Analysis of dust shield and detection system response to hypervelocity impacts for Comet Interceptor Dust Impact Sensor and Counter

Introduction: The dust ejected by cometary nuclei encodes valuable information on the formation and evolution of the early Solar System. Several short-period comets have already been studied in situ[1], but their pristine condition was modified by multiple perihelion passages. Dynamically new comets (DNCs) remain pristine bodies since they never visited the inner Solar System, stationing more than 2000A.U. far away from the Sun in the Oort cloud.

Comet Interceptor (CI) is the first F-class space mission selected by the European Space Agency to study a DNC or an interstellar object entering the inner Solar System for the first time[2]. The Dust Impact Sensor and Counter (DISC) is an instrument included in the Dust Field and Plasma (DFP) suite, part of the CI payload, dedicated to characterizing the dust encountered by the spacecraft (S/C) during its flyby in the coma of the target DNC. DISC will measure hypervelocity impacts (HVIs), in the range 10–70km/s, with cometary dust particles of 1–400μm diameter. It aims to characterize the mass distribution of dust particles in the range 10^-15–10^-8kg, and retrieve information on dust structural properties from impacts duration[3].

DISC design: DISC is a 121×115.5×46mm³ aluminum box containing both the detection system and the electronics (Fig.1). The former consists in a 100×100×0.5mm³ aluminum plate with three piezoelectric traducers (PZTs) at its corners. HVIs induce shockwaves in the sensing plate. Far from the impacted area, such waves become acoustic Lamb waves that propagate up to the PZTs, which start to vibrate at their resonant frequency. A couple of electronic boards at the bottom of the unit allows to retrieve the particles momentum and kinetic energy from PZTs vibration signal.

Fig.1: DISC sensing element and dust shield design.

DISC detection system is derived from the GIADA Impact Sensor measurement subsystem, that was designed to measure impacts of slow particles[4]. During CI flyby, some hypervelocity dust particles might perforate DISC outer sensing diaphragm and represent a serious hazard for the instrumentation. A dedicated mechanical element preliminarily designed as made of four 1cm-thick aerogel blocks and a 1mm-thick aluminum frame was integrated into DISC design to shield the entire S/C from such dangerous impacts.

Two key aspects need to be verified to ensure that the instrument is suitable for CI aims:

• DISC capability to survive the expected coma dust environment;
• DISC capability to measure the momentum/energy of impacting particles in the aforementioned size and mass ranges.

Dust shield assessment: We verified DISC dust shield performance using a two-stage Light-Gas Gun (LGG) (Open University, Milton Keynes) to shoot mm-sized particles of various materials at speeds around 5km/s[5,6]. This facility allowed to test the instrument resistance to momenta in the range 10^-2–10^-1kg·m/s and to energies of the order of 10²J. The dust shield showed good resistance up to energies of about 200J, released by a 3mm nylon bead at 5.5km/s. DISC resistance to higher-energy particles can be improved by increasing the aerogel thickness, without any further modifications to the general design.

These experiments proved that DISC is compatible with the foreseen coma dust environment. Integrating a thicker aerogel layer in the design will reduce the risk of failure due to higher-energy particles to low enough values even for the S/C more exposed to the dust flux. The S/C beneath DISC unit is further protected by DISC lower layers.

DISC performance: DISC will measure momenta in the range 10^-11–10^-3kg·m/s[7]. The LGG facility allows to reach high momentum values by shooting heavy particles, but their collision dynamics is very different from what expected for cometary dust. A different strategy to simulate the foreseen impact momentum range is needed.

A Van der Graaf (VdG) gun can shoot μm-sized dust particles up to 20km/s, reproducing momenta of 10^-9–10^-7kg·m/s[8].

The tested impact parameters range can be extended by simulating HVIs effects with a high-power pulsed laser beam. Laser intensity, beam dimension, and pulse duration can be regulated to respectively match impact pressure, section, and shock duration of the corresponding particle[9]. Laser intensities of 10⁹–10¹⁰W/cm² can generate surface pressures from kbar to Mbar, typical of cometary dust particles colliding at 3–6km/s. Using our Nd:YAG laser (λ=1064nm), which emits τ=6ns pulses with pulse energy of E_pulse=1.2J, we can cover a momentum range of 10^-10–10^-5kg·m/s. Since laser simulated and VdG real impacts share part of the released momentum range, laser shots can be calibrated and their representativity verified with real collisions.

The energy range expected for dust impacts measured during CI flyby is 10^-7–10²J. Laser simulated impacts cannot reach the higher energy values. However, the energy/pulse duration range is pretty vast and with some attenuators and pulse reducers the central/left part of the parameters space (around mJ energy and ns pulse time) could be reasonably covered.

Fig.2. shows the optical setup: a polarizer attenuator splits the beam and allows to regulate its power; a couple of mirrors prevents backwards reflections to get to the laser output aperture; a beam expander enlarges the beam, which enters a vacuum chamber and is focused by a plano-convex lens on the DISC breadboard mounted on a 3-axis translational stage. The vacuum chamber is fundamental to prevent plasma generation in air around the focus.

Fig.2: Optical setup for high-power pulsed laser simulated HVIs.

By properly tuning the laser parameters, this strategy allows to achieve representative simulations of cometary dust HVIs. In addition to assess DISC performances, simulating the same impact many times provides large statistics to calibrate DISC detection system and momentum/kinetic energy retrieval methodology with great accuracy.

References: [1] Keller H. U. and Kührt E. (2020) Space Sci. Rev., 216(1), 1–26. [2] Snodgrass C. and Jones G. H. (2019) Nat. Commun., 10(1), 1–4. [3] Della Corte V. et al. (2021) LPSC LII, Abstract #2332. [4] Esposito F. et al. (2002) Adv. Space Res., 29(8), 1159–1163. [5] McDonnell J. A. M. (2006) Int. J. Impact Eng., 33(1–12), 410–418. [6] Hibbert R. et al. (2017) Procedia Eng., 204, 208–214. [7] Di Paolo F. et al. (2021) LPSC LII, Abstract #1238. [8] Friichtenicht J. F. (1962) Rev. Sci. Instrum., 33(2), 209–212. [9] Pirri A. N. (1977) Phys. Fluids, 20(2), 221–228.