SB7

Comets are undoubtedly extremely valuable scientific targets, as they largely preserve the ices formed at the birth of our Solar System. In June 2019, the multi-spacecraft project Comet Interceptor was selected by the European Space Agency, ESA, as its next planetary mission, and the first in its new class of Fast (F) projects [Snodgrass, C. and Jones, G. (2019) Nature Comms. 10, 5418]. The Japanese space agency, JAXA, will make a major contribution to Comet Interceptor. The mission’s primary science goal is to characterise, for the first time, a yet-to-be-discovered long-period comet (LPC), preferably one which is dynamically new, or an interstellar object. An encounter with a comet approaching the Sun for the first time will provide valuable data to complement that from all previous comet missions, which visited short period comets that have evolved over many close approaches to the Sun. The surface of Comet Interceptor’s LPC target will be being heated to temperatures above the its constituent ices’ sublimation point for the first time since its formation.

Following launch, in 2029, the spacecraft will be delivered with the ESA Ariel mission to the Sun-Earth L2 Lagrange Point , a relatively stable location suitable for later injection onto an interplanetary trajectory to intersect the path of its target. This allows a relatively rapid response to the appearance of a suitable target comet, which will need to cross the ecliptic plane in an annulus which contains Earth’s orbit.

A suitable new comet would be searched for from Earth prior to launch, and after launch if necessary, with short period comets serving as a backup destinations. With the advent of powerful facilities such as the Vera Rubin Observatory, the prospects of finding a suitable comet nearing the Sun are very promising. The possibility may exist for the spacecraft to encounter an interstellar object if one is found on a suitable trajectory.

An important consequence of the mission design is that the spacecraft must be as flexible as possible, i.e. able to cope with a wide range of target activity levels, flyby speeds, and encounter geometries. This flexibility has significant impacts on the spacecraft solar power input, thermal design, and dust shielding that can cope with dust impact speeds ranging from around 10 to 70 km/s, depending on the target comet’s orbital path.

Comet Interceptor has a multi-spacecraft architecture: it is expected to comprise a main spacecraft and two probes, one provided by ESA, the other by JAXA, which will be released by the main spacecraft when approaching the target. The main spacecraft, which would act as the primary communication point for the whole constellation, would be targeted to pass outside the hazardous inner coma, making remote and in situ observations on the sunward side of the comet. The two probes will be targeted closer to the nucleus and inner coma region.

Planned measurements of the target include its nucleus surface composition, shape, and structure, its dust environment, and the composition of the gas coma. A unique, multi-point ‘snapshot’ measurement of the comet- solar wind interaction region is to be obtained, complementing single spacecraft observations made at other comets.

We shall describe the science drivers, planned observations, and the mission’s instrument complement, to be provided by consortia of institutions in Europe and Japan.

How to cite: Jones, G., Snodgrass, C., and Tubiana, C. and the The Comet Interceptor Team: The Comet Interceptor Mission, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-606, https://doi.org/10.5194/epsc2021-606, 2021.

Abstract

Comet Interceptor is the first Fast mission of the European Space Agency (ESA); it has been selected in June 2019 and is conceived to study a long-period comet, or interstellar object.

The EnVisS (Entire Visible Sky) camera is being designed for mapping and studying the coma of the selected mission target via imaging the whole sky.

1. Introduction

Comet Interceptor will fly-by a truly pristine comet, i.e. an object very likely entering the inner Solar System for the first time, or, possibly, an interstellar object originating at another star [1].

The present Comet Interceptor mission configuration comprises a spacecraft and two probes. The spacecraft, called A, will make remote and in-situ observation of the target from afar. The two probes, one provided by the Japan Aerospace Exploration Agency (JAXA), called B1, and the other one provided by ESA, called B2, will venture near to the target.

The EnVisS instrument is foreseen to be mounted on B2. EnVisS is an all-sky camera specifically designed to study the coma of the object over the entire sky.

The B2 probe will be a spinning spacecraft, thus a rotational push-broom or push-frame imaging technique can be adopted for EnVisS to scan and image the whole scene around the spacecraft. Filter strips directly bonded to the detector, or mounted very near to it, can be foreseen for studying the target in different wavelength ranges ([2] [3]) or performing polarimetric imaging. Having no moving parts, this solution allows for a compact, low mass and low complexity camera to be implemented. Moreover, since EnVisS is imaging the whole sky, no specific pointing requirements are to be requested for the fly-by geometry.

The EnVisS instrument features a fish-eye lens [5] coupled to a commercial space-qualified detector from 3D-Plus [4] and ad-hoc power and data handling units and software. A prototype of the EnVisS fish-eye lens optical head will be realized, in the coming months, by Leonardo SpA in Florence (Italy) [6].

2. The EnVisS camera concept

EnVisS will feature a push-frame imaging technique, thus acquiring slices of the sky, while the probe rotates (see Figure 1); the slices will be stitched together after acquisition to obtain a full sky image.

Figure 1: Illustration of EnVisS full sky scanning concept.

The camera has a FoV of 180° in the across track direction, i.e. in the direction of the spinning axis, and a global “instantaneous” FoV of 45° in the along track direction, i.e. in the direction of the motion of the scene, considering all together the filter strips coverage (see Figure 2).

The direction of the apparent motion of the scene due to the S/C B2 rotation is parallel to the horizontal direction in Figure 2, while the vertical direction corresponds to the S/C B2 direction of motion.

Figure 2: Schematics of the filters strip on the detector.

For EnVisS to study the comet dust and its polarization, three broad-band filters are foreseen at present. They are:

- one broadband filter positioned to be centered on the detector (orange central strip PAN in Figure 2);

- two polarimetric filters with polarization angles -/+ 60° (the two blue strips POL1 and POL2 in Figure 2).

Due to scientific requirements and technical instrument constraints, the wavelength range for all the filter strips has been selected to be 550-800 nm.

The probe B2 spin-axis will be pointing to the comet nucleus for most of the time, so that the vertical direction will be sampling different phase angles for the coma, i.e. from 0° to 180°.

For a 4s spinning rate of the B2 spacecraft, in 1 ms the scene is moving by one pixel on the detector. The expected acquired signal for the 1 ms exposure time, taking into account the present estimation of the comet coma radiance, is not sufficient to obtain the desired SNR, i.e. at least 10 for the broad-band filter and 100 for the polarimetric measurements.

A flexible approach has been devised to obtain the required SNR for each type of observation. In the direction of the apparent motion of the scene, the signal from the coma is not expected to change too much and a high spatial resolution is not needed from a scientific point of view. Thus the integration time for each filter strip can be tuned allowing for some smearing in the direction of the rotation of the scene. This approach has the beneficial effect that the spatial resolution is retained in the direction where the signal has its maximum variation and thus assuring a sampling of the comet phase function every 0.1° as required. It also allows for an adjustment of the exposure time if the radiance of the coma is different from expected.

For a very faint target, a further increase of the SNR can be achieved with binning in the rotation axis direction and co-addition of images taken in successive spacecraft rotations.

Acknowledgements

The Italian and Spanish authors acknowledge financial support respectively from the Italian Space Agency (ASI) through a contract to the Istituto Nazionale di Astrofisica (2020-4-HH.0) and the State Agency for Research of the Spanish MCIU through the ‘Center of Excellence Severo Ochoa” award to the Instituto de Astrofısica de Andalucıa (SEV-2017-0709) and from project PGC2018-099425-B-I00 (MCI/AEI/FEDER, UE).

References

[1] Snodgrass, C. and Jones, G. H., "The European Space Agency’s Comet Interceptor lies in wait", Nat. Commun. 10, 5418 (2019).

[2] Bell, J. F., III, et al., "Mars Reconnaissance Orbiter Mars Color Imager (MARCI): Instrument description, calibration and performance", J. Geophys. Res. 114, E08S92, (2009).

[3] Da Deppo, V. et al. "Optical design of the single-detector planetary stereo camera for the BepiColombo European Space Agency mission to Mercury", App. Opt. 49(15), 2910-9, (2010).

[4] https://www.3d-plus.com/

[5] Pernechele, C. et al., "Telecentric F-theta fisheye lens for space application", OSA Continuum 4(3), 783-789, (2021)

[6] Toffani, B. et al., "Design of the EnVisS instrument optical head", submitted for SPIE Optical Systems Design 2021.

How to cite: Da Deppo, V., Jones, G., Brydon, G., Pernechele, C., Zuppella, P., Nordera, S., Tubiana, C., Della Corte, V., Fulle, M., Bertini, I., Chioetto, P., Castro, J. M., Lara, L. M., Slavinskis, A., Praks, J., and Rotundi, A.: The EnVisS camera for the Comet Interceptor ESA mission, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-101, https://doi.org/10.5194/epsc2021-101, 2021.

The main goal of ESA’s F-1 class Comet Interceptor mission is to characterise, for the first time, a long period comet; preferably a dynamically-new or an interstellar object. The main spacecraft, will have its trajectory outside of the inner coma, whereas two sub-spacecrafts will be targeted inside the inner coma, closer to the nucleus. The flyby of such a comet  will offer unique multipoint measurement opportunity to study the comet's dusty and ionised environment in ways exceeding that of the previous cometary missions, including Rosetta.
 
The Dust Field and Plasma (DFP) instruments located on both the main spacecraft A and on the sub-spacecraft B2, is a combined experiment dedicated to the in situ, multi-point study of the multi-phased ionized and dusty environment in the coma of the target and  its interaction with the surrounding space environment and the Sun.
 
The DFP instruments will be present in different configurations on the Comet Interceptor spacecraft A and B2. To enable the measurements on spacecraft A, the DFP is composed of 5 sensors; Fluxgate magnetometer DFP-FGM-A, Plasma instrument with nanodust and E-field measurements capabilities DFP-COMPLIMENT, Electron spectrometer DFP-LEES, Ion and energetic neutrals spectrometer DFP-SCIENA  and Dust detector DFP-DISC. On board of spacecraft B2 the DFP is composed of 2 sensors: Fluxgate magnetometer DFP-FGM-B2 and Cometary dust detector DFP-DISC.
 
The DFP instrument will measure magnetic field, the electric field, plasma parameters (density, temperature, speed), the distribution functions of electrons, ions and energetic neutrals, spacecraft potential, mass, number and spatial density of cometary dust particles and the dust impacts.  
 
The full set of DFP sensors will allow to model the comet plasma environment and its interaction with the solar wind. It will also allow to describe the complex physical processes including wave particle interaction in dusty cometary plasma.

How to cite: Rothkaehl, H., Andre, N., Auster, U., Della Corte, V., Edberg, N., Galand, M., Henri, P., De Keyser, J., Kolmasova, I., Morawski, M., Nilsson, H., Prech, L., Volwerk, M., Goetz, C., Gunell, H., Lavraud, B., Rotundi, A., and Soucek, J.: Dust, Field and Plasma instrument onboard ESA’s Comet Interceptor  mission, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-399, https://doi.org/10.5194/epsc2021-399, 2021.

ESA’s Comet Interceptor mission is a low budget, fast track mission to a dynamically new comet (DNC). As a DNC enters the inner solar system for the first time, it is expected to feature strong activity and to display a fluid-scale plasma environment, rather than the kinetic-scale environment encountered at weakly active objects such as 67P.  In situ characterization of this plasma environment is therefore one of the main mission objectives and is the object of the Dust-Fields-Plasma instrument, a suite of sensors for the measurement of the dust, the plasma populations, and the magnetic and electric fields and waves, with the field sensors being mounted on booms, all within strict mass, power, and budget constraints. In this context a sensor has been developed that harbors a fluxgate magnetometer at the center of a hollow spherical Langmuir probe. Precautions have been taken to minimize the possible interference between both, while at the same time being very lightweight. An engineering model has been built, tested and characterized in detail. Together with a companion Langmuir probe and an additional magnetometer in gradiometer configuration, the probe-magnetometer combination (COMPLIMENT + FGM) provides data regarding magnetic and electric fields and waves, total ion and electron densities and electron temperature, as well as the ambient nanodust population. It also offers reference data for the other sensors, such as magnetic field direction, spacecraft potential and total plasma density at high cadence, and integrated EUV flux.

How to cite: De Keyser, J., Ranvier, S., Maes, J., Pawlak, J., Neefs, E., Dhooghe, F., Auster, U., Chares, B., Edberg, N., Fredriksson, J., Puccio, W., Henri, P., Le Duff, O., Peterson, J., and Oja, M.: A Langmuir Probe – fluxgate magnetometer combination for Comet Interceptor, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-285, https://doi.org/10.5194/epsc2021-285, 2021.

Introduction

The Comet Interceptor (CI) mission [1] will pass through a potentially hazardous region of a comet’s inner coma. It is therefore important to assess the dust impact risk to the spacecraft and their scientific instruments to aid hazard mitigation strategies. The purpose of the Engineering Dust Coma Model (EDCM) is to make predictions of which dust the three spacecraft will encounter during the active phase of the mission. The EDCM has been designed with a limited number of input parameters but keeping general physical realism of the phenomena in the inner coma.
We won’t know until after launch what the target comet of CI will look like. This is a particular problem for the dust coma because of the many parameters with unknown/unknowable values. The primary problem is thus not what model to use but what parameters to assume in a model of the inner dust coma. Because the different assumptions about the dust mass loss from cometary nuclei are strongly interdependent [2] it is not obvious a priori which set of parameters represent the best/worst case scenarios.
Instead of defining one set of parameters, we choose ranges for each parameter based on our knowledge of comets as e.g. Halley and other comets. All self-consistent combinations within those ranges will be run through our model to give a prediction of all possible coma environments within parameter space. This ensemble of dust environments is subsequently statistically evaluated to determine a probabilistic distribution of possible conditions which the spacecraft might encounter.
The EDCM is composed of three parts:

• the dust dynamical model that calculates the spatial distribution of dust,
• the scaling model that determines the absolute scaling of the dust densities,
• the instrument model that extracts the number density encountered along the spacecraft trajectories and performs the probabilistic calculations.

The dust dynamical model

The dust dynamical model describes the dust distribution within a cometary coma up to 1000 nuclues radii (R_N). It uses a minimal number of parameters for the description of a cometary dust coma, while keeping it physically realistic. This model physically consistently takes into account the expanding nature and asymmetry of the gas coma (caused by the gas production modulated by solar radiation) and considers the dust dynamics driven by the gas drag force, nucleus gravity, and solar radiation pressure. A series of general assumptions were made to simplify the model:

• The nucleus shape is assumed to be spherical.
• The gas is assumed to be an ideal perfect gas.
• The dust does not influence the gas flow (i.e. no back-coupling of the dust to the gas flow)
• The gas coma is constituted of one single species, H₂O.
• There is no extended gas/dust source/sink in the coma.
• The dust particles are spherical.

For the underlying gas dynamics model, we used the results by [3] who have calculated the gas field by solving the Euler equations. The dust dynamics model used in the EDCM is presented in detail in [4].

The scaling model

To determine the absolute scaling of the dust densities we chose to determine the dust-to-gas ratio, χ, by calculating Afρ for each set of parameters following the approach described in [5]. The dust column density of an aperture of 20R N is calculated. For points outside the simulation domain (1000 RN) a 1/r² extrapolation is applied. The column densities are then convolved with a power law (n ∼ a^−β ) and converted into reflectance using the scattering model of [6] as shown in [2]. The reflectance can then be used to calculate the Afρ as explained in [7]. The absolute scaling χ can then be determined by linearly scaling the model Afρ to the desired Afρ. I.e. if the model Afρ = 100 cm then an actual coma with Afρ = 200 cm is achieved with χ = 2.

The instrument model

In the final step, having determined the absolute scaling, χ, we extract the number density encountered along the spacecraft trajectories for each combination within parameters space. Again, for points outside the simulation domain (1000 R_N) a 1/r² extrapolation of density is applied.

At each point along the trajectory, we calculated the median number of particles predicted by all model variations as well as the 5^th, 10^th, 25^th, 75^th, 90^th, and 95^th percentile. The results for four size bins are shown in Fig. 1. The shaded areas illustrate the variation in the predicted number of particles based on the variation of the input parameters. These ranges thus reflect to a large degree the uncertainty of our knowledge of the future target of CI. As the dust size increases the expected number of particles decreases but the uncertainty increases. Further, the spike in particles around the closest approach (CA) highlights that most particles are encountered very close to CA. E.g. from cometo-centric distances of 10,000 km to CA at 1,000 km the dust densities increase by roughly 2.5 orders of magnitude.

Figure 1: Number of dust particles along the spacecraft trajectory of spacecraft A as a function of cometo-centric distance. The shaded areas show different percentile ranges within which cases fall.

References
[1] Snodgrass & Jones (2019), Nature Communications, 10, 5418.
[2] Marschall et al. (2020), Frontiers in Physics, 8, 227.
[3] Zakharov et al. (2021), Icarus, 354, 114091.
[4] Zakharov et al. (2021), Icarus, 364, 114476.
[5] Marschall et al. (2016), A&A, 589, A90.
[6] Markkanen et al. (2018), Astrophysical Journal Letters, 868, L16.
[7] Gerig et al. (2018), Icarus, 311, 1.

How to cite: Marschall, R., Zakharov, V., Tubiana, C., Kelley, M. S. P., and Della Corte, V.: Dust Hazard Assessment using the Engineering Dust Coma Model of the Comet Interceptor mission, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-540, https://doi.org/10.5194/epsc2021-540, 2021.

Introduction

Comet Interceptor is the first F-class mission developed by the European Space Agency (ESA). The goal of the mission is to intercept a long period comet or an interstellar object. The novelty of Comet Interceptor is, that it will be launched before its main target has been found. Because the target is unknown the spacecraft and its instruments need to be designed such that they can handle a wide range of targets, encounter geometries and potentially hazardous environments [1]. We study the attitude perturbations caused by the impacts of large dust particles during a cometary encounter. Specifically, a numerical model is used to make predictions in relation to Comet Interceptor and its main imaging system called Comet Camera (CoCa).

Method

Because Comet Interceptor is in an early phase we use a generic approach. The dust model is based on force-free radial outflow modelled after comet 1P/Halley. To compare our modelling of the dust coma we use the Engineering Dust Coma Model (EDCM), which will be used by ESA and the industrial consortia designing the Comet Interceptor spacecraft. For simplicity the GNC of our model is idealized, which means that it is able to correct any attitude perturbations instantaneously. Currently there is no knowledge about the implementation of the GNC available and we consider the modelled GNC to be a best case. Further, we assume that the spacecraft has a homogenious mass distribution. To get a statistical distribution of possible outcomes each scenario is simulated 1000 times.

Comparison to Giotto

To validate our model it was applied to the Giotto mission and compared to the measurements acquired during the approach to comet 1P/Halley.

In the table above the results of our model are compared to the total change in velocity Δv [2] and the nutation angle 50 seconds before closest approach of Giotto [3]. This shows that our model is able to produce results that are in the same order of magnitude than what Giotto measured. 

Comparison with EDCM

The EDCM contains a 1th, 5th, 10th, 25th, 50th, 75th, 90th, 95th and 99th percentile of the local dust number density at the specific point along the spacecraft trajectory. To compare our dust model with the EDCM we used the local dust density of a given percentile along the whole trajectory. As shown in the table below, this analysis showed, that our dust model lies in between the 50th and 75th percentile of the EDCM.

Free input parameters

The free parameters of our model are radius, height and mass of the spacecraft, dust production rate, relative velocity at the encounter, distance to the nucleus at closest approach and time interval between attitude correction. For target objects similar to comet 1P/Halley, we will show that without attitude control the nucleus is shifted out of the field of view of CoCa at approximately 40 seconds before closest approach.
We will show that out of the free input parameters the most crucial parameters are the encounter velocity, the spacecraft radius and the time interval between attitude control. Further, scaling laws of the free parameters will be shown. As an example, in Figure 3 the attitude perturbations in relation to the time interval between attitude correction and its scaling law fit is shown.

Conclusion

Based on our analysis we think that there is a high risk of loosing a few images, because the impact of a large particle shifts the nucleus partially or completely out of the field of view of CoCa. We will show that the rate of attitude corrections needs to be <10 seconds and that the total change in angular velocity that needs to be corrected is in the order of 10 °/s. To provide more insightful requirements the GNC needs to be modelled in more detail in the future.

Acknowledgement

This work has been carried out within the framework of the National Centre of Competence in Research PlanetS supported by the Swiss National Science Foundation. The authors acknowledge the financial support of the SNSF.

References

[1] Colin Snodgrass and Geraint H. Jones. The european space agency’s comet interceptor lies in wait. Nature Communications, 10(1):5418, 2019.

[2] P. Edenhofer, M. K. Bird, J. P. Brenkle, H. Buschert, E. R. Kursinki, N. A. Mottinger, H. Porsche, C. T. Stelzried, and H. Volland. Dust Distribution of Comet p/ Halley’s Inner Coma Determined from the Giotta Radio Science Experiment. , 187:712, November 1987.

[3] W. Curdt and H.U. Keller. Large dust particles along the giotto trajectory. Icarus, 86(1):305 – 313, 1990.

How to cite: Haslebacher, N., Gerig, S.-B., Thomas, N., Marschall, R., Zakharov, V., and Tubiana, C.: A numerical model of dust particle impacts during a cometary encounter with application to ESA's Comet Interceptor mission, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-269, https://doi.org/10.5194/epsc2021-269, 2021.

The NASA Double Asteroid Redirection Test (DART) mission will demonstrate asteroid deflection by a kinetic impactor. DART will impact Dimorphos, the secondary member of the (65803) Didymos system, in late September to early October, 2022 in order to change the binary orbit period. DART will carry a 6U CubeSat called LICIACube, contributed by the Italian Space Agency, to document the DART impact and to observe the impact ejecta. LICIACube will be released by DART 10 days prior to Didymos encounter, and LICIACube will fly by Dimorphos at closest approach distance of about 51 km and with a closest approach time delay of about 167 s after the DART impact. LICIACube will observe the structure and evolution of the DART impact ejecta plume and will obtain images of the surfaces of both bodies at best ground sampling about 1.4 m per pixel. LICIACube imaging importantly includes the non-impact hemisphere of the target body, the side not imaged by DART.

The LICIACube flyby trajectory, notably the closest approach distance and the time delay of closest approach, are designed to optimize the study of ejecta plume evolution without exposing the satellite to impact hazard. LICIACube imaging will determine the direction of the ejecta plume and the ejection angles, and will further help to determine the ejecta momentum transfer efficiency β. The ejecta plume structure, as it evolves over time, is determined by the amount of ejecta that has reached a given altitude at a given time. The LICIACube plume images enable characterization of the ejecta mass versus velocity distribution, which is strongly dependent on target properties like strength and porosity and is therefore a powerful diagnostic of the DART impact, complementary to measurements of the DART impact crater by the ESA Hera mission which will arrive at Didymos in 2026. Hera will measure crater radius and crater volume to determine the total volume of ejecta, which together with a ejecta mass-velocity distribution gives a full characterization of the DART impact.

Models of the ejecta plume evolution as imaged by LICIACube show how LICIACube images can discriminate between different target physical properties (mainly strength and porosity), thereby allowing inferences of the magnitude of the ejecta momentum. Measured ejecta plume optical depth profiles can distinguish between gravity-controlled and strength-controlled impact cases and help determine target physical properties. LICIACube ejecta plume images further provide information on the direction of the ejecta momentum as well as the magnitude, requiring full 2-D simulations of the plume images. We will present new simulation model optical depth profiles across the plume at arbitrary positions.

We thank NASA for support of the DART project at JHU/APL, under Contract # NNN06AA01C, Task Order # NNN15AA05T. The Italian LICIACube team acknowledges financial support from Agenzia Spaziale Italiana (ASI, contract No. 2019-31-HH.0 CUP
F84I190012600).

How to cite: Cheng, A., Dotto, E., Fahnestock, E., Della Corte, V., Chabot, N., Rivkin, A., Stickle, A., Thomas, C., Olivier, B., Michel, P., and Kueppers, M.: DART and LICIACUBE: Documenting Kinetic Impact, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-436, https://doi.org/10.5194/epsc2021-436, 2021.

The Hera mission is in development for launch in 2024 within the ESA Space Safety Program. Hera will contribute to the first deflection test of an asteroid, in the framework of the international NASA- and ESA-supported Asteroid Impact and Deflection Assessment (AIDA) collaboration. Hera will also offer a great science return.

1. Introduction
The impact of the NASA DART spacecraft on the 160 m-diameter natural satellite called Dimorphos of the binary asteroid 65803 Didymos in late September 2022 will change its orbital period around Didymos. As Didymos is an eclipsing binary, and close to the Earth on this date, the change can be detected by Earth-based observers. Before impact, DART will deploy the Italian LICIACube that will provide images of the first instants after impact. ESA’s Hera spacecraft will rendezvous Didymos four years after the impact. It will perform the measurements necessary to understand the effect of the DART impact on Dimorphos, in particular its mass, its internal structure, the direct determination of the momentum transfer and the detailed characterization of the crater left by DART.

2. Planetary Defense return
Hera will characterize in details the properties of a Near-Earth Asteroid that are fully relevant to planetary defense. Its objectives related to the deflection demonstration are the following:
•Measuring the mass of Dimorphos to determine the momentum transfer efficiency from DART impact.
•Investigating in detail the crater produced by DART to improve our understanding of the cratering process and the mechanisms by which the crater formation drives the momentum transfer efficiency. 
•Observing subtle dynamical effects (e.g.  libration imposed by the impact, orbital and spin excitation of Dimorphos) that are difficult to detect for remote observers. 
•Characterising the surface and interior of Dimorphos to allow scaling of the momentum transfer efficiency to different asteroids. 

3. Science return
Even if its requirements are driven by planetary defense, Hera will also provide unique information on many current issues in asteroid science. The reason is that our knowledge of these fascinating objects is still poor, especially for the smallest ones. The recent data obtained by the JAXA Hayabusa2 and NASA OSIRIS-REx missions have revolutionized our understanding of carbonaceous-type Near-Earth Objects. Hera has the the potential to do similar as it will rendezvous for the first time with a binary asteroid. Its secondary has a diameter of only 160 m in diameter. So far, no mission has visited such a small asteroid. Moreover, for the first time, internal and subsurface properties will be directly measured. From small asteroid internal and surface structures, through rubble-pile evolution, impact cratering physics, to the long-term effects of space weathering in the inner Solar System, Hera will have a major impact on many fields. How do binaries form? What does a 160 m-size rock in space look like? What is the surface composition? What are its internal properties? What are the surface structure and regolith mobility on both Didymos and Dimorphos? And what will be the size and the morphology of the crater left by DART, which will provide the first impact experiment at full asteroid scale using an impact speed close to the average speed between asteroids?  These questions and many others will be addressed by Hera as a natural outcome of its investigations focused on planetary defense.

4. Instruments
Hera is equipped with the following payload:
- The Asteroid Framing Cameras are both science and navigation cameras. They will provide the target global properties as well as local geomorphology and will investigate the crater. They will also measure the mass of Dimorphos through the “wobble” motion of Didymos.
- The Planetary ALTimeter (PALT) will measure the distance to the target and, from close distance, derive shape and topography information complementary to the shape information in framing camera images.
- A thermal infrared imager (TIRI) will provide information about thermal properties and spectral information in the mid-infrared.
- The Hyperscout-H hyperspectral imager will provide mineralogical information in the spectral range between 450 and 950 nm.
- Milani is a 6 unit cubesat that will carry the ASPECT Fabry-Perot imager to derive mineralogical information, and a thermogravimeter for measuring the abundance and constraining the composition of ambient dust particles.
- Juventas is a 6 unit cubesat that will carry a monostatic low-frequency radar, and a gravimeter to derive interior and surface properties of the asteroids. Its landing on Dimorphos will also allow an estimate of the surface response to a very slow impact.
- The radioscience experiment will measure the gravity field of the Didymos system. It will work in two ways: measurements of the acceleration of the Hera spacecraft by the asteroid pair through the radio link between earth and Hera will be used as well as the intersatellite link between Hera and the two cubesat, which will measure the gravitational parameters from the relative position and velocity of the three spacecraft.

5. NEO-MAPP
NEO-MAPP (Near Earth Object Modelling and Payload for Protection) is a project funded by the H2020 program of the European Commission. Hera is its reference mission, and most of the NEO-MAPP activities are aimed at supporting the preparation of Hera. The main goal of NEO-MAPP is to provide significant advances in our modeling of impact physics, binary dynamics and internal properties, as well as in instrumentations and associated measurements by a spacecraft (including those necessary for the physical and dynamical characterization in general). In particular, innovative and synergetic measurement and data-analysis strategies are developed that combine multiple payloads, to ensure optimal data exploitation for Hera and other NEO missions.

6. Conclusion
The measurements performed by Hera will thus provide unique information on many current issues in asteroid science and therefore, the scientific legacy of the Hera mission will extend far beyond the core aims of planetary defense. Hera is thus an amazing European contribution to the international planetary defense and asteroid exploration era.

Acknowledgements
We thank ESA and CNES for support. We also acknowledge funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 870377 (project NEO-MAPP).

How to cite: Michel, P., Kueppers, M., Fitzsimmons, A., Green, S., Lazzarin, M., Ulamec, S., Carnelli, I., and Martino, P. and the Hera Science Team: The ESA Hera mission to the binary asteroid (65803) Didymos: Planetary Defense and Science, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-71, https://doi.org/10.5194/epsc2021-71, 2021.

How to cite: Kohout, T., Cardi, M., Näsilä, A., Palomba, E., and Topputo, F. and the Milani team: Milani CubeSat for ESA Hera mission, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-732, https://doi.org/10.5194/epsc2021-732, 2021.

The Juventas CubeSat, will be delivered to the Didymos binary asteroid system by ESA's Hera mission within the context of the Asteroid Impact and Deflection Assessment (AIDA) international collaboration. AIDA is a technology demonstration of the kinetic impactor concept to deflect a small asteroid and to characterize its physical properties. Due to launch in 2024, Hera would travel to the binary asteroid system Didymos. It will explore the binary asteroid and the crater formed by the kinetic impact the NASA’s Double Asteroid Redirection Test (DART). HERA will carry two 6U CubeSats, one of which is the Juventas CubeSat developed by GomSpace Luxembourg with the Royal Observatory of Belgium as principal investigator. The spacecraft will attempt to characterize the internal structure of Didymos’ secondary body, Dimorphos, over a period of roughly 2 months using a low-frequency radar, JuRa. During this period, Juventas will also perform radio science measurements using its Inter-Satellite-Link to characterize the mass and mass distribution of Dimorphos. Afterwards, Juventas will attempt to land on Dimorphos, during which the spacecraft is expected to perform several bounces. Once landed, Juventas will use its gravimeter GRASS to obtain measurements of the surface acceleration on Dimorphos for a nominal duration of two orbits. Through the monitoring of dynamics for landing and bouncing impacts as well as measurements from the GRASS gravimeter payload while on the surface, Juventas will determine surface mechanical properties and provide further information on subsurface structure and dynamical properties of Dimorphos.

How to cite: Karatekin, Ö., Le Bras, E., Van wal, S., Herique, A., Tortora, P., Ritter, B., Scoubeau, M., and Manuel Moreno, V.: Juventas Cubesat for the Hera mision, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-750, https://doi.org/10.5194/epsc2021-750, 2021.

The ESA HERA mission approved by the last ESA council Space19+ will be launched in 2024 to deeply investigate the Didymos binary system and especially its moonlet. In 2022, DART/NASA will impact the moonlet to quantify the mechanical response of the body, mainly from ground-based observation [1]. Five years later, HERA/ESA is a unique opportunity to observe in detail the bodies, the crater and the ejecta in order to better constrain mechanical models providing a global characterization of the binary system: shape, density, dynamic properties, thermal properties and composition [2]. The Hera mothercraft will carry two CubeSat’s, Juventas and Milani.

The small spacecraft Juventas will investigate the asteroids’ internal structure. Information about the internal structure is crucial for science, planetary defense and exploration since our current knowledge relies entirely on inferences from remote sensing observations of the surface and theoretical modeling [3]. The Juventas Radar -JuRa- will fathom Didymoon and provide the first direct observations of the deep interior of an asteroid. JuRa is a monostatic radar, BPSK coded at 60MHz carrier frequency and 20MHz bandwidth, inherited from CONSERT/Rosetta [4], [5] and redesigned in the frame of the AIDA/AIM phase A/B [2], [6]. The instrument design is under validation for a flight model delivery in fall of 2022.

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 structures.

• The first objective of JuRA is to characterize the moonlet’s interior, to identify internal geological structure such as layers, voids and sub-aggregates, to bring out the aggregate structure and to characterize its constituent blocks in terms of size distribution and heterogeneity at from submetric to global scale.
• 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. Radar bypasses the near surface alteration by space-weathering and thermal-cycling as observed with optical remote sensing. The observation of the structure and composition of the moonlet will provide constraints on the mechanical model of the impact process.
• The same characterization applied to the main asteroid of the binary system is among the secondary objectives, to detect differences in texture and composition. When compared to the observation of the moonlet, it will constraint the model of binary system formation to discriminate between progressive versus catastrophic process and more generally on the stability conditions of the system.

In this talk, we will review the JuRa science objectives at the instrument development status. We will show the results of the engineering model end-to-end tests and the corresponding instrument performances. Then we will present the proposed operation strategy.

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 (LU), UGA/IPAG (FR), TU Dresden (DE), Astronika (PL) and FZ(CZ). Juventas is built by Gomspace (LU). Juventas navigation plan is developed by GMV (RO)
• 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. 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.

[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. 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.

[4] 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.

[5] 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.

[6] 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., and Kofman, W. and the JuRa instrument team: JuRa: the Juventas Radar on Hera to fathom Didymoon, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-348, https://doi.org/10.5194/epsc2021-348, 2021.

The thermal infrared imager TIRI onboard the ESA Hera spacecraft is being developed to investigate thermophysical properties of the S-type asteroid 65803 Didymos and its moon Dimorphos by mapping thermal inertia and compositional variations of them. TIRI is based on an uncooled micro-bolometer array of 1024 x 768 effective pixels and covers the field of view of 13.3° x 10.0°, with the resolution of 0.23 mrad per pixel. TIRI has an eight-position filter wheel to be used as one wide bandpath at 8-14 µm for thermal imaging, six narrow bands peaked at 7.8, 8.6, 9.6, 10.6, 11.65, and 13.1 µm for compositional information, and one closed plate both for a shutter and a temperature reference.

TIRI will be mounted on the top panel of the Hera spacecraft to point the target asteroids in the same direction with other instruments AFC, PALT, and Hyperscout-H, for the simultaneous observations. The asteroid surface temperature will change day and night according to thermal inertia and roughness of the surface layer, which will be consequently derived from the diurnal temperature profile. The maximum temperature in a day will also change according to the solar distance of the asteroid from ~1 to ~2 au at the beginning to the end of the nominal mission. During the early characterization phase (ECP) at 20 to 30 km from the asteroid, TIRI will take images from large solar phase angles from 50° to 70° with the spatial resolution of ~4.6 to 6.9 m per pixel to construct the asteroid shape model even in the night side and map the thermal inertia and composition. During the detailed characterization phase (DCP) at 10 to 20 km from the asteroid, TIRI will take images from the noon with the spatial resolution of ~2.3 m per pixel for more detailed thermal properties and compositional mapping. During the close-up operation phase (COP) at < 5 km from the asteroid, TIRI will take images from the noon with the spatial resolution of ~1 m per pixel. Higher spatial resolution will be achieved during the further close observations.

In the Hayabusa2 mission, thermal imaging has revealed the highly porous nature of C-type asteroid from global to local scales (Okada et al, 2020; Shimaki et al, 2020), but nobody knows the surface properties of S-type asteroids so that this is a unique opportunity to investigate the S-type asteroid Didymos in comparison with the C-type asteroid Ryugu. For the moon Dimorphos, it will be the smallest asteroid ever explored so that it is also a unique opportunity to investigate the small-sized asteroid, especially for the strength and porosity. TIRI will contribute to verifying Yarkovsky and YORP (B-YORP) effects, orbital and rotational evolution in relation to thermophysical modeling. The temperature profile and compositional difference between the inside and outside of the artificial crater formed by the kinetic impact of the NASA DART spacecraft will be the important target both for the purpose of planetary defense and science.

How to cite: Okada, T., Tanaka, S., Sakatani, N., Shimaki, Y., Arai, T., Senshu, H., Demura, H., Sekiguchi, T., Kanamaru, M., Kouyama, T., Blommaert, J., and Karatekin, Ö. and the Hera TIRI Team: Thermal infrared imaging experiment of S-type binary asteroids in the Hera mission, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-317, https://doi.org/10.5194/epsc2021-317, 2021.

MMX (Martian Moons eXploration) is the 3rd sample return mission of JAXA/ISAS following Hayabusa and Hayabusa2. The MMX spacecraft will be launched in 2024 by an H-III rocket and make a round trip to the Martian system ~5 years. In the proximity of the Martian moons for 3 years, MMX will observe them along with the Martian atmosphere and surrounding space and conduct multiple landings on Phobos to collect Phoboss-indigenous materials. Owing to the lack of definitive evidence, the origin of Phobos and Deimos is under debate between the two leading hypotheses: the capture of volatile-rich primordial asteroid(s) and the in-situ formation from a debris disk that generated by a giant impact onto early Mars. Whichever theory is correct, the Martian moons likely preserve key records on the evolution of the early solar system and the formation of Mars. Through close-up observations of both moons and sample return from Phobos, MMX will settle the controversy of their origin, reveal their evolution, and elucidate the early solar system evolution around the region near the snow line. Global circulation and escape of the Martian atmosphere will also be monitored to reveal basic processes that have shaped and altered the Martian surface environment. The MMX spacecraft consists of three modules with chemical propulsion systems. By releasing used modules at appropriate timings, the spacecraft mass is reduced to allow orbital tuning to quasi-satellite orbits around Phobos, landings on Phobos surface, and the escape from the Martian gravity to return to the Earth. MMX will arrive at the Martian system in 2025 and start close-up observations of Phobos from quasi-satellite orbits. Among the total of 7 mounted instruments for scientific observations, TENGOO (telescope camera) and LIDAR will conduct high-resolution topography mapping and OROCHI (multi-band visible camera), MIRS (infra-red spectrometer provided by CNES), MEGANE (gamma-ray and neutron spectrometer provided by NASA), and MSA (ion mass spectrum analyzer) will survey surface composition and its heterogeneity. Hydrous minerals and interior ice are important observational targets because they, if identified, strongly support the capture hypothesis. Data taken by these instruments will be also useful for the landing site selection and characterization. Before the first landing, a rover (provided by CNES/DLR) will be released near the sampling site to collect data on surface regolith properties to be referred for the mothership landing operation. The rover will carry cameras, miniRAD (thermal mapper), and RAX (laser Raman spectrometer) to collect data on the physical and mineralogical characteristics of the Phobos surface around the sampling site. In early 2027, Mars will come to its closest approach to the Earth which minimizes the communication delay between the spacecraft and the Earth station. Together with the timing relatively far from Sun-Mars conjunctions and the Martian equinoxes, this period is the most favorable for landing operations that need real-time communication with the ground station and solar illumination undisturbed by eclipses. MMX will use two sampling systems, the C-sampler using a coring mechanism equipped on the tip of a manipulator and the P-sampler (provided by NASA) using a pneumatic mechanism equipped on a landing leg. After the stay near Phobos, the MMX spacecraft will be transferred to Deimos-flyby orbits to conduct Deimos observations, and then the return module will depart the Martian system in 2028. During the stay in the Martian system, MMX will also conduct wide-area observations of the Martian atmosphere using imagers (OROCHI, MIRS, and TENGOO) to study the atmospheric dynamics and the water vapor and dust transport. Simultanenousely, MSA will survey ions not only released and sputtered from Phobos's surface but also escaped from the Martian upper atmosphere. CMDM (dust monitor) will continuously survey the dust flux around the moons to assess the processes of space weathering by micrometeoroid bombardments and the possible formation of dust rings along the moons’ orbits. The sample capsule will come back to the Earth in 2029. Complimentarily with remote sensing studies, returned samples will provide us strong cosmo-chemical constraints for the origin of Phobos as well as those for early solar system processes.   

How to cite: Kuramoto, K. and the The MMX Team: Overview and Science of MMX, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-878, https://doi.org/10.5194/epsc2021-878, 2021.

Phobos and Deimos will be globally spectral characterized by MIRS (MMX InfraRed Spectrometer)  onboard of the MMX mission.  MMX is an approved mission by JAXA to explore the two Martian moons, with the aim to return sample (>10 g) from Phobos to Earth. The spacecraft will be launched in 2024, arriving into Mars orbit in 2025 and bringing back Phobos’sample in 2029. Arrived at Mars system, the spacecraft will be injected into Phobos co-orbit and then in orbit around Mars like a Martian Moon in the so called quasi-satellite orbits (QSOs). The QSOs will be settled at different altitudes in the equatorial plane of Phobos to obtain data at various Phobos surface resolution and to reduce the effect of Phobos’gravity.

MIRS will work during the three years of the MMX stay in circum-Martian space. MIRS will provide global characterization of Phobos, Deimos and will observe Mars atmosphere to study spatial and temporal changes. In particular, MIRS instrument will be able to map the Phobos’ surface from High (from 90-190 km) and Medium (from 37-84 km) altitude orbits to determine Phobos composition, to observe in detail the landing site candidates at Low altitudes (from LA:17-38 km; LB: 8-21 km; LC: 6.6-16 km) to provide necessary  information to evaluate and select the two most interesting sampling sites and will perform close observations of the two landing sites, during the Vertical  descent phase. One of the main mission goals is to decipher the origin of the moons, which will provide important constraints on the formation and evolution processes of themselves and clues on planetary formation. The origin of Phobos and Deimos is still a debated question. Past space missions revealed that the properties of these two moons shared similarities with asteroids [1]. Their red spectra, without strong absorption features, are very different from those of Martian surface, and resemble to those of primitive C-D-type asteroids and support consequently the hypothesis of the captured asteroids [2]. On the other hand, for their orbits on the equatorial plane of Mars with very low eccentricity, recent numerical simulations seem to be more in favor of in-situ formation by a giant impact on Mars [3, 4]. 

To unveil the surface composition of the two moons at high spatial resolution and select the two sampling sites, MIRS will observe Phobos and Deimos in the 0.9-3.6 μm range with a spectral resolution better than 20 nm. MIRS will acquire spectra of Phobos at a spatial resolution of about 20 m for a latitude of +/-30° during the Medium altitude survey. The different landing sites will be selected at different resolutions up to few meters for the final five landing site candidates with priority order. A higher spatial resolution less than 1 m will be reached over an area within 50 m from the selected sampling sites. The spectral radiometric absolute accuracy is expected to be of 10%, and the relative accuracy of 1%. The high SNR (>100 up to 3.2 μm) and unprecedented spatial resolution achieved by MIRS will permit to characterize the detailed composition of both the red and blue units on Phobos and to investigate the local compositional heterogeneity associated with the different surface morphology. MIRS is expected to spectroscopically detect and characterize all present signatures, like water (ice) (absorption bands at 1.5, 2.0 and 3.0-3.2 μm), hydrous silicate minerals (features at 2.7-2.8 μm, and minor overtones at 1.4 and 1.8 μm), or anhydrous silicates (bands in the 0.9-1 and 2.0 μm regions) as well also organic matter (3.3-3.5 μm), if present. A detailed characterization of the detected absorption bands, with precise measure of the band center, depth and area, thanks to high S/N ratio, will allow to constrain the surface mineralogy and species’ abundances. Spectral observations of fresh areas, like small craters will provide insights on space weathering processes. MIRS will be also able to measure the spectral radiance of the surface within the instrument footprint. The spectral thermal tail (from about 2.5 μm) will permit to derive the surface temperature, consequently MIRS will allow to derive surface temperature variation and surface thermal inertia.

For Deimos, MMX will perform multiple flight-bys. At a distance of 300 km, MIRS will observe Deimos surface at spatial resolution of 100 m, which is comparable to that for Phobos from high QSO, and it will be able to detect the same major absorption bands as observed in Phobos. The most important scientific goal is to understand whether Deimos is made of the same material as Phobos. The data will allow to detect compositional heterogeneity that could be linked to topography and to the Martian phase aspect. As in the case of Phobos, a full characterization of the bands is possible for band depth as weak as 3%.  MIRS will improve the knowledge of the surface composition especially in terms of spatially resolved spectral data. Even if Deimos is not the target for the sampling, MIRS with the other onboard instruments will considerably improve the physical and chemical knowledge of its surface, and bring new insights about the history of the Martian environment.

MIRS will be able to characterize the global composition of Phobos and Deimos surface material (and subsurface through analysis of crater ejecta). These unprecedented data will allow a better understanding of the two Martian moons. MIRS data will help deciphering whether Phobos composition is closer to primitive dark asteroids and consequently similar to carbonaceous chondrites with possible presence of organics and/or ices which will imply a capture origin or containing, even if partially, high-temperature phase materials representing  a mixture from crust and mantle of Martian silicates [5], more similar to a devolatilized  and/or hydrated Martian mantle, which would indicate a giant impact origin.  

References

[1] Pang, K.D. et al. 1978, Science 199, 64; [2] Fraeman, A.A. et al. 2014, Icarus 229, 196; [3] Rosemblatt, P. et al. 2016, Nature Geoscience 9, 581; [4] Hyodo, R. et al. 2017, Astroph. J., 845; [5] Usui, T. et al. 2020, SSRv 216, 49.

How to cite: Barucci, M. A., Reess, J. M., Bernardi, P., Le Du, M., Doressoundiram, A., Fornasier, S., Sawyer, E., Iwata, T., Nakagawa, H., and Nakamura, T. and the MIRS Team: Phobos and Deimos as observed by MIRS spectrometer on board of Martian Moon eXploration (MMX) mission, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-37, https://doi.org/10.5194/epsc2021-37, 2021.

Introduction

In preparation of the Phobos Rover experiment as part of JAXA’s Mars Moon eXplorer (MMX) mission, we study the illumination conditions on the Martian moon, focusing on the effects of eclipses and radiation from Mars.

JAXA's Mars Moon eXplorer (MMX) mission is due to launch in 2024 and is planned to arrive in the Martian system in 2025 [1]. The probe will deliver a rover [2], built by CNES and DLR, with contributions by INTA and JAXA, to the equatorial area of Phobos (latitude in [-30°, 30°]) between Dec. 15, 2026 and Aug. 15, 2027 [3]. A period of operations of 100 Earth days is planned, i.e. the nominal end of the rover’s mission is at latest on Nov. 23, 2027. Current plans [3] foresee a landing on the sub-Mars hemisphere (longitude in [0°W, 90°W] ∪ [270°W, 360°W]), which necessitates studies on the occurrence of eclipses and on contributions of radiation from Mars to the total incoming flux.

As [4] have shown, eclipses occur around spring and autumn equinox. At the sub-Mars point they can last for up to 55 minutes, which is a significant fraction of Phobos’ orbital period of only 7h 39 min. Thus, eclipses have a significant impact on the amount of energy available to the rover as it is powered by solar panels. Radiation from Mars comprises direct solar flux scattered by Mars towards Phobos and thermal emissions by the Martian surface. Both effects can be of advantage for the rover by providing additional solar energy and generating significantly higher surface temperatures [5] that may help the rover to survive the nights. On the other hand, Mars radiation can be hindrance for the evaluation of data gathered by the rover’s instruments.

Baseline Data

We used the shape model by [6], the rotation model from [7] and - as the rotation model is determined depending on it - the JPL ephemerides MAR097 for this study. To compute the radiation from Mars we use the Mars Climate Database, version 5.3 [8, 9].

Results

Given a location on Phobos, i.e. a facet of the shape model, an eclipse occurs if the facet is inclined to the Sun, and if the Sun is hidden behind Mars. The irregular shape of Phobos effects that even on this small body the eclipse duration is not the same for all locations on the sub-Mars hemisphere. Figure 1 shows the maximum eclipse duration for the currently envisioned landing area during the rover’s nominal mission time. The longest eclipses, lasting for 54.82 minutes, occur for a large area about the sub-Mars point. For some locations, e.g. at the Eastern part of crater Stickney or west of the crater rim, the maximum eclipse duration is much shorter.

Figure 2 shows the eclipse duration for the sub-Mars point and a location in the Eastern part of crater Stickney (lat -3.47°, lon -38.07°) during the rover’s nominal mission time. At autumnal equinox (occuring on Oct. 21, 2027), the eclipse durations attain their maxima of 54.82 minutes and 11.12 minutes, resp. If the rover is landed in an extended region about the sub-Mars point, then, due to solar power requirements of the rover, it is most beneficial to plan rover operations between 62 to 199 days past Dec. 15, 2026 to completely avoid eclipses. At the location at the Eastern part of Stickney the eclipse-free period is much longer and adds up to 205 days.

Since Phobos moves in a bound rotation, the fraction of the Martian disk that is visible from a given location of Phobos, is almost constant. The small changes of up to 7% caused by physical librations, orbital eccentricity and a slight out of equatorial plane movement, can, for this study, be neglected. Figure 3 shows the visibility of the Martian disk for autumnal equinox of Mars year 39.

Due to the bound rotation of Phobos, independent from the epoch, the largest amount of Mars radiation arrives close to the sub-Mars point, going further away from this region the amount of Mars radiation becomes smaller. Figures 4 shows the scattered solar flux and thermal emissions from the Martian surface close to autumnal equinox.

For longitudes near longitude 0° the amount of Mars radiation (scattered solar flux and thermal emissions) is maximum roughly around midnight. For these regions Mars radiation has a big influence on the temperature since then there is no direct solar flux. Going further away from longitude 0° the peaks of both kinds of Mars radiation shift towards the evening (lon in [0°W,90°W]) or morning [270°W, 0°W], resp. [5].

However, due to different lengths of Phobos' orbital period and Mars' rotational period, the positions and magnitudes of Mars radiation change somewhat from orbit to orbit, see Figure 5.

Acknowledgements:

This work was partly supported by research grant No. OB 124/14-1 from the Deutsche Forschungsgemeinschaft (DFG).

References:

[1] Kuramoto, K., et al.: Martian Moons eXploration (MMX): architecture of its science, EPSC-DPS Joint Meeting 2019, EPSC-DPS2019-1636-1, 2019.

[2] Murdoch, N., et al.: The Martians Moons eXploration (MMX) Rover to Phobos, EPSC 2020, EPSC2020-247, 2020.

[3] 4th MMX Science Working Team Meeting, February/March 2021.

[4] Li, Z.Q., et al.: Lighting condition analysis for Mars' Moon Phobos, 2016 IEEE Aerospace Conference, 2016.

[5] Kührt, E. & Giese, B.: A thermal model of the Martian satellites, Icarus, 81, 102-112, 1989.

[6] Willner, K. et al.: Phobos’ shape and topography models, PSS, 102, pp. 51-59, 2014.

[7] Archinal, B. A. et al.: Report of the IAU Working Group on Cartographic Coordinates and Rotational Elements: 2015. Celest. Mech. Dyn. Astron 130, 2018.

[8] Forget, F. et al.: Improved general circulation models of the Martian atmosphere from the surface to above 80 km. JGR 104, 24155–24176, 1999.

[9] Millour, E. et al.: The Mars Climate Database (version 5.3). From Mars Express to ExoMars, 2018.

How to cite: Ziese, R., Willner, K., and Oberst, J.: Illumination conditions on Phobos for the MMX rover mission, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-427, https://doi.org/10.5194/epsc2021-427, 2021.

Asteroids smaller than a few hundred meters in diameter are generally considered to be the most common in the inner solar system evolution; however, they are usually too faint to be detected. This prevented developing a comprehensive list of them and characterizing their physical and chemical conditions. These small asteroids may be leftovers that survived from their parent bodies' impact-driven catastrophic disruptions (and re-accumulation) or escaped from other planetary bodies during multiple processes.  A better understanding of how such objects formed and evolved will constrain the material transport mechanisms in the solar system. Also, smaller asteroids frequently approach the Earth because of their highest population. Some may enter the atmosphere to cause fireball events (they explode while passing through a denser atmosphere) and impact events, leading to damage to civilization. However, it would be challenging to assess and mitigate such events without knowledge of their physical and chemical conditions. 

The Hayabusa2 extended mission, led by the Japan Aerospace Exploration Agency (JAXA), is a rendezvous mission that plans to rendezvous with asteroid 1998 KY26 in 2031 and fly by asteroid 2001 CC21 in 2026. While cruising, the spacecraft will also continue to observe the zodiacal light and exoplanets. The major science objectives of this mission are (1) constraining the origin and evolution mechanism of small asteroids and (2) establishing knowledge about planetary defense.

The rendezvous target 1998 KY26 has been observed by optical observations and ground radar in detail [e.g., Ostro et al., 1999]. This is a near-Earth asteroid with a 20 m – 40 m and a spin period of 10.7 min. Also, the radar-derived shape model infers that the shape is relatively round but still has a high uncertainty. The surface color is dark, implying the potential taxonomic classes associated with a mixture of carbonaceous materials and mafic silicates (B, C, F, G, D, P) [Ostro et al., 1999]. The reported roughness at centimeter-to-decimeter scales suggests exposed bear rocks [Ostro et al., 1999].

The centrifugal force is a dominant element controlling this asteroid’s geophysical properties. The bulk density should be higher than 2,800 kg/m3 to keep surface materials. If the bulk density is lower than that, the surface slope should reach 180 deg in the equatorial region, and thus particles on the surface can be lofted if there is no attraction. Identifying particles in this region will constrain the roles of cohesion. On the other hand, the polar regions may still be gravity-dominant, where small particles may preferably rest on the surface there. If particle ejection is active, those not escaping from the body may orbit for some period and eventually be accumulated in these regions. The internal structure is always tensile everywhere, although the tensile stress is low, and the bulk cohesive strength necessary to sustain the body is only ~20 Pa, which is within the reported strength of rubble pile bodies [Hirabayashi et al., 2021]. If this is the case, while the monolithic condition is favored, the rubble pile structure is still plausible. Furthermore, because of its size, the orbital and rotational conditions change due to the Yarkovsky and YORP effects in a short period [Vokrouhlický et al., 2000; Tholen, 2003; Nesvorný and Vokrouhlický, 2008]. 

Optical observations have shown that the fly-by target 2001 CC21 has a size of ~700 m and a spin period of ~5 h. While surface compositions are not well characterized, it may be L-type, which may be related to primordial compositions in the solar system, although an S-type is also possible. Lightcurve observations [Ries et al., 2005; Pravec, 2021, personal communication] imply that this asteroid is highly elongated, a ratio of the short axis to the long axis being ~0.6. If this is the case, and this asteroid has a rubble pile structure, the asteroid should have experienced unique re-accumulation processes after catastrophic disruptions. Physical processes such as violent impacts, mass wasting, space weathering have likely changed the surface conditions; the variations in morphologies, roughness, and chemical compositions constrain such physical processes.

This work presents an overview of science investigations in the Hayabusa2 extended mission targeting 1998 KY26 and 2001 CC21.

References:

• Hirabayashi et al. (2021), Hayabusa2 extended mission: New voyage to rendezvous with a small asteroid rotating with a short period, Advances in Space Research, In Press.
• Nesvorný and Vokrouhlický (2008), ANALYTIC THEORY FOR THE YARKOVSKY–O'KEEFE–RADZIEVSKI–PADDACK EFFECT ON OBLIQUITY, The Astrophysical Journal, 136, 1.
• Ostro et al. (1999), Radar and optical observations of asteroid 1998 KY26. Science 285, 557–559.
• Ries et al. (2005), Photometry of Near Earth Asteroids at McDonald Observatory, Frank N. Bash Symposium 2005: New Horizons in Astronomy ASP Conference Series, Vol. 352, 2006.
• Tholen (2003), Recovery of 1998 KY26: Implications for Detecting the Yarkovsky Effect, American Astronomical Society, DPS meeting #35, id.31.05; Bulletin of the American Astronomical Society, 35, 972
• Vokrouhlický et al. (2000), Yarkovsky Effect on Small Nearth-Earth Asteroids: Mathematical Formulation and Examples, Icarus 148, 1, 118-138.

How to cite: Hirabayashi, M., Hirata, N., Kamata, S., Kitazato, K., Kouyama, T., Morota, T., Sakatani, N., Shimaki, Y., Tanaka, S., Wada, K., and Watanabe, S.: Hayabusa2 Extended Mission: Rendezvous with 1998 KY26, one of the most common but unexplored near-Earth asteroids, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-442, https://doi.org/10.5194/epsc2021-442, 2021.

DESTINY+ (Demonstration and Experiment of Space Technology for INterplanetary voYage with Phaethon fLyby and dUst Science) was selected in 2017 as a mission for JAXA/ISAS small class program. It will be launched in 2024 by an Epsilon S rocket and flyby Phaethon in January, 2028. It is a joint mission of technology demonstration and scientific observation. The engineering mission is led by ISAS/JAXA and the science mission is led by PERC, Chiba Inst. of Technology (ChiTech). It will test high performance electric propelled vehicle technology and high-speed flyby of asteroid (3200) Phaethon and possibly asteroid 2005UD, a likely break-up body from Phaethon, as an extended mission. Engineering challenges include an up-close encounter at a distance of 500 km from Phaethon with radio-optical hybrid navigation guidance and control, and autonomous imaging based on optical information for target tracking during a high-speed flyby of about 35km/sec. The science goal is to understand the nature and origin of cosmic dust brought onto the Earth, in the context of exogenous contribution of carbon and organics for possible prebiotic seeds of the terrestrial life. Phaethon is a parent body of Geminid meteor shower, and thus a known source to periodically provide dust to the Earth, via its dust stream. The science objectives are two folded: (1) in-situ analyses of velocity, arrival direction, mass and chemical composition of interplanetary and interstellar dust particles around 1 au, the dust trail, and nearby Phaethon, and (2) flyby imaging of Phaethon to study its geology, for understanding dust ejection mechanism of active asteroid and the surface feature and composition which are affected by extensive solar heating. Science payloads include a panchromatic, telescopic camera with a tracking capability (TCAP), a visible-NIR multi-band camera with four bands of 425, 550, 700, 850 nm (MCAP), and a dust analyzer (DDA), which is an upgrade version of Cassini Cosmic Dust Analyzer (CDA). While the two cameras are developed by PERC/Chitech, DDA is developed by Univ. of Stuttgart, as an international collaboration with DLR. Ground calibration for DDA is being performed with German/Japanese joint efforts. International observation campaign for Phaethon was conducted in December 2017, and that of asteroid 2005 UD in October, 2018. Also, international observation campaign for stellar occultation by Phaethon was performed in 2019. Here, we present the current status and science of DESTINY+ mission.

How to cite: Arai, T. and the DESTINY+ Team: Overview and Science of DESTINY+, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-877, https://doi.org/10.5194/epsc2021-877, 2021.

The DESTINY+
spacecraft (Demonstration and Experiment of Space Technology for
INterplanetary voYage with Phaethon fLyby and dUst Science) will be launched to the
active asteroid (3200) Phaethon by the Japanese Space Agency JAXA in 2024. The main
mission target will be Phaethon with a close flyby in 2028. Together with two cameras, the
DESTINY+ Dust Analyzer (DDA) on board will perform close observations of this rockcomet type object to solve essential questions related to the evolution of our inner Solar
System, especially the heating processes of small bodies. Phaethon is believed to be the
parent body of the Geminids meteor shower and is considered to be a comet-asteroid
transition object. Such objects likely play a major role to better understand the nature and
origin of mass accreted on to Earth. The DDA dust analyzer is an upgrade of the Cassini
Cosmic Dust Analyzer (CDA) which very successfully investigated the dust environment of
the Saturnian system. The DDA instrument is an impact ionization time-of-flight mass
spectrometer with integrated trajectory sensor, which will analyse sub-micrometer and
micrometer sized dust particles. The instrument will measure the particle composition (mass
resolution m/Δm ≈ 100-150), mass, electrical charge, impact velocity (about 10% accuracy),
and impact direction (about 10° accuracy). In addition to dust analysis in the vicinity of
Phaethon during the close flyby at this small asteroid, DDA will continuously measure dust
in interplanetary space in the spatial region between 0.9 and 1.1 AU during the
approximately four years spanning cruise phase from Earth to Phaethon. We give a progress
report of the instrument development together with an update on the preparation of the
scientific measurements planned during the DESTINY+ mission.

How to cite: Krüger, H. and the DESTINY+ Dust Analyzer Team: Dust Science with the DESTINY+ Dust Analyzer, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-871, https://doi.org/10.5194/epsc2021-871, 2021.

Twinkle’s rapid pointing and non-sidereal tracking capabilities will enable the observation of a diverse array of Solar System objects, including asteroids and comets. Twinkle aims to provide a visible and near-infrared (0.5-4.5 micron) spectroscopic population study of asteroids and comets to study their surface composition and monitor activity. Its wavelength coverage and position above the atmosphere will make it particularly well-suited for studying hydration features that are obscured by telluric lines from the ground as well as searching for other spectral signatures such as organics, silicates and CO₂.

I will present an overview of Twinkle’s capabilities and discuss the broad range of targets the mission could observe, including the measurements it will take to support JAXA's Martian Moons eXploration (MMX) mission, demonstrating the broad scientific potential of the spacecraft.

How to cite: Edwards, B., Tessenyi, M., Stotesbury, I., Archer, R., Wilcock, B., and Joshua, M.: Twinkle: a low-Earth orbit, visible and infrared observatory for exoplanet and solar system spectroscopy, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-588, https://doi.org/10.5194/epsc2021-588, 2021.

With the introduction of visible and infrared Imaging Spectrometers about 20 years ago, we have assisted to a dramatic enhancement of the scientific return achieved by space missions launched for the exploration of the Solar System. This innovative class of instruments are in fact able to conjugate together the imaging capabilities of the cameras with the spectral resolution accomplished by spectrographs. By exploiting such capabilities, planetary scientists can retrieve and map the physical and chemical properties of a target body and correlate them with surface morphological features or with dynamical structures of the atmospheres. It is not by chance that such instruments have become an essential payload on many planetary exploration missions since they have demonstrated their capabilities to allow a better understanding of the composition, processes and evolution of Solar System bodies. In the meanwhile, ongoing photonic research has made available new devices whose technology is mature enough to allow to investigate new optical concepts for color cameras and imaging spectrometers architectures to be implemented in a single integrated instrument (⨏ISPEx), which we present here. The ⨏ISPEx camera design uses a Liquid Crystal Tunable Filter (LCTF) to ensure high flexibility in the selection of the bandpass (between 0.42-0.73 µm) and bandwidth (35 nm at 0.55 µm) of the color filters, which can then be adapted to very different scientific targets. The camera has a broad 3.3°x1.6° FOV and an IFOV of 14 µm allowing to reach a spatial resolution of 0.14 m/px from 10 km distance. Conversely, the ⨏ISPEx VIS (0.4-1.05 µm spectral range, 3.2 nm/band sampling) and IR (0.95-5.0 µm, 8 nm/band) integral field imaging spectrometers use Coded Mask Optical Reformatters (CMOR) based on optical fibers bundles to collect the full hyperspectral cube through a single acquisition resulting in a great reduction of the acquisition time with respect to traditional whiskbroom and pushbroom instruments. The FOV/IFOV are 0.4° (circular)/100 µrad and 0.28°x0.28° (square)/225 µrad for the VIS and IR channel spectrometers, respectively. These values correspond to a 70 m-wide circular hyperspectral image made of 4500 pixels at 1 m/px resolution for the VIS channel and to a 50x50 m image made of 484 pixels at 2.25 m/px for the IR channel for acquisitions made from a distance of 10 Km. These requirements are optimized for remote sensing of asteroids and comets from close distances. Camera’s and spectrometers’ FOV are co-located on the same boresight thanks to the use of a common Three Mirror Afocal Telescope operating with a 2.4X beam reduction factor (entrance pupil diameter 84 mm). By means of a beamsplitter and a dichroic filter the collimated beam generated by the telescope is split to the camera and spectrometers’ pupils. Thanks to the solutions we are developing, the camera and imaging spectrometers will operate at the same time on the same boresight, resulting in a great simplification of their operations. In this respect, the ⨏ISPEx will offer the advantage to allow a better exploitation of the data collected by the three channels resulting in a tremendous advantage for many scientific investigations. With this synergic approach it will be possible to analyze high resolution images to constrain morphology interpretation of a target, while hyperspectral data collected at the same time allow the retrieval of composition and physical properties. The availability of camera images makes possible to apply sharpening algorithms to spectrometer’s ones. Apart this, an integral-field spectrometer will keep the capabilities of more traditional whiskbroom and pushbroom spectrometers but it will overcome them when the target scene is rapidly evolving and changing during the acquisition: traditional instruments are limited by the fact that the cube acquisition process may take a time longer than the temporal scale of the investigated event. This is the case of observations enquiring into the dynamical evolution of planetary atmospheres, lightning events, outbursts, hypervelocity impact or fast-moving targets. By operating with fast readout detectors, an Integral Field spectrometer can adequately resolve the four dimensions of data (2D spatial, spectral and temporal) opening the possibility to perform time-resolved hyperspectral movies. Another substantial advantage of Integral Field spectrometers is their better operability during fast flyby phases, where the distance from the target and illumination geometry are rapidly changing, resulting in limited time periods suitable to observe the target with optimal conditions. By collecting the entire hyperspectral cube in a fraction of the time necessary to complete the scan for a traditional scanning spectrometer, an Integral Field spectrometer can reach a level of imaging flexibility similar to the one achieved by a camera. Within this study we are defining the configuration of the ⨏ISPEx space model operating in the 0.4-5.0 µm spectral range including optical performance analyses, and thermomechanical and electronic architecture. Moreover, we are realizing a development breadboard limited to the 0.4-1.0 µm spectral range to conduct performance tests at system level on LCTF and CMOR devices. We gratefully acknowledge financial contribution from the Agreement ASI-INAF n.2018-16-HH.0.

How to cite: Filacchione, G., Tarabini, M., Mazzotta Epifani, E., Ciarniello, M., Piccioni, G., Raponi, A., Saggin, B., Kanuchova, Z., Palumbo, P., Guerri, I., Taiti, A., Novi, S., Barilli, M., Zambelli, M., Biondi, D., Boccaccini, A., Nuccilli, F., and Giusti, M.: ƒISPEx, the Integral-Field Imager and Spectrometer for Planetary Exploration, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-23, https://doi.org/10.5194/epsc2021-23, 2021.

Abstract

We conduct experiments using a single-wheel testbed and simulations using the Soft-Sphere Discrete Element Method to study wheel-regolith interactions on small-body surfaces. We analyze wheel sinkage and traction on different surface materials and we discuss the influence that low-gravity has on rover maneuverability. 

Introduction

Wheeled locomotion systems have been successfully operated on the surfaces of the Moon and Mars for decades. Though wheeled rovers have been proposed for small-body exploration [1], they have yet to be tested on the surface of a small moon or asteroid. Driving on a granular surface in a milli or micro-gravity environment will be challenging, but possible. In fact, a rover will be deployed to the surface of Phobos as part of the upcoming Martian Moons eXploration mission [2]. The MMX rover will not only demonstrate the feasibility of driving in low-gravity, but it will greatly improve our understanding of regolith dynamics on small body surfaces.

In addition to the navigation cameras, the MMX rover includes two additional cameras, known as the WheelCams. These cameras will capture the direct interaction between the rover’s wheels and the surface of Phobos [2]. The WheelCam images will be used to determine surface properties like grain size distribution and shape, friction angle, cohesion, and bearing strength. In anticipation of the MMX mission, we are using experimental and numerical methods to develop techniques to analyze and interpret the WheelCam images. In fulfillment of this objective, we are conducting single-wheel driving tests on different surface materials and are studying the influence that friction, cohesion, and gravity have on the flow behavior of the regolith and the driving performance of the rover.

Experiments

The WheelCam testbed at ISAE-Supaéro (Fig. 1) [3] is being used to analyze wheel and regolith behavior from the perspective of the MMX rover WheelCams. In this set-up, a motor is used to rotate a 3D printed wheel at a constant speed, and a set of horizontal and vertical sliders allow the wheel to move freely in the horizontal and vertical directions. Various sensors are used to measure the sink and slip of the wheel, and two cameras are positioned to capture the wheel-regolith interaction from the same perspective as the MMX WheelCams. Tests are conducted in different granular materials in order to catalogue the resulting trench forms and sinkage levels, and to capture any observable disturbances caused by the wheel’s grousers.

Simulations

In addition to conducting experiments, we also simulate the rover wheel traversing a bed of granular material using the Soft-Sphere Discrete Element Method (SSDEM) and the open-source code Chrono [4, 5]. The wheel geometry is based on a simplified version of the MMX rover wheel design [6], and the granular bed consists of approximately 200,000 spherical particles which are 5.5-6.5 mm in diameter (Fig. 2). In the simulations, the wheel sinks into the bed under gravity and then rotates at a constant and controlled velocity. The wheel’s behavior is evaluated using metrics like sinkage and slip, and its performance is compared for different types of surface materials (i.e., different friction and cohesion levels) and different gravity levels.

Results

By combining the experimental and numerical results, we can analyze wheel-regolith interactions for a wide range of material cases. We see that wheel sinkage varies drastically according to the friction angle and the cohesion of the surface material. However, the sinkage remains constant across different gravity levels. This observation is consistent with findings from parabolic flight experiments [7]. The numerical simulations allow us to study the flow of the regolith directly under the rover wheel in more detail. Recent impact experiments have shown that granular material becomes fluidized more quickly under reduced gravity conditions [8]. Here, we look for the transition from the quasi-static to the inertial flow regime when the rover starts moving to see if fluidization of the material in low-gravity restricts the maneuverability of the rover.

Acknowledgements

We would like to thank ISAE-Supaéro and the Centre National d’Études Spatiales (CNES) for financially supporting this research effort. The simulations were conducted using HPC resources from CALMIP under grant allocation 2019-P19030.

References

[1] Wilcox, B. and Ross M. J. "The MUSES-CN nanorover mission and related technology." IEEE Aerospace Conference. Proceedings (Cat. No. 00TH8484). Vol. 7. IEEE, 2000.

[2] Michel, P., et al. "The MMX rover: performing in-situ surface investigations on Phobos." Earth, Planets, and Space. Accepted. 2021.

[3] Passoni, L., et al. "A Single-Wheel Testbed for Regolith Science Studies." Lunar and Planetary Science Conference. No. 2548. 2021.

[4] Sunday, C., et al. "Validating N-body code CHRONO for granular DEM simulations in reduced-gravity environments." Monthly Notices of the Royal Astronomical Society 498.1. 2020.

[5] Tasora, A., et al. “Chrono: An open source multi-physics dynamics engine.” International Conference on High Performance Computing in Science and Engineering, Springer, Cham , 2015.

[6] Sedlmayr, H., et al. "MMX-Development of a Rover Locomotion System for Phobos." IEEE Aerospace Conference. IEEE, 2020.

[7] Wong, Y., et al. "Predicting the performances of rigid rover wheels on extraterrestrial surfaces based on test results obtained on earth." Journal of Terramechanics 49.1. 2012.

[8] Murdoch, N., et al. "Low-velocity impacts into granular material: application to small-body landing." Monthly Notices of the Royal Astronomical Society 503.3. 2021.

How to cite: Sunday, C., Murdoch, N., Imhof, G., Tardivel, S., Ulamec, S., and Michel, P.: Wheel-regolith interactions on small-body surfaces, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-473, https://doi.org/10.5194/epsc2021-473, 2021.

EI+FWI Method for Reconstructing Interior Structure of Asteroid Using Lander-to-Orbiter Bistatic Radar System

Jian Deng¹,Wlodek Kofman^2,3, Pei-min Zhu¹, Alain Herique ²,Ruidong Liu⁴, Shi Zheng⁴

¹ China University of Geoscience AQ:5 (CUG), Wuhan 430074, China (e-mail: dengjian_cug@126.com).

²  CNRS, CNES, IPAG, Université Grenoble Alpes, 38000 Grenoble, France, wlodek.kofman@univ-grenoble-alpes.fr).

³  Centrum Badan KosmicznychPolskiej Akademii Nauk (CBK PAN), 00-716 Warsaw, Poland (e-mail:

⁴ China Academy of Space Technology, Xi’an Branch, Xi’an 710100, China (e-mail: shizheng_xjtu@163.com).

Since the missions to asteroids including Dawn, Hayabusa2 [2 - 4],  OSIRIS-Rex [5] and so on, we had accumulated some preliminary knowledge on the surface structure and components of asteroids. But the internal structure which is the key to understanding and modeling asteroid accretion and evolution history is still fairly unknown and has never been measured accurately for any asteroid. In view of this, China National Space Administration (CNSA) is actively preparing a mission named “Zhenghe” [6] aiming to reveal the inner structure of Asteroid 2016HO3. Consequently, study on radar tomography for imaging the interior of asteroids is becoming one of the most in-demand technologies.

The conventional radar tomography is usually based on ray tracing method (RTM), which utilizes only a small portion of the signal information and is unable to generate complete results especially under the acquisition geometry of limited data density. To get the accurate dielectric permittivity distribution within the asteroid, full waveform inversion (FWI) [7] which utilizes the whole information (including time, amplitude and phase) of the signal, is proposed as a promising method which could be applied in the future asteroid interior detection missions.

Despite the high resolution, FWI is a highly nonlinear and ill-posed inverse problem which is sensitive to inaccuracies of the starting model especially when bandwidth is limited. Because of the bandwidth limitation, the range-compressed radar signal with carrier oscillates rapidly in the time domain, which makes the object function of FWI contains numerous local minimums. Thus, it is hard for FWI to converge to the global minimum of model space without an accurate initial model. Concerning this problem, we propose the envelope inversion (EI) [8], which is more robust to the choice of initial model because the time-domain envelope signal is in the baseband frequency range and varies slower than the range-compressed signal with carrier, as a supplement to FWI to recover the inner permittivity distribution of asteroid. The conventional FWI compares in the object function complete wave forms, while EI compares the time-domain envelope between the synthesized and observed signal.

It has been discussed that asteroid radar tomography could be performed in different observation geometries, such as monostatic orbiter, bistatic orbiters and lander-to-orbiter system, and our work mainly focuses on the bistatic lander-to-orbiter scenario. Such analogous scenario (Fig. 1) has been applied in the CONSERT (Comet Nucleus Sounding Experiment by radio wave transmission) [9] mission to the Comet 67P/CG.

Fig. 1.  Lander-to-orbiter configuration. This artistic view is based on CONSERT/Rosetta

In our work, the effects of initial model, noise level and carrier-frequency on FWI and EI algorithm for asteroid tomography are analyzed based on 2D numerical experiments [8],. The 2D numerical experiments shows that the EI+FWI combination, where EI serves to define the initial model for FWI, constrained by total variation regularization, has good potential for asteroid tomography. Based on EI+FWI strategy, a series of 3D numerical experiments are conducted to test the influence of orbital measurement density and landing site on the tomography. The synthetic asteroid model and the observation geometry are presented in Fig. 2.

The 3D numerical experiments demonstrate that, if the signal is less contaminated by noise, EI+FWI has the ability to reconstruct the interior structure with a sufficient accuracy even under a sparse observation (like only 20 orbital measurement points); but if the signal is highly contaminated by noise, dense observation will be indispensable to ensure the reconstruction accuracy. Besides, the results also indicate that the single lander system has the potential to reveal the interior of asteroid, but the landing site could have a significant influence on the tomography. The partial results of the 3D experiments are shown as Fig. 3.

Fig. 2. (a) 3D synthetic asteroid model, the model contains 3 anomalies (red) embedded in the background (green),  the anomalies and the background is 4 and 3, respectively. (b) Schematic illustration of spiral observation geometry, it contains 480 measurement points (blue dots), and the angle θ is between the rotation axis and the line defined by the lander and the barycenter of the asteroid

Fig. 3.  EI+FWI reconstructions under the spiral observation illustrated in Fig. 2(b). (a) and (b) present the reconstructions without noise and with the SNR= –10dB (before the processing of range compressing the BPSK signal), respectively. The inversions are started with a homogenous asteroid of which the  (i.e. 10% lower than the background of true model).  

References.

[1] C. T. Russell and C. A. Raymond, “The dawn mission to vesta and  ceres,” Space Sci. Rev., vol. 163, nos. 1–4, pp. 3–23, Dec. 2011.

[2] Y. Tsuda, S. et al.,  “Flight status of robotic asteroid sample return mission Hayabusa2,” Acta Astronautica, vol. 127, pp. 702–709, Oct. 2016.

[3] S.-I. Watanabe et al., “Hayabusa2 mission overview,” Space Sci. Rev., vol. 208,

nos. 1–4, pp. 3–16, Jul. 2017.

[4] D. Perna et al., “Spectral and rotational properties of near-Earth asteroid (162173) Ryugu, target of the Hayabusa2 sample return mission,” Astron. Astrophys., vol. 599, p. L1, Mar. 2017.

[5] D. S. Lauretta, “OSIRIS-REx asteroid sample-return mission,” in Hand- book of Cosmic Hazards and Planetary Defense, Eds. Cham, Switzerland: Springer, 2015, pp. 543–567.

[6] E. Gibney, “China plans mission to Earth’s pet asteroid,” Nature, to be published.

[7] P. Sava and E. Asphaug, “3D radar wavefield tomography of comet interiors,” Adv. Space Res., vol. 61, no. 8, pp. 2198–2213, Apr. 2018.

[8], J. Deng,et al., “EI + FWI Method for Reconstructing Interior Structure of Asteroid Using Lander-to-Orbiter Bistatic Radar System”  IEEE TGRS in press

[9] W. Kofman et al., “Properties of the 67P/Churyumov-Gerasimenko interior revealed by CONSERT radar,” Science, vol. 349, no. 6247, Jul. 2015, Art. no. aab0639.

How to cite: Deng, J., Kofman, W., Zhu, P., Herique, A., Liu, R., and Zheng, S.: EI+FWI Method for Reconstructing Interior Structure of Asteroid Using Lander-to-Orbiter Bistatic Radar System, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-50, https://doi.org/10.5194/epsc2021-50, 2021.