DISC-the Dust Impact Sensor and Counter on-board Comet Interceptor: characterization of the dust coma of a dynamically new comet

36 37 The Comet Interceptor space mission, selected by ESA in June 2019 as the first F-Class 38 mission, will study a dynamically new comet or an interstellar object by a unique multi-point 39 ’snapshot’ measurement. The mission design will allow to complement previous single 40 spacecraft’s fly-by cometary observations . The Dust Impact Sensor and Counter (DISC), devoted 41 to the dust coma characterization, is part of the payload selected for Comet Interceptor. It will be 42 mounted on-board two of the three spacecraft, as part of the Dust-Fields-Plasma (DFP) suite, 43 dedicated to understand further: 1) dust in the coma; 2) magnetic field; 3) plasma and energetic 44 neutral atoms. DISC architecture originates from the Impact Sensor subsystems, part of the Grain 45 Impact Analyzer and Dust Accumulator (GIADA) that successfully flew on-board the 46

1 Istituto di Astrofisica e Planetologia Spaziali, Istituto Nazionale di Astrofisica, Via fosso del cavaliere, 100, 00133 14 Rome, Italy (vincenzo.dellacorte@inaf.it) 15 2 Dipartimento di Scienze e Tecnologie, Università degli Studi di Napoli "Parthenope", CDN, IC4, 80143 Naples, 16 Italy, 17 3 LESIA, Observatoire de Paris, Universite PSL, Sorbonne Universite, Universite de Paris, 5 place Jules Janssen, 18 92195 Meudon, France.   7 Osservatorio Astronomico di Trieste, Istituto Nazionale di Astrofisica, Trieste 24 8 Osservatorio Astronomico di Roma, Istituto Nazionale Astrofisica, Monteporzio Catone 25 9 Centrum Badań Kosmicznych PAN Warszawa, Poland, 26 10 Department of Physics and Astronomy, School of Physical Sciences, Ingram Building, University of Kent, 27 Canterbury, Kent, CT2 7NH. 28 11 School of Physical Sciences, The Open University, Milton Keynes, MK7 6AA, UK. 29  to count dust particles with mass >10 −15 kg; 3) to constrain dust particle density/structure. 3 In this paper, we describe DISC design, aims, methods, feasibility and performances 4 evaluations, carried out by real and simulated dust impacts and by retrieving the number of 5 particles, and their corresponding momentum, using the Comet Interceptor's Engineering Dust Introduction 10 11 Giotto, the first deep space ESA mission [Reinhard, 1986], returned the most detailed images 12 of comet 1P/Halley "small and dark" nucleus, characterized by intense activity from discrete areas 13 [Keller et al. 2020]. Three decades later, the Rosetta/ESA space mission observed comet 14 67P/Churyumov-Gerasimenko, continuously for two years, inbound to and outbound from 15 perihelion, contributing to a major advance in comet science and planetary formation processes 16 [  23 following perihelion passage would not be feasible due to their orbital periods of thousands of 24 years. Comet Interceptor solves this issue because, after launch, the spacecraft will be inserted in 25 a "parking orbit" to L2 of the Sun-Earth system, awaiting target discovery and selection. This 26 mission profile is designed thanks to a powerful ground-based telescope, the Vera Rubin Large 27 Synoptic Survey Telescope (VR-LSST) that from 2023 will continuously monitor the sky and will 28 identify incoming DNCs with a few years' notice [Jones et al., 2020]. 29 Comet Interceptor is composed of a primary spacecraft, S/C A, which also acts as a communication 30 hub, and two sub-spacecraft, housed on S/C A during the L2 parking and cruise phases, S/C B1 31 (provided by JAXA) and S/C B2 (provided by ESA). This configuration will allow multi-point 32 observations of the DNC and its surrounding environment [Snodgrass et al., 2019]. After DNC 33 detection, the multiprobe will move from L2 towards the selected target. When close to the target, 34 the three S/C will separate, with S/C A remaining at larger distance (flyby closest approach at 35 about 1000 km) and S/Cs B1 and B2 will flyby closer (at about 300 km) to the DNC. 36 The payload on-board the three S/C includes:  measure their momentum. DISC on S/C A, due to the large distance from the DNC nucleus is 1 expected to detect a smaller dust flux. Thanks to the reduced number of dust impacts and less 2 restriction on data volume for S/C A, DISC data will be more informative: downloading complete 3 data acquisition will allow a deep post-download analysis of PZT signals that will provide 4 additional information on dust physical properties (e.g. density, porosity, tensile strength). 1) What is the composition of the gas and dust in the coma? 22 2) How does the coma connect to the nucleus, i.e. how does cometary activity work? 23 3) What is the nature of coma-solar wind interaction? 24 To answer these questions, the high-level scientific requirement is to characterize the dust in the 25 coma and to determine how it interacts with plasma. 26 DISC will contribute to these scientific aims by means of measurement requirements and with 27 expected performances reported in the dust related science traceability matrix (Table 1).

Dust measurement requirements DISC requirements
Characterise the dust in the coma and how it interacts with plasma.
Determine dust fluxes and particles' mass and size.
Dust detection rate up to 200 particles/second. Compare electron, ion and dust densities, to constrain dust charging.
Dust flux and dust spatial distribution. At max flyby speed (70 km/s) the spatial resolution will be 70 m, at lowest flyby speed (10 km/s) it will be 10 m.  3 boards, housed at the bottom of the mechanical box, and the sensing plate, located at the top of the 4 box and exposed to the dust environment ( Figure 1). A dust shield is mounted between the sensing 5 plate and the electronics to protect it from hypervelocity dust particles, possibly crossing the DISC 6 sensing plate. To save mass and improve efficiency, the dust shield is made of an aerogel layer (2 7 cm thick) installed within a lightweight aluminum frame. The dust shield, as demonstrated by real 8 hyper-speed dust impacts [Ferretti et al., 2022], is able to protect the undermounted electronic 9 boards from dust impacts with kinetic energy up to 400 Joule. The aerogel capability to slow down 10 and stop hyper-speed particles was demonstrated by the NASA/Stardust space mission [Brownlee 11 et al., 2006]. The use of an aerogel dust shield allowed us to maintain DISC mass (0.5 kg) within 12 the allowed budget, saving about 90% of mass with respect to a conventional dust shield design. 13 The design of the DISC sensing device is inherited by the GIADA-IS (Grain Impact Analyser 14 and Dust Accumulator-Impact Sensor), flown successfully on-board the ESA/Rosetta space probe  Figure 1C). Three PZTs mounted in the selected configuration is the minimum 19 number of PZTs to measure position and momentum of the dust impact. A larger number of PZTs, 20 e.g. as for GIADA-IS, would have guarantee measurement redundancy, which is not mandatory 21 for Comet Interceptor mission concept. The PZTs detect the bending Lamb waves, i.e. elastic 22 waves whose particle motion lies in the plane that contains the direction of wave propagation and 23 the direction perpendicular to the plate [Lamb, 1917], generated by the dust impact and 24 propagating across the plate. The resulting electrical signal has an amplitude proportional to the      to disentangle two dust particle populations in the DNC coma: 1) those ejected from the 13 nucleus and directly impacting DISC (direct particles); and 2) those impacting DISC 14 after deflection due to solar radiation pressure and/or gravity (reflected particles). This 15 will be possible as DISC will measure the dust mass distribution contemporarily at In Table 2   • a dust dynamical module, to calculate the dust spatial distribution, considering the 12 expanding nature and asymmetry of the gas coma and the dust dynamics, driven by the 13 gas drag force, the gravity and the solar radiation pressure; 14 • a module to scale the dust volume densities, which performs dust column integration, 15 considering the dust scattering and a size distribution, to obtain the brightness Af (defined The maximum impact rate detectable by DISC, evaluated by HIV impact numerical simulation, is 7 about 200 particles/s (see next paragraph). We compared this intrinsic DISC performance with the 8 expected dust environment. From the derived dust spatial density (Figure 4) expected along the 9 fly-by (S/C A and S/C B2), we can conclude that: 1) for S/C A the resulting number of dust impacts 10 per second is far from the DISC limit (200 particles/s), even in the case of the maximum flyby 11 speed (70 km/s); 2) for S/C B2 the number of particles per second is close to the DISC limit only 12 when the maximum flyby speed is considered. We can conclude that DISC performances are 13 suitable for the predicted dust environment. 14  N°of particles for each size bin hitting DISC exposed surface (both on S/C A and S/C B2) Max 2 3 To study the feasibility of using the GIADA Impact Sensor working principle, designed and tested 4 in a low speed impact regime, to the Hyper Velocity Impact regime in which DISC is expected to 5 work, we ideated a multi-methodology study. The study allowed us also to evaluate DISC 6 performances in the frame of Comet Interceptor fly-by. 7 8 Comet Interceptor flyby speed will depend on DNC selection, but for sure it will be within the 9 range 10 -70 km/s, thus coinciding with dust speed; DISC is designed to measure the momentum 10 of particles between 1 and 200 μm in size. These speed and size intervals result in particle momenta 11 to be measured by DISC ranging between 10 -11 -10 -3 kg m/s. These momentum values imply that 12 the DISC sensing plate will be impacted by particles with energies up to 10 3 J [Di Paolo et al., 13 2021]. These high energy impacts prevent making a DISC performances evaluation and calibration 14 in laboratory. For this reason, we have devised a performance evaluation approach implying 15 different techniques/methods: 16 17 1. Hyper Velocity Impacts of real projectiles on the DISC breadboard at accelerator facilities. 18 2. Hyper Velocity Impacts simulated by means of a high-power laser on the DISC breadboard. 19 3. Numerical simulation of Hyper Velocity Impacts on a simulated DISC sensing plate. 20 This approach allows to: 1) overcome the unfeasibility of performing in laboratory HVI in the 21 speed range foreseen for Comet Interceptor; 2) anticipate DISC performances; and 3) consider the 22 processes involved in HVI regime, e.g. light flash, ionization, formation of ejecta, that can modify 23 the momentum transfer to the DISC sensing element. 24 25 26 Hyper Velocity Impacts with real projectiles on the DISC breadboard at accelerator facilities. 27 Hyper Velocity Impact tests are performed on the DISC breadboard using two different accelerator  30 achieving impact velocities up to 20 km/s [e.g., Friichtenicht, 1962]. While the former facility is 31 used mainly to verify DISC mechanical performances, e.g. to investigate the dust shield efficiency, 32 the latter is used mainly to verify DISC-PZTs performances in an HVI regime, in combination 33 with the other two methods listed above. Table 3 Table 3. Particle parameters used as projectiles at the Light Gas Gun and Van de Graaff accelerators to 1 study DISC response in the lower range of the hyper velocity regime. For the reported particle compositions, 2 individual particles can only be shot for diameters down to ~few hundred-microns. For smaller particles at 3 the LGG accelerator a buckshot projectile configuration must be used and only multiple particle impacts 4 can be tested. With respect to the LGG, at the VdG facility smaller particles (close to the lower limit of DISC 8 measurement range) can be used, the projectiles have to be denser than ≈ 800 kg m −3 , i.e. than 9 those expected for cometary dust [Fulle et al., 2017]. The combination of particle physical 10 properties results in a particle momentum regime well suited for DISC performance. We use the 11 outcome of these Hyper Velocity Impact tests as a benchmark for the simulated impacts approach. 12 In addition, these tests are used to verify the effectiveness of the DISC dust shield (see the DISC 13 design section). 14 15 Hyper Velocity Impacts simulated by means of a high-power laser on the DISC breadboard. 16 In the frame of DISC performances evaluation, in order to extend the range of impact parameters, 17 e.g. momentum, speed, energy, we designed and realized an experimental set up exploiting laser- 18 simulated Hyper Velocity Impacts. This approach takes advantage of the late stage equivalence 19 [Pirri, 1977]: by choosing appropriate laser intensity, beam radius, and pulse duration, the effect 20 of HVIs on a DISC breadboard is reproduced. By means of a Nd:YAG (wavelength 1064 nm) 21 pulsed laser, energies up to about 1.2 J and pulses around 3-6 ns can be reached, reproducing HVI 22 of particles with different velocities, sizes and contact times. In Figure 5 we report the results of 23 the study we performed to identify particle speed and radius ranges to reproduce hyper velocity 24 dust impacts by means a Nd:YAG pulsed laser. We identified the velocity/radius ranges at different 25 particle densities, representative of cometary dust: fluffy aggregates (in blue) ; 26 porous aggregates (in cyan and red) [Flynn et al., 2013]; and olivine grains with zero porosity (in 27 black) [Marsh, 1980]. 28 29 We determined, considering the particle density interval 1-3214 kg/m 3 , the ranges of particle 30 momentum and mass that can be simulated by a Nd:YAG pulsed laser. We report these ranges in 31 Figure 6 (gray area) together with the corresponding momentum and mass values of real particles 32 used for HVIs at the LGG (magenta) and VdG (green) facilities. It is noticeable that the laser-33 simulated HVI and the VdG real particle impacts cover a common range of particle parameters;

Figure 5
Particle velocity/radius ranges, for different particle densities, whose hyper 3 velocity impacts can be simulated by a Nd:YAG pulsed laser. Numerical simulation of Hyper Velocity Impacts on a simulated DISC sensing plate. 1 We setup a dedicated numerical simulations chain able to characterize the effects of an HVI on a 2 surface, describing in details the impact mechanical process and retrieving the simulated PZTs 3 signal. Combining particle-based Smoothed-Particle Hydrodynamics (SPH) and element-based 4 Finite Elements (FE), the chain provides a balanced model configuration to guarantee both 5 calculation efficiency and results accuracy. In fact, SPH alone can well characterize the material 6 distortion and compression induced by HVI, but it would lead to unmanageable calculation time. 7 The strategy we followed implies to apply: 1) SPH to the region close to the impact, characterized 8 by heavy distortion and compression of the impacted material; 2) FE for the discretization of the 9 whole impacted surface with linear elastic response. The DISC response to HVI was analyzed 10 applying in cascade SPH and FE by means of AUTODYN TM and ANSYS TM platforms, 11 respectively. The effect of a particle impacting on the DISC sensing plate is simulated in two 12 steps: 13 1. As a first step we apply SPH to simulate dust particle impact in a region very close to the 14 impact point, starting from the contact time up to about 10 -6 s after it. The simulation 15 reproduces the impact and the consequent changes of the impacted surface due to the 16 shockwaves. Large deformations and material detachments occur until, relatively far from the 17 impact point, Lamb waves form and the elastic regime is set ( Figure 7). 18 2.  In green is reported the SPH discretization of the simulated sensing plate and its fragments.
pag. 14  The simulations were also used to evaluate the dead time of the DISC sensing plate. We carried 1 out simulation till when the signal registered at PZT extinguished. With this procedure we 2 obtained, for the upper limit of particles momentum, a maximum extinction time of about 2 3 milliseconds. DISC dead time is linked to the extinction time of the Lamb waves, i.e. the time 4 needed to extinguish the vibration in the sensing plate after each impact. 5 6 In Figure 8 we report the comparison between Lamb waves generated with different methods: 1) 7 registered by a PZT after a real low velocity dust impact (< 10 m/s) on the DISC sensing plate 8 (upper panel); 2) induced on the DISC sensing plate by a HVI simulated by means of a high power 9 laser pulse (middle panel); and 3) resulting from simulated HVI on the DISC sensing plate. The 10 comparison confirms that the DISC sensing plate has a similar elastic behavior for simulated HVI 11 (far from the impact point) and for low-speed impacts [Liu et al 2016, Piccirillo et al. 2021]. The 12 comparison of signals from GIADA Impact Sensor measurements with those obtained by 13 simulated Hyper Velocity Impacts (HVI) on the DISC sensing plate shows the suitability of the 14 GIADA Impact Sensor working principle in the frame of Comet Interceptor requirements. 15 16 17 Conclusions 18 19 The DISC instrument is dedicated to the in-situ dust characterization of the coma environment of 20 a Dynamically New Comet, the target of the Comet Interceptor/ESA space mission. DISC is a 21 direct evolution of the GIADA-Impact Sensor subsystem, successfully flown on-board the 22 ESA/Rosetta space probe. In this paper, we showed that the working principle conceived for the 23 GIADA-Impact Sensor, foreseen and tested for slow to medium velocity impacts, is valid also in 24 the Hyper Velocity Impacts (HVI) regime. We report the DISC expected performances, in terms 25 of detectable events, sensitivity limits and measurement capabilities, that we derived by means of 26 the Engineering Dust Coma Model. In addition, we evaluated the DISC performances, concluding 27 that they fulfill the main scientific requirements to obtain the coma dust mass distribution of a 28 Dynamically New Comet, with a multi-methods strategy: 29 1) HVI tests firing real particles onto the DISC breadboard sensing plate by means of two