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# In Situ Cosmic Dust Detection for Heliophysics

Hsiang-Wen Hsu, Andrew Poppe, Jamey Szalay, Mihaly Horanyi, Veerle Sterken, Thomas Chen, Ingrid Mann, Mitchell Shen, Priscilla Frisch, Sigrid Elschot, et al.

► To cite this version:

Hsiang-Wen Hsu, Andrew Poppe, Jamey Szalay, Mihaly Horanyi, Veerle Sterken, et al. In Situ Cosmic Dust Detection for Heliophysics. Bulletin of the AAS, 2023, 55 (3), pp.e-id. 176 (2023). 10.3847/25c2cfeb.b165eaae. obspm-04920467

HAL Id: obspm-04920467

https://hal-obspm.ccsd.cnrs.fr/obspm-04920467v1

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# Science Opportunities Enabled by In Situ Cosmic Dust Detection Technology for Heliophysics and Beyond

A White Paper submitted to the Decadal Survey for Solar and Space Physics (Heliophysics) 2024-2033

Hsiang-Wen Hsu<fnref n="1" /> (sean.hsu@lasp.colorado.edu; +1 720 470 3148)

Co-authors (by alphabetical order):

Nicolas Altobelli², Lennart R. Baalmann³, Lisse Carey⁴, Thomas Y. Chen⁵, Andrzej Czechowski⁶, Sigrid Elschot⁷, Priscilla Frisch⁸, William Goode<fnref n="1" />, Mihály Horányi<fnref n="1" />, Sascha Kempf<fnref n="1" />, Harald Krüger⁹, William Kurth¹⁰, Yanwei Li¹¹, David Malaspina<fnref n="1" />, Ingrid Mann¹², Nicole Meyer-Vernet¹³, Michiko Morooka¹⁴, Libor Nouzak<fnref n="1" />, Andrew Poppe¹⁵, Frank Postberg¹⁶, Mitchell M. Shen¹⁷, Jonathan Slavin¹⁸, Ralf Srama¹¹, Veerle J. Sterken², Zoltan Sternovsky<fnref n="1" />, Jamey Szalay¹⁷, Jakub Vaverka¹⁹, Xu Wang<fnref n="1" />

## Synopsis

We discuss a suite of instruments cable of carrying out the next generation of in situ cosmic dust measurements from the heliosphere into interstellar space in support of the Interstellar Probe mission concept. A Dust Analyzer should be considered as the highest priority for its coverage of both compositional and dynamical information of the bulk of interstellar dust (ISD) and interplanetary dust particle (IDP) populations, essential to address the major science questions. A PVDF Dust Counter and Plasma Wave Antenna instrument could additionally provide critical improvement through the detection of larger, rarer dust populations, to constraining the mass density of ISD as well as providing additional directionality coverage. A Neutral Mass Spectrometer bridges the measurement gap between microscopic dust grains and gas species, potentially relevant for understanding the nature and interactions of the very local interstellar medium and our heliosphere. The Dust Analyzer, PVDF Dust Counter, and Plasma Wave Antenna instruments can be calibrated using dust accelerator facilities (University of Colorado, USA, see Shu et al., 2012; and Universität Stuttgart, Germany) with ISD-relevant materials at realistic mass and speed ranges.

<footnote n="1">¹University of Colorado Boulder, USA; ²European Space Agency; ³ETH Zürich, Switzerland; ⁴Johns Hopkins University Applied Physics Laboratory; ⁵Columbia University; ⁶Polish Academy of Sciences, Poland; ⁷Standard University; ⁸University of Chicago; ⁹MPI for Solar System Research, Germany; ¹⁰University of Iowa; ¹¹University of Stuttgart, Germany; ¹²Arctic University of Norway, Norway; ¹³Observatoire de Paris, France; ¹⁴Swedish Institute of Space Physics, Sweden; ¹⁵University of California, Berkeley; ¹⁶Freie Universitaet Berlin, Germany; ¹⁷Princeton University; ¹⁸Harvard University; ¹⁹Charles University, Czech Republic.</footnote>
## Introduction and Open Science Questions

Cosmic dust is a general term encapsulating microscopic solid objects in space. Their classification generally depends on their current locations and orbital properties, e.g., interstellar dust (ISD) comes from sources outside the solar system, whereas interplanetary dust particles (IDP) originate from bodies with orbits bound to the Sun. As solid grains, cosmic dust behaves differently, yet interactively, with neutral gas, plasmas, and the electromagnetic environments of the heliosphere and interstellar space. Solid particles can serve as both sources and sinks for gas and plasmas, depending on the spatial distribution and ambient conditions. As the Sun moves through the Milky Way Galaxy, the heliosphere encounters and interacts with the very local interstellar medium (VLISM) upstream. These interactions define the configuration of our heliosphere and determine the interstellar medium (ISM) influx delivered to the inner solar system (e.g., Thomas, 1978). Apart from in situ field-and-particle measurements carried out by the Voyager spacecraft (e.g., Richardson et al., 2008; Krimigis et al., 2019), our knowledge about such interactions and possible recycling between phases of materials in the ISM is largely based on remote sensing observations and theoretical studies (e.g., Thomas 1978; Grün et al., 1994; Linde and Gombosi, 2000, Draine 2003; Mann 2010; Slavin et al., 2012, 2015; Sterken et al., 2012; Frisch et al., 2022).

<figure><img src="image_3.png" /></figure>

**Figure 1.** Predicted ISD mass distribution based on the Earth-based remote sensing observations of the VLISM (Draine, 2009; solid black curve), compared to the inflowing ISD measured in situ by Ulysses and Galileo in the inner heliosphere (Landgraf et al., 2000; blue squares). The difference suggests significant filtering of small ISD by the heliosphere. The bottom panel displays the detectable dust grain mass ranges of the three dust detection methods. This figure is modified from the ISP mission concept report.

So far, observational constraints about the nature of ISD in VLISM are largely based on measurements made from the inner heliosphere (Grün et al., 1993; Witte et al., 1993). The Sun shields much of the solar system from interstellar elements via the heliospheric magnetic field carried by the solar wind. Incoming ISD is filtered and only a selective fraction (with low charge-to-mass ratio) passes through relatively unimpeded and can be detected in the inner heliosphere. Most small ISD do not reach the inner heliosphere due to electromagnetic interactions (Grün et al., 1994; Linde and Gombosi, 2000; Slavin et al., 2012; Sterken et al., 2013, also Fig. 1) as are organic materials (Altobelli et al., 2016; Kimura et al., 2020). **The Interstellar Probe (ISP) mission concept (McNutt et al., 2021; Brandt et al., 2022) provides the first ever opportunity to reach into the ISM and directly sample unaltered ISD without solar influence.** This unique opportunity will help to address many overarching science questions related to heliophysics and beyond. A companion white paper (Poppe and Szalay, 2022) discusses three critical science questions related to ISD to be addressed in the next decades:
* What are the physical characteristics of interstellar dust, including their composition, size distribution, flux magnitude, and charge distribution?

* What are the dynamics of interstellar dust as they interact with the heliosphere?

* What effects do interstellar dust grains have on the structure, dynamics, and composition of the heliosphere?

In this white paper, we discuss the viability of existing in situ dust detection technology feasible to measure the desired compositional and dynamical properties of dust populations from inner heliosphere to interstellar space to address the overarching science questions directly relevant to heliophysics, planetary sciences, and astrophysics. The synergies between dust and heliosphere science are also discussed in a separate white paper (Sterken et al., 2022).

<table><caption>Table 1. An overview of existing in situ dust detection instruments. All these instruments are included in the example payloads of the Interstellar Probe mission concept. The Sensitive Area, Mass & Power, and data rate are adopted from the ISP mission concept report.</caption><thead><tr><th></th><th>Dust Analyzer</th><th>PVDF<br>Dust Counter</th><th>Plasma Wave<br>Antenna</th><th>Neutral Mass<br>Spectrometer</th></tr></thead><tbody><tr><th>Measurables</th><td>- Composition<br>- Grain mass/size<br>distribution<br>- Number density<br>profile<br>- Impact velocity<br>- Electric charges</td><td>- Grain mass/size<br>distribution<br>- Number Density<br>profile</td><td>- Grain mass/size<br>distribution<br>- Number density<br>profile</td><td>- Nanograin<br>composition<br>- Number density<br>profile</td></tr><tr><th>Sensitive<br>Area</th><td>400 cm<sup>2</sup></td><td>1 m<sup>2</sup></td><td>Spacecraft bus<br>conductive<br>surface area</td><td>≈ 1 cm<sup>-2</sup></td></tr><tr><th>Mass &<br>Power</th><td>10 kg / 12 W</td><td>1 kg / 2W</td><td>~11 kg / 11 W</td><td>10 kg / 11 W</td></tr><tr><th>Pointing</th><td>Spacecraft ram</td><td>Spacecraft ram</td><td>No constraint</td><td>Spacecraft ram</td></tr><tr><th>Data Rate</th><td>1bps (30kbit per<br>spectrum)</td><td>1bps</td><td>1 - 2 bps</td><td>10<sup>5</sup> bits per<br>spectrum</td></tr></tbody></table>

## In Situ Dust Detection in ISP Mission Concept Study

The development of in situ dust detection technology is rooted in heliophysics. The first in situ measurements of interstellar dust in the heliosphere were performed by the impact ionization dust detector on board the Ulysses mission (Grün et al., 1993, also rediscovered in *Helios* data, e.g., Altobelli et al., 2006). Instruments dedicated to in situ ISD analysis are presented as an integral part of the notional payload design in the ISP Mission Concept Study (McNutt et al., 2021; Brandt et al., 2022). In situ characterization of the spatial and compositional distributions of solid components in the outer heliosphere and the VLISM will provide direct constraints on their interactions with the heliosphere. Understanding the role of ISD in VLISM and their heliospheric interactions will further our knowledge about the evolution of other astrospheres at different epochs and locations around the Galaxy.

Most in situ dust detection methods rely on the impact ionization process, that is, the resulting ionization of projectile material by kinetic energy of a hypervelocity impact. One important
property regarding the ISP mission concept is the high spacecraft speed of ~30 km s⁻¹ in order to reach interstellar space within a reasonable time frame. The impact speed of ISD, considering its relative speed with respect to the Sun, could be > 50 km s⁻¹ if ISP heads towards the nose of the heliosphere. Since the amount of impact plasma production increases significantly with impact speeds, modern dust instruments are well suited to analyze the predominant sub-µm-sized ISD populations down to < 100 nm (Fig. 1) and provide dynamical and compositional information of each incident particle.

## In situ Dust Detection Methods

In the following sections, we discuss the existing in situ dust detection methods viable to provide the required measurables relevant for heliophysics science. All four types of instrumentations discussed here are considered in the ISP mission concept. Here we discuss their measurement capabilities, heritages, and scientific relevance. Collectively, they provide a broad measurement range of grain size, compositional, and dynamical properties of dust populations from the heliosphere to interstellar space. Table 1 highlights the measurables, data rate, and example instrument specification of these dust detection methods. We also provide updates about the latest instrument developments and analyses relevant for heliophysics studies, as well as synergetic perspectives with existing and upcoming space missions.

## Dust Analyzer

The Dust Analyzer is an impact ionization dust instrument with an integrated Time-of-Flight (ToF) mass spectrometer. Through multi-coincidental signals generated by hypervelocity impacts of incident dust grains inside instrument, a modern Dust Analyzer provides the most precise and comprehensive information about the mass/size, speed, and composition of individual dust grain. A Dust Analyzer is sensitive to sub-µm-sized grains at impact speeds > 10 km s⁻¹. The amplitude of the impact charge signal collected at the instrument target provides information about the mass and impact speed of the incident dust grain. The grain composition can be measured by the integrated ToF mass spectrometer. The development of a settable gain adjustment, mentioned in the ISP mission concept report, will allow measurements to cover a broad range in grain size/impact speed parameter space. If equipped with an entrance velocity grid system (more details later), the electric charges (amount and polarity) and impact velocity vector of incident grains can also be measured (Auer et al., 2002).

The first in situ measurement of interstellar dust in the heliosphere was measured by the impact ionization dust detector on board the Ulysses mission (Grün et al., 1993). While the Ulysses dust detector was not able to measure grain composition, the decade-long dataset provided essential constraints on the dynamical properties of ISD and how it interacts with the heliosphere, i.e., flux and directionality variabilities as a function of solar cycles (Krüger and Grün 2009; Sterken et al., 2015; Strub et al., 2015). A next generation dust instrument, the Cosmic Dust Analyser (CDA, Srama et al., 2004) on board the Cassini mission, was equipped with a linear ToF mass spectrometer. While the mass resolution of CDA was relatively low ($m/\Delta m \sim 30$), where *m* is the ion mass), the compositional information has been proven critical for characterizing dust populations at Saturn and beyond (e.g., Kempf et al., 2005; Hillier et al., 2007; Postberg et al., 2009, 2018; Srama et al., 2011; Altobelli et al., 2016; Hsu et al., 2015, 2018; Khawaja et al., 2019). Throughout the Cassini mission, CDA also collected 36 silicateous ISD mass spectra of particles ~0.1 to 0.3 µm in radius, whose rock-forming elements (Mg, Si, Fe, Ca) show only minor grain-
to-grain differences (Altobelli et al., 2016), pointing at re-processing in the ISM. This contrasts
with the seven diverse larger ISD grains returned from the Stardust mission (Westphal et al., 2014).

The next-generation Dust Analyzer, as referred in the ISP mission concept report (Interstellar Dust Analyzer, IDA) and Table 1, shares a similar design of the Surface Dust Analyzer (SUDA) on board the Europa Clipper. At the time of writing, SUDA is en route to be integrated onto the Europa Clipper spacecraft scheduled for a 2024 launch. A SUDA-like Dust Analyzer uses a Reflectron-type ToF mass spectrometer (Srama et al., 2005; Fig. 2) and a **mass resolution of m/Δm > 200 can be achieved. Abundant isotopes, such as the stable isotopes of carbon, iron, and magnesium, can be directly measured for individual grains (Fig. 2)**. The typical mass range is 1-500 u. This is sufficient for the ISP measurement conditions because all dust grains impacting at ISP speeds will be reduced to elemental form. Improved upon CDA, a SUDA-like instrument can select to measure ToF mass spectra of either cation or anion species. The dual-polarity capability will provide comprehensive information to reconstruct the composition distribution of encountered dust populations. A tentative derivation of deuterium to hydrogen (D/H) ratio using anion ToF mass spectrometry has been achieved in laboratory experiments (Ulibarri, 2022).

<figure><img src="image_4.png" /><figcaption>**Figure 2.** (a) The schematic of a SUDA-like Dust Analyzer as presented in the ISP mission concept report. Note that this configuration does not include a velocity grid system. (b) Adopted from the ISP mission concept report, a Time-of-Flight mass spectrum of a pyroxene grain recorded by a Dust Analyzer prototype. The carbon isotopes (u = 12, 13), magnesium isotopes (u = 24, 25, 26), and potassium isotopes (u = 39, 41) are clearly identified. (c) Resolved magnesium isotope mass lines registered by SUDA showing isotope amplitudes consistent with correct abundance ratios (provided by S. Kempf).</figcaption></figure>

The in situ compositional and dynamical characterization of ISD, carried out by ISP, will provide
critical information in understanding the origin and relevant processes of ISD in the heliosphere,
the VLISM, and interstellar space further out. Such an instrument will enable characterization of
the unaltered elemental composition of ISD for the first time, providing direct constraints on
material exchanges between solid (dust), gas and plasma species in interstellar space, as well as
ISD's origin and formation processes. Measurements carried out upstream will help to examine
the ISD interactions at the boundary of the heliosphere (e.g., Richardson et al., 2008) and the dynamical and compositional filtering of ISD exerted by the heliosphere. This will be done by comparison with Cassini CDA measurements at Saturn, and future ISD measurements in the inner heliosphere starting 2024-2025 carried out by Dust Analyzers on board Interstellar Mapping and Acceleration Probe (IMAP; McComas et al., 2018) and Japan Aerospace Exploration Agency (JAXA)'s DESTINY⁺ missions<fnref n="1" />, as well as serendipitous measurements by the Europa Clipper mission/SUDA en route and/or around Jupiter.

Another useful sensor addition to be placed in front of a Dust Analyzer is a velocity grid system – a series of grids to detect image charges of incident grains to provide their charge and velocity information. **The velocity grid measurements will provide unique constraints about the dynamics and physical nature of individual grains.** Regarding grain dynamics, in addition to the instrument pointing, the measured grain velocity can be used to derive constraints about the grain trajectories and possible sources. Regarding ISP, grain velocity measurements will help to distinguish small, fast ISD from larger interplanetary dust populations in bound orbits during the traverse of the Kuiper Belt (Poppe 2016; Bernardoni et al., 2022). The dynamical information, such as flux variation and velocity distribution, will also provide critical constraints to understand how charged ISD grains interact with complex electromagnetic environments of the heliosphere and VLISM (e.g., Opher et al., 2006, 2017; Kleimann et al., 2022). Figure 3 shows the velocity grid signal from the SUDA flight model of a grain carrying an electric charge of 0.5×10⁻¹⁵ C, i.e., an order of magnitude improvement in sensitivity over CDA. The measured grain speed was 15 km s⁻<fnref n="1" />. Detection of faster grains to be encountered by ISP is achievable with minor modifications.

<figure><img src="image_5.png" /><figcaption>**Figure 3.** A grain charge signal time series measured by SUDA during laboratory calibrations. The detection (red) of the incident charged grain with a speed of 15.2 km s⁻<fnref n="1" /> and about 0.5 fC electric charge is marked on top of the velocity grid signal (light green). The time 0 marks the dust impact on target.</figcaption></figure>

The measured grain charge, coupled with impact signals and ambient plasma properties (carried out by other instruments), will provide direct constraints on the grains' electric potential and charge-to-mass ratio that can only be derived in situ. Precise grain charge-to-mass ratio

<footnote n="1">¹ Both missions carry SUDA-like Dust Analyzers. The Interstellar Dust Explorer (IDEX) onboard IMAP is developed by the Laboratory of Atmospheric and Space Physics, University of Colorado, USA. The Destiny⁺ Dust Analyzer (DDA) is developed by the University of Stuttgart, Germany.</footnote>
measurements will provide unprecedented constraints in modeling the dynamical evolution of ISD from the VLISM into the heliosphere as well as in the Galaxy. Improved understanding of ISD dynamics may even transform ISD measurements as a tool to probe and constrain the large-scale electromagnetic structure of the heliosphere and VLISM (Czechowski and Mann, 2003a, b; Mann et al. 2004; Sterken et al., 2022). Grain charge information also helps to examine the physical properties of ISD that cannot be measured otherwise, such as the bulk density and grain shape, relevant to studying the dynamical effects caused by radiation pressure and Lorentz forces (Kimura et al., 2002; Hunziker et al., 2022).

In an augmented scenario for ISP to perform a flyby of a significant Kuiper Belt Object (KBO), a Dust Analyzer can perform the so-called “surface compositional mapping”, i.e., reconstruct a surface composition map along the spacecraft ground track, based on composition measurement of ejecta lofted by micrometeoroid impacts (Postberg et al., 2011; Goode et al., 2021). This is in addition to the collection of thousands of mass spectra from IDP released from KBOs though meteoroid impacts. Both these measurements will be the first in situ composition analysis of surface materials from KBOs and will provide direct constraints about the KBO surface composition and their evolution.

## PVDF Dust Counter

A polyvinylidene fluoride (PVDF) thin film dust detector is a reliable, low-cost dust detection system that has been used on various missions, including the Cassini (CDA/High Rate Detector, Srama et al., 2004), Stardust (the Dust Flux Monitor Instrument, Economou et al., 2013), New Horizons (Student Dust Counter (SDC), Horanyi et al., 2008; Bernardoni et al., 2022), and JAXA's IKAROS solar sail mission (Hirai et al., 2014, 2017), among others. Dust detection is registered by the signal generated through the removal of polarized film material by dust impact cratering, with the signal amplitude proportional to the removal mass and thus the incident grain mass. At impact speeds of 10s km s⁻¹, the grain masses that a PVDF detector is sensitive to ranges from 10⁻¹⁸ - 10⁻¹¹ g, corresponding to ~0.1 - 10 µm in radius.

Complementing the impact ionization dust analyzer, a large-area PVDF detector can be implemented under reasonable costs with significant science gains. The sensitive area of the example Dust Counter in the ISP example payload design is 1 m², i.e., 25 times larger than the provisional Dust Analyzer (400 cm²). A large-area PVDF Dust Counter mounted towards the spacecraft ram direction is best used for low dust density environment in the outer heliosphere and interstellar space to provide better number density measurements of larger, rarer grain populations and the overall grain size/mass distribution. **Since the total ISD mass is dominated by the larger grain populations, a precise measurement of this population is thus critical to determine the ISD mass density and the gas-to-dust ratio in the VLISM** (Frisch and Slavin, 2003; Krüger et al., 2015). Combined with the New Horizons SDC measurements, an IDP density profile by ISP through the Kuiper Belt will also help to constrain the IDP / zodiacal cloud models (Vitense et al. 2014; Poppe, 2016; Bernardoni et al., 2022) and help to understand the debris disc around other stars (Lisse et al., 2007). The low power consumption (2W) and low data rate (<1 bps) are compatible with the ISP design for exploring the outer heliosphere and interstellar space.

## Plasma Wave Antenna

Dust detection by Plasma Wave Antenna instruments relies on the impact ionization process and has been used since the Voyager era (Gurnett et al., 1983). Starting as serendipitous measurements, antenna dust detections have been reported from various heliophysics and planetary science
missions, including Wind (Malaspina et al., 2014), Cassini (Gurnett et al., 2004; Kurth et al., 2006; Ye et al., 2019), STEREO (Bale et al., 2008; Bougeret et al., 2008; Meyer-Vernet et al., 2009; Zaslavsky et al., 2012), MAVEN (Andersson et al., 2015a,b), Juno (Waves, Kurth et al., 2017; Ye et al., 2020), MMS (Torbert et al., 2018; Vaverka et al., 2019, 2021), Parker Solar Probe (Bale et al., 2016; Malaspina et al., 2020, 2022; Page et al., 2020; Pusack et al., 2021; Szalay et al., 2020, 2021), and Solar Orbiter (Maksimovic et al., 2020; Zaslavsky et al., 2021).

The major benefits of dust detection using antenna instruments are: the large sensitive area (up to the full projected spacecraft surface area, e.g., Ye et al., 2014), the ability to make observations in any spacecraft orientation, and capability to measure larger impactor sizes compared to dedicated dust instruments. The major drawbacks of antenna-based dust detection are: the lack of mass-velocity discrimination, lack of composition information, and less constrained sensitive area. Because all drawbacks can be overcome by dedicated dust instruments, including both antenna detection and dedicated dust instruments results in the most comprehensive measurements.

Dust detections were reported using both stacer/boom antennae (e.g., Cassini/RPWS and STEREO/WAVES) and wire antennae (e.g., spin plane antennae on Wind and double probe instruments on MMS). Based on the latest antenna dust impact signal analysis, dust directionality can also be extracted using signals from multiple antennae (Page et al., 2020; Pusack et al., 2021; Shen et al., 2021a). Antenna dust detection provides large number statistics, especially for larger grain populations from various directions, complimenting results from dedicated dust instruments. With a known impact speed, the grain mass distribution can be constrained from the amplitude of impact signals. For ISP consideration, the grain mass range sensitive to antenna detection is roughly > 10⁻¹⁸ g (> 0.1 µm), to 10s µm (e.g., Malaspina et al., 2022) or even larger. Under low-density plasma conditions in the outer heliosphere and VLISM, dust impact signals registered by Plasma Wave Antennae will become longer and stronger, making them easier to analyze (Shen et al., 2021b; Vaverka et al., 2021). **Antenna operations under monopole mode provides higher dust detection sensitivity** (Mann et al. 2019, Vaverka et al. 2021), **whereas directional information can be extracted from dipole mode measurements** (Page et al., 2020; Pusack et al., 2021; Shen et al., 2021a, 2022). Onboard signal processing can be developed to reduce required data rates (e.g., Shen et al., 2021a; Kvammen et al. 2022; Rackovic Babic et al., 2022).

## Neutral Mass Spectrometer

Extremely small grains with sizes of a few nanometers and below, ambiguously classified between solid grains and macromolecules, cannot produce sufficient impact signals to be detected by conventional dust instruments even at ISP speeds. As discussed in the ISP mission concept report, a Neutral Mass Spectrometer, such as the Neutral gas and Ion Mass spectrometer (NIM, Föhn et al., 2021) on board the European Space Agency's Jupiter Icy Moon Explorer (JUICE), could measure the composition of impacting nanograins along with gas detection signals. Dust detections by a Neutral Mass Spectrometer have been reported by the Cassini Ion and Neutral Mass Spectrometer during the crossing of dust-rich plume of Enceladus (Teolis et al., 2010). Detection of charged nanometer-sized grains were also reported by plasma spectrometers on the Cassini and Rosetta missions (Jones et al., 2009; Gombosi et al., 2015). While this measurement will not provide information about individual nanograin detection, an in situ search of nanometer-sized grains will help to address (1) the electromagnetic filtering of nm-sized ISD by the heliosphere (Grün et al., 1994; Linde and Gombosi, 2000; Sterken et al., 2013), (2) the discrepancy between observations of ISD within and outside the heliosphere (Fig. 1), and (3) the ISD size distribution and evolution.
## References

Altobelli, N. et al., A&A, 448, 243-252, 2006.

Altobelli, N. et al., Science, 352, 2016.

Andersson, L et al., Space Sci. Rev., 195, 173-198, 2015a.

Andersson, L et al., Science, 350, 6261, 2015b.

Auer, S. et al., Planet. Space Sci., 50, 773-779, 2002.

Bale, S. D. et al., Space Sci. Rev., 136, 529-547, 2008.

Bale, S. D. et al., Space Sci. Rev., 204, 49-82, 2016.

Bernardoni, E. et al., Planet. Sci. J., 3, 3, 2022.

Bougeret, J. L. et al., Space Sci. Rev., 136, 487-528, 2008.

Brandt, P. C. et al., Heliophysics White Paper, 2022.

Czechowski, A. & Mann, I., A&A, 410(1), 165-173, 2003a.

Czechowski, A. & Mann, I., JGR, 108(A10), 2003b.

Draine, B. T., Annu. Rev. Astron. Astrophys., 41:241-89, 2003.

Draine, B. T., in From the Outer Heliosphere to the Local Bubble, Space Science Series of ISSI, Springer, 2009.

Economou, T. E. et al., Icarus, 222, 526-539, 2013.

Frisch, P. C. & Slavin, J. D., Astrophys. J., 594, 2003.

Frisch, P. C. et al., ApJS, 259, 48, 2022.

Föhn, M et al., IEEE, 978-1-7281-7436-5/21, 2021.

Gombosi, T. I. et al., Astro. Astrophys., 583, 2015.

Goode, W. et al., Planet. Space Sci., 208, 2021.

Grün, E. et al., Nature, 362, 428-430, 1993.

Grün, E. et al., Astron. Astrophys., 286, 1994.

Hillier, J. K. et al., Icarus, 190, 643-654, 2007.

Gurnett, D. A. et al., Icarus, 53, 2, 236-254, 1983.

Gurnett, D. A. et al., Space Sci. Rev., 114, 395-463, 2004.

Hirai, T. et al., Planet. Space Sci., 100, 87-97, 2014.

Hirai, T. et al., Adv. Space Res., 59, 6, 1450-1459, 2017.

Horányi, M. et al., Space Sci. Rev., 140, 2008.

Hsu, H.-W. et al., Nature, 519, 207, 2015, doi: 10.1038/nature14262.

Hsu, H.-W. et al., Science, 362, 49, 2018, doi: 10.1126/science.aat3185.

Hunziker, S. et al., Planet. Space Sci., 220, 105536, 2022.

Jones, G. H. et al., GRL, 36, L16204, 2009.

Kempf, S. et al., Nature, 433, 2005.

Khawaja, N. et al., MNRAS, 489, 5231, 2019, doi: 10.1093/mnras/stz2280.

Kimura, H. et al., Icarus, 157, 349-361, 2002.

Kimura, H. et al., A&A, 643, A50, 2020, doi: 10.1051/0004-6361/201526964.

Kleimann, J. et al., Space Sci. Rev., 218, 36, 2022.

Krimigis, S. M. et al., Nature Astro., 3, 997-1006, 2019.

Krüger, H. & Grün, E., Space Sci. Rev., 143, 2009.

Krüger, H. et al., Astrophys. J., 812, 139, 2015.

Kurth, W. S. et al., Planet. Space Sci., 54, 988-998, 2006

Kurth, W. S. et al., Space Sci. Rev., 213, 347-392, 2017.

Kvammen, A. et al., EGUSphere, 1-27, 2022.

Landgraf, M. et al., JGR, 105, 2000.

Linde, T. J. & Gombosi T. I., JGR, 105, A5, 10411, 2000.

Lisse, C. M. et al., Astrophys. J., 658, 584-592, 2007.

Maksimovic, M. et al., Astro. Astrophys., 642, 2020.

Malaspina, D. M. et al., GRL, 41, 266-272, 2014.

Malaspina, D. M. et al., Astrophys. J., 892, 115, 2020.

Malaspina, D. M. et al., Astrophys. J., 925, 2022.

Mann, I. et al., Adv. Space Res. 34(1), 179-183, 2004.

Mann, I., Annu. Rev. Astron. Astrophys., 48, 173, 48, 2010.

Mann, I. et al., Annales Geophysicae 2019, 37 (6), 2019

McComas, D. J. et al., Space Sci. Rev., 214, 8, 2018.

McNutt J., R. L. et al., Interstellar Probe, 2021.

Meyer-Vernet, N. et al., Solar Phys., 256, 1-2, 463, 2009.

Opher, M. et al., Astrophys. J., 640, L71-74, 2006.

Opher, M. et al., Astrophys. J. Lett., 839, L12, 2017.

Page, B. et al., Astrophys. J. S., 246, 2, 51, 2020.

Poppe, A. R., Icarus, 264, 369-386, 2016.

Poppe, A. R. et al., Heliophysics White Paper, 2022.

Postberg, F. et al., Nature, 459, 2009.

Postberg, F. et al., Planet. Space Sci., 59, 1815-1825, 2011.

Postberg, F. et al., Nature, 564, 2018.

Pusack, A. et al., Planet. Sci. J., 2, 186, 2021.

Rackovic Babic, K. et al., A&A, 659, A15, 2022.

Richardson J. D. et al., Nature, 454, 2008.

Shen, M. M. et al., JGR, 126, 9, 2021a.

Shen, M. M. et al., JGR, 126, e2020JA028965, 2021b.

Shen, M. M. et al, JGR, submitted 2022.

Shu, A. et al., Rev. Sci. Instrum, 83, 075108, 2012.

Slavin, J. D. et al., Astrophys. J., 760, 2012.

Slavin, J. D. et al., Astrophys. J., 803, 2015.

Srama, R. et al., Space Sci. Rev., 114, 465-518, 2004

Srama, R. et al., Vol. 587 ESA SPECIAL PUBLICATIONS, 2005.

Srama, R. et al., CEAS Space J., 2, 1-4, 3-16, 2011.

Sterken, V. J. et al., Astron. Astrophys., 538, 2012.

Sterken, V. J. et al., Astron. Astrophys., 552, 2013.

Sterken, V. J. et al., Astrophys. J., 812, 141, 24, 2015.

Sterken, V. J. et al., Heliophysics White paper, 2022.

Strub, P. et al., Astrophys. J., 812, 2015.

Szalay, J. R. et al., Planet. Sci. J., 2:185, 2021.

Teolis, B. D. et al., JGR, 115, A09222, 2010.

Thomas, G. E., Ann. Rev. Earth Planet. Sci., 6, 173-204, 1978.

Torbert, R. B. et al., Space Sci. Rev., 199, 105, 2016.

Ulibarri, Z., PhD thesis, 2022.

Vaverka J. et al., JGR, 124, 8179-8190, 2019.

Vaverka J. et al., Astrophys. J., 921, 127, 2021.

Vitense, C. et al., Astron. Astrophys., 540(A30), 2012.

Westphal, A. J. et al., Science, 345, 2014

Witte, M. et al., Adv. Space Res., 13, 6, 121, 1993.

Ye, S.-Y. et al., JGR, 119, 6294, 2014.

Ye, S.-Y. et al., GRL, 46, 10941, 2019.

Ye, S.-Y. et al., JGR, 125, 2020.

Zaslavsky, A. et al., JGR, 117, A05102, 2012.

Zaslavsky, A. et al., A&A, 656, A30, 2021.