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.