation.  Supporting shape modeling and the determination of the dynamic state through radar ranging are other secondary objectives.

Thus, the observation of the structure and composition of moonlet will provide constraints on the mechanical model of the impact process. By When compared to the observation of the main body, it will constraint the model of binary system formation to discriminate between progressive versus catastrophic process and more generally on the stability condition of the system.

Acknowledgments

Hera is the ESA contribution to the AIDA collaboration. Juventas and JuRa are developed under ESA contract supported by national agencies. JuRa is built by EmTroniX (Lux), UGA/IPAG (Fr), TUD (Gr), Astronika (Pl) and BUT (Cz). Juventas is built by Gomspace (Lux).

This project has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No 870377 (project NEO-MAPP).

References

[1] A. Herique et al., « Direct observations of asteroid interior and regolith structure: Science measurement requirements », Advances in Space Research, vol. 62, n^o 8, p. 2141‑2162, oct. 2018, doi: 10.1016/j.asr.2017.10.020.

[2] P. Michel et al., « Science case for the Asteroid Impact Mission (AIM): A component of the Asteroid Impact & Deflection Assessment (AIDA) mission », Advances in Space Research, vol. 57, n^o 12, p. 2529‑2547, juin 2016, doi: 10.1016/j.asr.2016.03.031.

[3] A. F. Cheng et al., « Asteroid Impact & Deflection Assessment mission: Kinetic impactor », Planetary and Space Science, vol. 121, p. 27‑35, févr. 2016, doi: 10.1016/j.pss.2015.12.004.

[4] S. D. Raducan, T. M. Davison, et G. S. Collins, « The effects of asteroid layering on ejecta mass-velocity distribution and implications for impact momentum transfer », Planetary and Space Science, vol. 180, p. 104756, janv. 2020, doi: 10.1016/j.pss.2019.104756.

[5] W. Kofman et al., « The Comet Nucleus Sounding Experiment by Radiowave Transmission (CONSERT): A Short Description of the Instrument and of the Commissioning Stages », Space Science Reviews, vol. 128, n^o 1‑4, p. 413‑432, mai 2007, doi: 10.1007/s11214-006-9034-9.

[6] W. Kofman et al., « Properties of the 67P/Churyumov-Gerasimenko interior revealed by CONSERT radar », Science, vol. 349, n^o 6247, p. aab0639, juill. 2015, doi: 10.1126/science.aab0639.

[7] A. Herique et al., « A radar package for asteroid subsurface investigations: Implications of implementing and integration into the MASCOT nanoscale landing platform from science requirements to baseline design », Acta Astronautica, mars 2018, doi: 10.1016/j.actaastro.2018.03.058.

How to cite: Herique, A., Plettemeier, D., Goldberg, H., and Kofman, W. and the JuRa Team: JuRa: the Juventas Radar on Hera to fathom Didymoon, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-595, https://doi.org/10.5194/epsc2020-595, 2020.

The AIDA (Asteroid Impact Deflection Assessment) mission involves DART (Double Asteroid Redirection Test) and Hera spacecraft. DART is led by the NASA and will impact the smaller body of Didymos, a binary asteroid. HERA [1], on the other hand, is a European project and will rendezvous Didymos in 2026 following up the impact produced in 2022 by DART.

Didymos is a near-Earth binary asteroid composed by Didymain, the main asteroid, and its moon called Didymoon. Didymain has an estimated radius of 387 m and a rotation period of 2.26 hours. Didymoon, whose approximate radius is 103 m, has a circular tidally locked orbit of 1180 m around Didymain and an orbital period of 11.92 hours.

The Austrian contribution to Phase B2 Part 1 of the HERA mission was carried out by JR, VRVis, and two science collaborators under GMV contract. It consisted mainly of the design of tools needed to help define optimized images of Didymos (Didymain and Didymoon) for tactical and strategic purposes as well as to determine the asteroid shape. For both purposes, the further development of PRoViP [2] and PRo3D [3], tools already available for the ExoMars mission, was required. To validate the 3D reconstruction chain, a simulation approach was designed and implemented, see Figure 1:

Image rendering

In order to generate synthetic images of Didymos in the most representative and faithful way, the information about the geometry of the Asteroid Frame Camera (AFC) aboard the HERA spacecraft, its planned positions and orientations along the available trajectories (e.g.: Early Characterization Phase –ECP–, Detailed Characterization Phase –DCP-1–, Detailed Characterization Phase –DCP-2– and Very Close Fly-Bys –VCFB–), the shape parameters of Didymos as well as solar illumination direction had to be considered.

For image rendering, the PRo3D Viewer [3] was used and interfaced via a .json file with these parameters for each individual image to be rendered, including information about the AFC geometry as "fieldOfView" and "resolution" entries. AFC poses, Sun, Didymain and Didymoon positions and orientations were established by means of the HERA Spice Kernels, delivered by GMV, where the barycenter of Didymain is defined as the origin of the coordinate system and its position is always fixed at (0, 0, 0).

For rendering with PRo3D, the shape and dimensions of Didymos were necessary and provided as *.txt files by GMV (Figure 2: Left Bottom). To enhance the realisms of the synthetic images JR added texture and relief to this data (Figure 2, right). For this purpose, the texture and 3D information of Earth Moon surface (as arbitrary texture to test the workflow) were used (Figure 2, Left top and Left centre). In this step, the *.txt files were converted into *.opc (Ordered Points Cloud) files [4], i.e.: the mandatory format for the PRo3D Viewer.

A subset (463/731) of the resulting rendered images of Didymain along DCP-1 trajectory can be seen in Figure 3. The HERA approach to Didymain (from 23 to 10 Km) along the trajectory can be observed.

Shape reconstruction

In order to determine the Didymain shape, the images rendered with PRo3D (e.g.: Figure 3) were supplied to ColMap [5] tool, embedded in PRoViP. ColMap performs 3D reconstruction by means of Structure from Motion (SfM) technique, which assumes that the object to be reconstructed (Didymain) or the sensor (AFC) remains motionless. As Hera AFC moves along the trajectory (e.g. DCP-1) at the same time that Didymain performs its rotation movement, this assumption is violated. In order to establish correct SfM conditions, all AFC poses and orientations calculated with the Spice kernels had to be transformed with the inverse rotations of Didymain to simulate a moving camera around a static scene. Once the rotations were computed the new transformation matrices were provided to ColMap in the “image.txt” file, with following format:

<#>

“#” denotes the frame number, “Q0-Q3” specify the rotation in (Hamilton) quaternion notation and “T0-T2” the translation for the current frame. “CID” indicates the used camera, for this case always the AFC and “filename” defines the corresponding image file, i.e.: the rendered image. In the same way, the intrinsic parameters of the AFC were given in the “cameras.txt” file, which format is as follows:

<#>  

Here “#” specifies the camera id (i.e.: the AFC). “CAMERA_TYPE” defines the used camera model (i.e. Pinhole, Radial etc.), set to SIMPLE_PINHOLE for the AFC. “Width” and “Height” define the image width in pixels following the AFC specifications (1024 by 1024 pixels) and “PARAMS” contains (in the case of SIMPLE_PINHOLE cameras) the FoV in pixels and the principal point.

The reconstructed camera poses calculated with ColMap are represented in Figure 4. They are expressed in the Didymain reference frame and, as mentioned before, do not represent the real AFC positions along the DCP-1 trajectory as calculated with the kernels, but the transformed cameras’ positions required for SfM.

Several views (9) of the resulting 3D reconstructed dense Didymain model (“Didymain_DCP1_meshed-poisson.ply” file) are represented in Figure 5. The resulting statistics of the reconstruction can be seen in Table 1: