Space plasma physics science opportunities for the lunar orbital platform - Gateway

The Lunar Orbital Platform - Gateway (LOP - Gateway, or simply Gateway) is a crewed platform that will be assembled and operated in the vicinity of the Moon by NASA and international partner organizations, including ESA, starting from the mid-2020s. It will offer new opportunities for fundamental and applied scientific research. The Moon is a unique location to study the deep space plasma environment. Moreover, the lunar surface and the surface-bounded exosphere are interacting with this environment, constituting a complex multi-scale interacting system. This paper examines the opportunities provided by externally mounted payloads on the Gateway in the field of space plasma physics, heliophysics and space weather, and also examines the impact of the space environment on an inhabited platform in the vicinity of the Moon. It then presents the conceptual design of a model payload, required to perform these space plasma measurements and observations. It results that the Gateway is very well-suited for space plasma physics research. It allows a series of scientific objectives with a multi-disciplinary dimension to be addressed.


Introduction
The Moon is a unique location to study the deep space plasma environment. During most part of its orbit around the Earth the Moon is directly exposed to the solar wind. Due to the absence of a substantial intrinsic magnetic field and of a collisional atmosphere, solar wind and solar energetic particles (SEPs) arrive almost without any deviation or absorption and impact directly on its surface, interacting with the lunar regolith and the tenuous lunar exosphere (e.g. Geiss et al., 2004;Futaana et al., 2018). The same phenomenon occurs also with the galactic cosmic rays (GCRs), which present fluxes and energy spectra typical of interplanetary space (e.g. Sohn et al., 2014). Downstream from the Moon, a structured plasma umbra and penumbra region is formed, characterized by the gradual decrease of the ion and electron densities (Bosqued et al., 1996;Nishino et al., 2010). The Moon's vicinity is an ideal environment to study galactic cosmic rays, solar wind and solar energetic particles. This environment is typical of deep space (Plainaki et al., 2016), apart from the fact that the Moon itself forms an obstruction to the GCRs.
During 5 -6 days every orbit, however, the Moon crosses the tail of the terrestrial magnetosphere (Figure 1). It is then exposed not to the solar wind but to the terrestrial magnetotail plasma environment, offering the possibility to study in-situ magnetotail dynamics and its dependence on solar and geomagnetic activity (e.g. Kallio and Facskó, 2015;Kallio et al., 2019). Phenomena such as plasmoids released from the near-Earth magnetotail and propagating anti-Sunward, bursty bulk flows (BBFs), energetic particle bursts, plasma waves, magnetic reconnection and plasma sheet dynamics can thus be studied in-situ (e.g. Parks et al., 2001;Nakamura, 2006;Taylor et al., 2006;Nagai et al., 2009;Du et al., 2011;Cao et al., 2013;Grigorenko et al., 2019;Sitnov et al., 2019;Kronberg et al., 2021).
The Moon is then also very well situated to study atmospheric escape from the Earth into space (Lammer et al., 2008;Harnett et al., 2013;Wei et al., 2020;Dandouras, 2021;André et al., 2021;Wang et al., 2021), in the form of heavy ions upwelling from the terrestrial ionosphere and transported and lost into the deep magnetotail. The wealth of data supplied from the THEMIS-ARTEMIS and from the Kaguya (SELENE) spacecraft confirmed the observation of such ions, of terrestrial origin, in the lunar environment (Poppe et al., 2016;Terada et al., 2017). The THEMIS-ARTEMIS data, however, did not include the crucial information on the plasma composition , and the Kaguya plasma measurements where limited to a less than two-year mission and to low-energy plasma (Saito et al., 2010). Far magnetotail studies performed by the Geotail spacecraft supplied key information on the dynamics of ion beams streaming downtail (Christon et al., 1994(Christon et al., , 2020Seki et al., 1998), but lacked the ion composition measurements at low energies (below ~10 keV).
When the Moon gets again outside of the magnetotail, terrestrial magnetosphere dynamics can be monitored through remote sensing, using a variety of magnetospheric imaging techniques. These include Energetic Neutral Atom (ENA) imaging, which conveys information on the interaction between energetic ions and the terrestrial exosphere (e.g. C:son Brandt et al., 2002;Vallat et al., 2004), solar wind charge exchange X-rays imaging of the interaction between the solar wind / magnetosheath plasma and the terrestrial exosphere (Branduardi-Raymont et al., 2012;Sibeck et al., 2018), plasmasphere EUV imaging (Sandel et al., 2003), or exosphere Lyman-α imaging (e.g. Zoennchen et al., 2017).
But, most important, the lunar environment offers a unique opportunity to study the Moon surfacebounded exosphere (Figure 2), its production mechanisms, its dynamics, its interaction with the solar wind and with the terrestrial magnetotail plasma, and its escape into space (Potter et al., 2000;Wurz et al., 2007Wurz et al., , 2022Futaana et al., 2008;Chaufray, 2011, Lammer et al., 2022). The LADEE (Lunar Atmosphere and Dust Environment Explorer) and LRO (Lunar Reconnaissance Orbiter) observations have provided a glimpse of the complexity of the lunar exosphere and of the associated physical mechanisms (Stern et al., 2013;Elphic et al., 2014;Benna et al., 2015;Hurley et al., 2016).
In preparation of its scientific payload, ESA set up international science teams to prepare and to support the definition of payload studies, including a topical team in the field of space plasma physics. In the first part of this article (sections 2 and 3) we report on the outcome of this topical team, which was entitled "Space Plasma Physics Science Opportunities for the Lunar Orbital Platform -Gateway". This part focuses on the science objectives identified by the topical team (section 2), and on the corresponding instrumentation required to address them (section 3). In the second part (section 4) we present a conceptual design study for a "Space Plasma Physics Payload Package onboard the Gateway" (SP4GATEWAY) we undertook for ESA, addressing these objectives and compatible with the technical requirements.

Specific Objectives and Goals
The "Space Plasma Physics Science Opportunities for the Lunar Orbital Platform-Gateway" topical team, set up by ESA in 2019, brought together the key expertise required for defining the space plasma parameters to measure from lunar orbit, and the appropriate instrumentation required to perform these observations. The science objectives that were identified include:

Monitoring the solar wind and the lunar energetic particle environment
Due to the absence of a substantial intrinsic magnetic field and of a collisional atmosphere, the Moon is directly exposed to: − Solar Wind: ~keV particles − Solar Energetic Particles (SEPs): ~MeV particles − Galactic Cosmic Rays (GCRs): ~GeV particles The monitoring of the solar wind (e.g. von Steiger, 2008), at the lunar environment, aims to evaluate its role as a driver for the dynamics of the terrestrial and the lunar exospheres, of the dynamics of the terrestrial magnetosphere, and of the lunar surface sputtering and charging.
The monitoring and characterization of the SEPs and GCRs, at lunar orbit, aims to evaluate the radiation environment of the Moon and also the role of SEPs and GCRs as lunar surface sputtering sources. Since the Moon does not have a substantial magnetic field it is possible, with an appropriate particle detector, to measure the low energy part of the GCR spectrum (< 1 GeV) with high precision.

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This offers an advantage with respect to low-Earth orbits, where most of the advanced GCR observatories like PAMELA and AMS-02 are located, where this low energy part is filtered out by the Earth's magnetosphere.
Typical SEP proton intensities, measured during a solar event, are shown in Figure 3 (adapted from Quinn et al., 2017). Some of the SEP protons (~MeV energy range) can interact, in the high solar corona, with partially stripped coronal ions, charge exchange with them and produce ~MeV ENAs (Energetic Neutral Atoms) (Mewaldt et al., 2009).
GCR Hydrogen and Oxygen nuclei fluxes are shown in Figure 4, presenting a clear solar cycle modulation (adapted from Mrigakshi et al., 2012). The interaction of these SEPs and GCRs with the lunar regolith produces albedo energetic particles, resolvable with current instruments up to a few ~100 MeV, and with fluxes that are sensitive to the regolith hydration (Looper et al., 2013;Schwadron et al., 2016;Zaman et al., 2022), cf. Figure 5. The separation of the pristine energetic particle fluxes from the albedo energetic particles (e.g. by zenith centered / nadir centered looking directions respectively) appears thus as a requirement, in order to provide information on the deep space SEP and GCR environment and on the interaction of the lunar regolith with this environment.

Monitoring the terrestrial magnetosphere and exosphere
When the Moon is within the terrestrial magnetotail, in-situ measurements of the plasma sheet and plasma sheet boundary layer dynamics are enabled. These consist of magnetic field and energetic ion and electron monitoring, including the measurement of energetic ions of terrestrial origin streaming downtail.
The evolution of the flux of O + downtail streaming beams, as a function of the tailward distance from the Earth, is shown in Figure 6 (from Seki et al., 1998). During high geomagnetic activity conditions these beams include heavy atomic and molecular ions (Christon et al., 1994(Christon et al., , 2020. Closer to the Moon, O + downtail streaming beams have been observed by the Kaguya Lunar Orbiter (Terada et al., 2017). The spectral characteristics of these streaming O + ions show a clear distinction between the O + ions of lunar origin (few 10 eV to ~100 eV) and the terrestrial magnetospheric O + ions (few keV), cf. Figure 7 (adapted from Terada et al., 2017). Particle tracing simulations performed by Harnett et al. (2013) using a 3D multi-fluid model, and by Poppe et al. (2016) using the MHD Open Global Geospace Circulation Model, show how heavy ions, originating from the Earth's inner magnetosphere, can be ejected downtail during high geomagnetic activity events, reaching energies of several keV to several 10 keV at lunar distances, cf. Figure 8.
When the Moon is outside of the magnetotail, terrestrial magnetosphere dynamics and response to solar wind conditions can be monitored through remote sensing. This includes: − Ring current and near-Earth plasma sheet monitoring, by imaging of the ENAs produced by charge exchange between the plasma sheet or ring current energetic ions (few ~keV to few ~10 keV) with the geocorona neutral hydrogen atoms, e.g. Brandt et al. (2004), Vallat et al. (2004), Goldstein et al. (2022). − Magnetopause and cusps monitoring by detecting and imaging the SWCX (solar wind charge exchange) soft X-rays produced by charge exchange between highly-charged heavy ions, originating from the solar wind, and the exospheric neutral atoms, e.g. Branduardi-Raymont et al. (2012, Sibeck et al. (2018). − Plasmasphere imaging, by resonant scattering of the solar EUV (30.4 nm) by the plasmaspheric He + ions, e.g. Sandel et al. (2003), Darrouzet et al. (2008), He et al. (2016).

Monitoring the Moon's surface-bounded exosphere
The Moon surface-bounded exosphere constitutes a complex multi-scale system (Figure 2), characterised by its interactions with the solar radiation, the solar wind and terrestrial magnetotail plasma, the meteoritic flux, dust, and the regolith (Futaana et al., 2018). The low number densities of this very tenuous atmosphere, particularly of the minority species, and the complexity and multiplicity of the source and loss mechanisms have resulted in a poor understanding of it (Wurz et al., 2007Poppe et al., 2022). Figure 9 provides the altitude density profiles of the major species, separately for the atoms and molecules released thermally from the regolith and for the atoms released through sputtering. In addition to atomic and molecular hydrogen, O, OH, CH4, noble gases (He, Ne, Ar, Kr, Xe), metallic atoms (Na, K, Mg, Al) and other elements populate the lunar exosphere (Leblanc and Chaufray, 2011;Benna et al., 2015;Grava et al., 2015Grava et al., , 2016Grava et al., , 2021Halekas et al., 2015;Hurley and Benna, 2018;Leblanc et al., 2022;Wurz et al., 2022).
These neutral exospheric atoms and molecules can be subsequently ionized by the solar UV radiation and generate pickup ions. These ions are promptly accelerated from their birthplace by the ambient electric field E and drift across the magnetic field B. The unique orbital characteristics of the pickup ions (cycloidal motion consisting of a combination of E × B drift and a gyration around B) make it possible to infer important details about their sources (Hartle and Killen, 2006). Such lunar pickup ions have been detected in the terrestrial magnetotail lobes (Poppe et al., 2012a) and in the solar wind (Wang et al., 2011).
As the measurements performed onboard the LADEE and LRO spacecraft have shown, the lunar exosphere can be monitored, either by in-situ measurements, using a neutral mass spectrometer, or by remote sensing using a UV spectrometer (Chin et al., 2007;Elphic et al., 2014).
Other techniques for studying the lunar exosphere are: − Remote sensing of the lunar exosphere by detecting and imaging the ENAs produced by charge exchange interactions between the solar wind protons and the exospheric neutral atoms (Futaana et al., 2008). The energies of these ENAs are comparable to the energies of the parent solar wind protons, i.e. of the order of ~keV. − Remote sensing of the lunar exosphere by detecting and imaging the SWCX soft X-rays produced by charge exchange between highly-charged heavy solar wind ions and the exospheric neutral atoms (Robertson et al., 2009). − In-situ measurement of freshly ionized pickup ions, originating from the lunar exosphere neutral species. (Hartle and Killen, 2006;Yokota et al., 2009;Wang et al., 2011;Poppe et al., 2022). At high altitudes above the lunar surface, as those of the Gateway orbit (cf. section 4.1), this method can provide higher sensitivity in the detection of low number density species than the direct sampling of the parent neutrals Poppe et al., 2022).

Monitoring the interaction of the solar wind with the Moon's surface
Solar wind protons, arriving at the Moon's surface, can be absorbed, or scattered, or can remove another atom from the lunar regolith by sputtering or desorption (Wieser et al., 2009;McComas et al., 2009a;Futaana et al., 2012). It results that a large fraction of the solar wind protons, up to 20%, is reflected back to space as neutral hydrogen atoms (ENAs). It is noteworthy that backscattering of neutralized solar wind protons occurs not only when the Moon is in the pristine solar wind, but also when the Moon enters into the terrestrial magnetosheath and is then exposed to the shocked and thermalized solar wind (Allegrini et al., 2013).

Figure 10
shows typical energy spectra of the reflected hydrogen ENAs, compared to the parent solar wind protons energy spectra. As shown, the flux of the reflected ENAs closely follows the variations of the flux of the parent proton population. The energies of these ENAs are however a fraction of the parent solar wind protons.
Since the solar wind proton trajectories are modulated by the surface electrostatic potential and by the eventual local magnetic field anomalies (cf. Figure 11), the detection and imaging of these reflected ENAs provides a tool to investigate the lunar surface electric and magnetic fields Vorburger et al., 2013Vorburger et al., , 2015Vorburger et al., , 2016Bamford et al., 2016). Local crustal magnetic anomalies (or "swirls") constitute "mini-magnetospheres", shielding locally the lunar regolith from the solar wind protons and from the resulting space weathering (Wieser et al., 2010;Wang et al., 2012;Deca et al., 2015;Glotch et al., 2015;Hemingway et al., 2015;Poppe et al., 2015;Pieters and Noble, 2016;Hemingway and Tikoo, 2018).
The solar wind protons that do not scatter back, but are absorbed in the lunar regolith (top 20 -30 nm of the lunar grains), diffuse within the regolith. They can then interact with the oxygen atoms in the regolith oxides and form OH (McCord et al., 2011;Farrell et al., 2017;Tucker et al., 2019;McLain et al., 2021). These solar wind-produced hydroxyl radicals contribute to the formation and release of molecular water, and thus to a solar wind-induced water cycle on the Moon (Crider and Vondrak, 2003;Liu et al., 2012;Futaana et al., 2018;Jones et al., 2018;Honniball et al., 2021).
The exposure of the lunar surface to the solar radiation and to the flux of charged particles results also in an electrostatic surface charging. An electric potential thus develops between the lunar surface and the ambient plasma, which manifests itself in a near-surface plasma sheath with a scale height of the order of the Debye length (Halekas et al., 2011;Stubbs et al., 2013;Burinskaya, 2015;Harada et al., 2017). This near-surface electric field becomes very complex and highly variable in the vicinity of the terminator, with the surface polarity changing from mostly positive (few 10 V) on the dayside, due to photoelectron emission, to highly negative (of the order of the ambient electron temperature, i.e. up to several -100 V) on the nightside, and in the trailing lunar wake region . Local surface topography is also a factor contributing to a complex near-surface electrostatic and plasma environment, particularly in the vicinity of permanently-shadowed craters (Poppe et al., 2012b;Nénon and Poppe, 2021). As the THEMIS-ARTEMIS observations have shown, the lunar surface charging can be remotely sensed from a Moon orbiting spacecraft, even several 1000 km away from the lunar surface, through the shifted energy spectra of the detected plasma particles when the spacecraft crosses magnetic field lines connected to the lunar surface (Halekas et al., 2011).
Dust is another component of the lunar plasma environment Grün et al., 2011;Horányi et al., 2015;Popel et al., 2018Popel et al., , 2022. Dust grains on (or near) the lunar surface can either be ejected from the regolith, due to the impact of interplanetary micrometeoroids, or be electrostatically levitated due to grain charging, as discussed in the previous paragraph. This creates a dusty plasma system consisting of neutrals of the lunar exosphere, solar-wind ions and electrons, ions and electrons of the Earth's magnetotail (when the Moon gets inside the terrestrial magnetotail), photoelectrons formed due to the interaction of the solar radiation with the lunar surface, and charged dust grains flying over the lunar surface.

Lunar Gateway Space Plasma Physics
8 This is a provisional file, not the final typeset article

Measurement Requirements
Following the identification of the scientific objectives in the field of space plasma physics, that can be addressed using instrumentation onboard the Lunar Orbital Platform -Gateway (cf. section 2), the ESA topical team identified the physical parameters needed to be measured in order to address these objectives, and the corresponding instrumentation required to perform these observations. The topical team addressed thus the following two questions (Dandouras et al., 2020b): − What plasma physics science questions can be addressed in the vicinity of the Lunar Orbital Platform -Gateway? − What are the instrument / payload requirements to achieve such science?
It identified measurements that can be performed either directly from the Gateway platform (3 200 × 70 000 km altitude lunar orbit), or from instrumented cubesats that could be released from the platform and placed into lower lunar orbits, or directly from the Moon surface. Here we will focus on the measurements that can be performed by instrumentation mounted onboard the Gateway, and which can be either in-situ measurements or remote sensing observations, and then we briefly mention the other two possibilities. Space plasma physics measurements that could be performed directly from the Moon surface will be the object of a dedicated forthcoming paper. Table 1 provides an overview of the physical parameters / observables identified, in the field of space plasma physics, that can that be monitored by instrumentation onboard the Gateway.
Tables 2 and 3 are for the observations that could be performed, on a longer term, from lower lunar orbits and from the Moon's surface, respectively. Table 4 focuses then on the science questions that can be addressed from instrumentation onboard the Gateway, and shows how each science objective, identified by the topical team, translates into a measurement requirement, and then to the corresponding instrument / payload requirement.
Additional objectives that could be eventually addressed by remote sensing instrumentation onboard the Gateway, and could point to targets of opportunity, include: aurora imaging, heliosphere imaging (through the ENA imager), and lunar surface imaging (e.g. meteor impact flashes).

Conceptual design for a Space Plasma Physics Payload Package onboard the Gateway
Following the work of the topical team, and the identification of the measurement requirements, ESA issued an Invitation to Tender for a "Deep Space Gateway Plasma Physics Payload Conceptual Design" (ESA AO/1-9789/19/NL/FC). In response to it we proposed to ESA, were selected and then undertook a conceptual design study for a "Space Plasma Physics Payload Package onboard the Gateway" (SP4GATEWAY), addressing these objectives while being compatible with the technical requirements.
The Gateway modules that are best-suited for hosting the in-situ measurement plasma instruments were first identified, following a simulation we performed of the interaction between the Gateway and its plasma environment (section 4.2). The proposed model payload, and its accommodation on the Gateway modules, are presented in sections 4.3 and 4.4 respectively. The fields-of-view (FOVs) of the remote sensing instruments, as projected on the sky and on the celestial objects, were then analyzed by simulating their evolution along the Gateway orbit (section 4.5).

Gateway configuration, orbit and attitude
The Gateway will evolve during its lifetime, different modules being added during the successive phases of the project. For the purpose of this study we considered a typical "Gateway Phase 2" configuration, with the Orion spacecraft attached, shown in Figure 12.
The Gateway orbit will be a Near Rectilinear Halo Orbit (NRHO) around the Moon, with periapsis × apoapsis altitudes 3 200 × 70 000 km (Whitley and Martinez, 2016). The orbital period is ~6.5 (Earth) days, and the orbital inclination ~90°. The periapsis will be above the north pole of the Moon. This orbit provides constant Earth visibility (9:2 resonance with the lunar synodic period).
The Gateway attitude will be with the +X axis (longitudinal axis, cf. Figure 12) pointed towards the Sun. The +Z axis will be normal to the Moon orbit plane, pointing southwards. The pointing accuracy requirement is that Orion remains in a tail-to-Sun attitude ±20°, i.e. the +X axis has a ±20° pointing accuracy.

Simulation of the Gateway plasma environment
The simulation of the interaction between the Gateway and the plasma environment was performed by ONERA and the Artenum company (Hess et al., 2020). A 3D mesh model with approximately 64 000 elements was developed to represent the Gateway, and the properties of the surface materials of the different Gateway modules were taken into account. The SPIS (Spacecraft Plasma Interaction System) software tool was then used to simulate the Gateway interaction with its ambient plasma environment. This open-source software, available at https://www.spis.org, computes the potential at the surface of a spacecraft according to its exchange of charges with the space plasma, i.e. the collection of charge from the plasma and the re-emission of photoelectrons and of secondary electrons due to impacting energetic particles. It also simulates the perturbation induced by this electrostatic charging on the natural plasma. This software was further developed to simulate the charging of the regolith and the motion of lunar dust particles  and to simulate the perturbation of the measurements by plasma instruments due to the charging .
Two cases were simulated, that correspond to the two situations that will be typically encountered: In each case both a nominal Gateway attitude (Gateway major axis aligned to the solar direction, cf. section 4.1) and an extreme attitude excursion, with the Gateway major axis tilted by 20° with respect to the solar direction, were considered. The simulation runs generated, for each case, maps of the electrostatic potential (volume values in the Gateway environment and surface values on the Gateway modules), and maps of the density values of H + , O + , photoelectrons and secondary electrons. The details are given in the report by Hess et al. (2020).
The main results of this study are:

Gateway in the solar wind
The volume electrostatic potential distribution, when the Gateway is in the solar wind and the major axis of the station is aligned to the solar direction (nominal attitude), following 600 s of interaction time, is shown in Figure 13.
The Gateway structure gets to a 3.5 V equilibrium potential, while the major part of the solar panels goes to a 10 V potential on the Sun facing side and -46 V on the rear side ( Figure 13A). The PPE (Power and Propulsion Element), bearing the two main solar panels, is thus inappropriate for space plasmas instrumentation for low-energy plasmas. The wake effect, due to the solar wind flow, is particularly visible behind the main solar panels, whereas in the front modules of the station the potential perturbation appears to be moderate. To highlight the potential values away from the solar panels (i.e. where the plasma instruments should be mounted), the surface and volume potentials are plotted also in a scale saturated between +5 V and -3 V ( Figure 13B). As shown there, the thickness of the sheath formed by the plasma flow around the Gateway, on the station parts exposed to the solar wind and away from the solar panels, is typically ~1.8 m and the electrostatic potential perturbation is moderate (a few volts). This implies that the effect on the ion and electron measurements will be very moderate, and only the lowest energy particles (< ~100 eV) will be affected. Solar wind ions, which have energies of typically ~1 keV, will be almost not affected. It implies also that a boom of ~2 -3 m length is adequate for placing sensors as a magnetometer and a wave antenna outside of the sheath.
The ambient proton density around the Gateway is shown in Figures 13C and 13D. The left panel ( Figure 13C) corresponds to the nominal Gateway attitude (Gateway major axis aligned to the solar direction), whereas the right panel ( Figure 13D) corresponds to an extreme attitude excursion of 20° with respect to the solar direction. Note, in both cases, the plasma wake downstream of the station. The tilted axis simulations show a small asymmetry between the illuminated and the shadowed sides of the Gateway and, as expected, a tilted plasma wake.

Gateway in the terrestrial magnetotail
Here the Gateway is exposed to the terrestrial plasma sheet / magnetosheath plasma streaming downtail. The volume electrostatic potential distribution, under these conditions and when the major axis of the station is aligned to the solar direction (nominal attitude), following 400 s of interaction time, is shown in Figure 14A. The surface equilibrium potential here is 6.5 V, while the major part of the solar panels goes to a 13 V on the Sun facing side and -31 V on the rear side. Due to the lower density of the ambient plasma, the sheath forming around the station is more extended, but the potential barrier is weaker (-1.1 V) and more isotropic, compared to the solar wind case. However, the overall results are not very different and the conclusions made in the solar wind case apply also here. Figure 14B shows the emitted photoelectron density.
The ambient H + and O + ion densities (terrestrial ions streaming downtail during active geomagnetic conditions, cf. section 2.2) are shown in Figures 14C and 14D respectively. The O + density distribution shows here a high similarity with the proton density distribution in the solar wind, presenting a very clear wake effect due to the higher ion mass.

Gateway -plasma environment interaction: synthesis
The interaction of the Gateway with its plasma environment has been simulated for the two cases that will be encountered: Gateway in the solar wind and Gateway in the terrestrial magnetotail. In both cases the surface potential of the Gateway away from the solar panels is moderate (3.5 V in the solar wind and 6.5 V in the magnetotail). A sheath is formed by the plasma flow around the Gateway, which for the solar wind case has a thickness of ~1.8 m when the Gateway is aligned to the solar direction. However, when the Gateway major axis (X-axis) is tilted by 20° with respect to the solar direction, which corresponds to an extreme excursion from the nominal attitude, this plasma sheath becomes asymmetrical and much thicker in the "shadowed" side.
These results are very encouraging, because they allow to identify the Gateway modules on which the perturbation of the natural plasma environment by the Gateway will be minimal, and are thus well-suited for placing the plasma instruments. Figure 15 shows the positions identified for instrument mounting in a color code, from green (most favorable) to red (least favorable positions).
The US Habitat and the International Habitat present small surface charging, are surrounded by a thin plasma sheath and do not suffer from any plasma wake effect. They are thus suitable for placing the plasma instruments sensitive to electrostatic charging, as the magnetospheric ion and electron spectrometers (green / light green markers in Figure 15).
However, these positions on the cylindrical surfaces of the US Habitat and of the International Habitat are tangent to the solar wind flow. When the Gateway is tilted with respect to the solar direction, the solar wind flow is detached and thus not measurable from these positions (cf. Figure  13D). Solar wind measurements require, not only limited (less than ~10 V) surface charging and absence of local plasma wake effects, but also a direct "face exposure" to the solar wind. The +X side of the Logistics Module (lower light green marker in Figure 15) is thus the most suitable position for the solar wind instruments.
Concerning the wave and field instruments, their positioning on ~2 -3m booms, on the "green / light green markers", allows having them outside of the plasma sheath.
The remaining positions can be used for energetic particle and magnetospheric imaging instruments, which are not sensitive to plasma charging effects.
The least favorable positions for placing plasma instruments (positions to avoid) are the PPE (Power and Propulsion Element) and the close to it HALO (Habitation and Logistics Outpost), cf. red markers in Figure 15, due to the large solar panels and associated circuitry, their "downstream" positioning (with respect to the solar wind flow), the high surface charging, and the proximity to the ion propulsion engine.

Model payload
In order to address the scientific objectives identified by the topical team, we first defined a model payload, consisting of a suite of instruments corresponding to the requirements shown in Table 4 (cf. section 3). These measurement instruments are largely based either on existing flight-proven This is a provisional file, not the final typeset article instruments, adapted here for the lunar plasma environment, or on tested and validated laboratory prototypes (TRL (Technology Readiness Level) ≥ 5). Table 5 lists these instruments and provides an overview of their main characteristics. The detailed description of the characteristics of the instruments is given in a series of three ESA reports, corresponding respectively to a Requirements Inventory (De Keyser et al., 2020), Conceptual Design Report (Devoto and Dandouras, 2020), and Programmatic Assessment (Futaana et al., 2020). Here we present their principal characteristics.

cMAGF: 3-axis Fluxgate Magnetometer
This instrument will provide the ambient vector magnetic field (in solar wind, terrestrial magnetotail, Moon vicinity, lunar wake, etc.). The proposed magnetometers package consists of three different types of units. The main one has three pieces of boom-mounted 3-axis fluxgate magnetometers, on two ~3 m retractable booms: one sensor at the tip of each boom, and a third sensor at the common root of the two booms. This allows having two main sensors outside the plasma sheath formed by the plasma flow around the Gateway (cf. section 4.2), while the boom-root sensor provides the possibility for removing eventual Gateway-induced perturbations by using the gradiometer technique. The presence of two boom-tip mounted sensors, on two booms, provides for further corrections for eventual perturbations. These three units will provide the main measurements while supporting the cleaning and processing of the measured data.
In order to monitor the perturbations from the station in more detail, several single magnetometer sensors will be mounted on various places directly on the station (Constantinescu et al., 2020). Additionally, we propose two current monitors, monitoring the currents flowing from the solar panels, which are expected to contribute the largest magnetic field perturbations.

cSWIS: Solar Wind Ion Spectrometer
The cSWIS instrument is a solar wind ion spectrometer that will determine the velocity distribution functions (VDFs) of the solar wind ions and will provide the solar wind density, velocity and temperature.
A top-hat electrostatic analyzer instrument is considered, covering the 0.1 -40 keV/e energy range and having a field-of-view (FOV) of 96° × 48° aligned with the solar wind arrival direction: 96° angular range in azimuth (+24° to -72° in the ecliptic plane, as the Gateway points to the Sun but it may sometimes drift away from this direction, after which it catches up) and 48° in elevation (between -24° to +24°). The spatial resolution is 3° in both azimuth and elevation, and the energy resolution is ΔE/E = 8%. In order to achieve high VDF acquisition cadence, we propose to use solar wind beam tracking, along the lines of the Cold Solar Wind (CSW) instrument (Cara et al., 2017;De Keyser et al., 2018), that was designed for the THOR (Turbulent Heating ObserveR) mission which was proposed to ESA as a medium-class M4 mission.

cSWFC: Solar Wind Faraday Cup
The cSWFC instrument will be used to determine the solar wind density, velocity and relative alphaparticle content, based on simultaneous measurements of the collector currents provided by six identical Faraday cups.
The energy of incoming ions is determined by the high voltages applied onto the control grids. The Faraday cups are organized into three units, each of them containing two cups. One unit serves for the determination of the total ion flux vector, the second unit uses high voltages applied on the control grids and provides two points of energy distribution that are used for the determination of the proton velocity and temperature in the Maxwellian approximation. The last unit serves for the measurement of the 1D velocity distribution (integral distribution) of protons and alpha particles. Each of the six Faraday cups will have a 45° × 45° FOV which, as for the cSWIS instrument, will be aligned with the solar wind arrival direction. The energy resolution is 1% (< 50 eV).
The proposed cSWFC instrument is based on the BMSW (Bright Monitor of the Solar Wind) Faraday cup instrument, that flew onboard the Spektr-R mission (Šafránková et al., 2013).

cMISP: Magnetospheric Ion Spectrometer
cMISP is a mass-discriminating ion spectrometer, that determines the velocity distribution functions of the ambient plasma ions: terrestrial magnetosphere ions, lunar exosphere pickup ions and solar wind ions.
The proposed instrument is a time-of-flight ion mass spectrometer capable of obtaining ion distributions (about 10 eV/e to 40 keV/e) with a high-resolution mass-per-charge composition determination (m/Δm > 15). Ions are selected as a function of their E/q (energy per charge) ratio, by sweeping the high voltage applied between the two hemispheres of a rotationally symmetric toroidal electrostatic analyzer (360° ×5° instantaneous FOV). Then they go through a post-acceleration of about 5 kV and they subsequently enter into the time-of-flight (TOF) section, where the velocity of the incoming ions is measured, which allows then the calculation of their m/q (mass per charge) ratio. A specially designed thin microchannel plate (MCP), through which the ions pass, is used as a conversion surface for the production the "start" TOF signal secondary electrons. The "stop" TOF signal is provided by the ion detection on another MCP. The instrument provides for a ΔE/E ~7 % energy resolution and a 22.5° angular resolution.
cMISP is based on the MIMS (MCP Ion Mass Spectrometer) instrument, that was designed for the ESCAPE (European SpaceCraft for the study of Atmospheric Particle Escape) mission, which was proposed to ESA as a medium-class M5 mission (Dandouras et al., 2018). MIMS in its turn was based on a successfully tested prototype developed at IRAP (Devoto et al. 2008). MIMS is an evolution of the CIS-CODIF instrument, flying onboard Cluster (Rème et al., 2001), but with higher mass resolution.
Since MIMS was designed for a spinning spacecraft, where it would take advantage of the spacecraft rotation to obtain a full 3D ion distribution within one spacecraft spin, cMISP on the Gateway, which is a 3-axis stabilized space station, requires the addition of electrostatic deflection plates at the instrument entrance to scan the FOV over a 360° ×120° solid angle (±60° with respect to the central entrance plane).

cMESP: Magnetospheric Electron Spectrometer
cMESP is an electron spectrometer that will determine the velocity distribution functions (VDF) of the solar wind electrons (pristine or reflected from lunar crustal magnetic field anomalies) and of the plasma sheet electrons, when the Gateway gets into the terrestrial magnetosphere.
A top-hat electrostatic analyzer instrument covering the ~5 eV to ~20 keV energy range is proposed. As for the cMISP instrument, the addition of electrostatic deflection plates at the instrument entrance to scan the FOV over a 360° ×120° solid angle is required.
The proposed cMESP instrument is based on the SWEA (Solar Wind Electron Analyzer) instrument, flying onboard the MAVEN spacecraft (Mitchell et al., 2016).

cENPD: Energetic Particles Detector
The cENPD instrument will detect and measure the fluxes of the energetic charged particles, ions and electrons: Solar Energetic Particles (SEPs), low-energy Galactic Cosmic Rays (GCRs) and terrestrial plasma sheet energetic particles. The instrument will also investigate the spectra of the secondary high energy ions, released from the lunar surface following its irradiation by GCRs and/or SEPs (albedo energetic particles, cf. section 2.1). It will cover the ~40 keV -~100 MeV energy range for ions and ~20 keV -~30 MeV for electrons. It will provide a ΔE/E ≤ 10 keV energy resolution and supply, for ions, a measure of the composition (protons to iron nuclei).
In order to cover both pristine and albedo energetic particles, it will consist of two identical detection heads, each with a 60° × 60° FOV: one pointing to the lunar zenith and the other pointing to the opposite direction (lunar nadir). Each detection head will be composed of a collimator and a 1 cm 2 , 1 mm thick silicon detector. In front of the detector a filter wheel will allow to place either a thick foil, a pinhole or an obturator to allow the reconfiguration of the detection head to various scientific modes to measure the combined spectra of electrons and ions, to measure the electron spectrum, to protect the detector from sunlight or to avoid saturation of the detector.
The proposed cENPD instrument will benefit from the heritage of the IPD instrument, flown onboard the DEMETER satellite (Sauvaud et al., 2006), and of the IDEE instrument, developed for the TARANIS satellite (Lefeuvre et al., 2008).

cGCRD: Galactic Cosmic Rays Detector
The cGCRD instrument will measure the spectra and the composition of the Galactic Cosmic Rays and that of the Solar Energetic Particles, covering the 0.1 to ~ 5 GeV energy range. It will thus be complementary to the cENPD instrument, covering the higher energies.
The proposed cGCRD instrument is the Mini.PAN penetrating particle analyzer, which is an approved H2020-FETOPEN project that will build a demonstrator of the Penetrating particle ANalyzer (PAN) for deep space applications (Wu et al., 2019).
Mini.PAN is based on the particle detection principle of a magnetic spectrometer, with novel layout and detection concepts to optimize the measurement precision for both high flux and low flux particles. As above several hundred MeV/nuc standard methods for measuring particle energies (TOF, dE/dx, ΔE-E) become less efficient, the use of magnetic spectrometry (the charged particle energy is derived from the degree of bending of its trajectory in the magnetic field) is used as the principal particle analysis method. In Mini.PAN the bending of the particle in the magnetic field is measured by precise silicon strip tracking detectors, while the elemental identity of the particle is determined by its charge and Z, which is measured with the dE/dx method at multiple points. Mini.PAN is designed to precisely measure the momentum, the charge, the direction and the time of energetic particles between 100 MeV/nuc and a few GeV/nuc.
Mini.PAN offers much higher energy resolution (compared to integral measurements), especially in the > 100 MeV range, and is appropriate for precision energy and species measurements in the 100 MeV/nuc to low GeV/nuc range, which contains both albedo particles and the low energy part of the ambient GCR spectrum. This part is not well resolved by past solar wind observatories (e.g. ACE) or by massive GCR detectors in low Earth orbit (e.g. PAMELA). Mini.PAN is also a new type of miniaturized, advanced energetic particle detector that can be adapted and adjusted for deep space missions, where mass limitations exist.
As a bonus, the proposed concept for the cGCRD detector can also detect MeV ENAs (likely of heliospheric origin, cf. Mewaldt et al., 2009), because the detection method combines a strong magnet, the ΔE-E technique and particle tracking through successive, pixelated SSDs. Few-MeV hydrogen ENAs would give the characteristic ΔE-E signal on the SSD stack, but across a straightline trajectory, since the magnet does not influence them, i.e. they can be separated from charged species (which also get detected) and from the very high energy GCRs (which are less detected but penetrate deeper).

cHENA: High-Energies ENA Imager
cHENA is a high-energies ENA (Energetic Neutral Atoms) imager, for detecting and imaging the ENAs produced by charge exchange interactions between the terrestrial plasma sheet or ring current energetic ions and the geocorona neutral hydrogen atoms. It will cover the ~10 -500 keV energy range, and will be equipped with a collimator to both delimit the FOV (120° × 90° or narrower) and reject the charged particles. The transmitted ENAs then go through a TOF system and are detected by an MCP (64 × 64 pixels). Pointing the instrument optical axis towards the terrestrial inner magnetosphere requires mounting cHENA on an azimuthal (1-axis) articulation.
The proposed cHENA instrument is based on the MIMI-INCA ENA imager, flown onboard Cassini (Krimigis et al., 2004) and the HENA ENA imager, flown onboard the IMAGE mission (Mitchell et al., 2000).

cMENA: Medium-Energies ENA Imager
cMENA is a medium-energies ENA imager, for detecting and imaging the ENAs produced by charge exchange interactions between the terrestrial plasma sheet ions and the geocorona neutral hydrogen atoms. It will thus be complementary to the cHENA instrument, extending the coverage to lower energies (~1 keV -100 keV). An additional objective for MENA is the detection and imaging of ENAs produced in the lunar environment, from the charge exchange interactions between the solar wind protons and the lunar exosphere. This requires flexibility in the instrument pointing (Earth or Moon pointing), which implies also mounting cMENA on its own azimuthal (1-axis) articulation.
The proposed instrument has a 90° × 10° instantaneous FOV and it uses, as cHENA, a collimator to both delimit the FOV and reject the charged particles. The collimator includes also a UV filter. The instrument provides a 5° × 10° angular resolution, i.e. one-dimensional images. It is based on the heritage of the wide-angle imaging neutral-atom spectrometer onboard the TWINS mission (McComas et al., 2009b) and of the SERENA-ELENA neutral atom imager onboard the MPO Mercury Planetary Orbiter of the BepiColombo mission (Orsini et al., 2021). This is a provisional file, not the final typeset article 4.3.10 cLENA: Low-Energies ENA Imager cLENA completes the suite of ENA imagers by covering the lowest energies (down to ~10 eV). These low-energy ENAs have two main sources: the charge exchange interactions of the solar wind protons with the lunar exosphere and the charge exchange interactions with the lunar surface. Moon pointing for its FOV is thus required.
The proposed instrument has a 15° × 15° field-of-view consisting of a single pixel and uses a conversion surface to ionize incoming ENAs and then feed them into an electrostatic wave system, which acts as a filter to pass only particles within the proper energy range. The particles then go through a TOF system. The instrument is capable of high-cadence observations of the solar windlunar surface interaction within the ~10 eV to ~3.3 keV energy range and with a ~50% ΔE / E energy resolution. It is based on the LNT instrument that has been designed for the Luna-Resurs-Orbiter (Luna 26) mission. Earth pointing requires, as for cHENA and for cMENA, mounting the instrument on its own azimuthal (1-axis) articulation. The articulation allows also pointing the instrument to the Moon, as a function of the scientific target of each observation session.

cUVIS: UV Imaging Spectrometer
The proposed instrument is based on the heritage of the PHEBUS UV / EUV imaging spectrometer onboard the MPO Mercury Planetary Orbiter of the BepiColombo mission (Chassefière et al., 2010).

cLPEF: Langmuir Probe and E-field
cLPEF is a Langmuir probe instrument for providing ambient plasma diagnostics: a conductive probe, either biased or floating, is immersed into the plasma and the resulting electron / ion fluxes to the conducting surface provide electric current or voltage measurements with respect to the spacecraft. From these measurements the main plasma characteristics can be derived, including the plasma density, the electric field or the spacecraft floating potential. In order to provide unperturbed plasma measurements the probe has to be located well outside of the plasma sheath that forms around the spacecraft with a thickness proportional to the local Debye length.
The proposed cLPEF instrument will employ two spherical probes (~ 8 -10 cm in diameter), each placed on the tip of a retractable boom. Since this requirement is identical to the one for the two main magnetometer sensors of the cMAGF instrument, and in order to optimize the resources and simplify the interfaces of the whole space plasma package, it is proposed to combine the sensors of these two instruments and house a tri-axial fluxgate magnetometer sensor within each of the two Langmuir spherical probes (cf. section 4.4 for the CAD figures). Such a combined Langmuir probe / magnetometer concept has been originally introduced as a part of an integrated plasma and dust package study conducted under the ESA Contract No. 4000103352/11/NL/AF in the framework of the proposed Lunar Lander mission, and it is the approach used on ESA's Comet Interceptor mission (Ratti et al., 2022).
An additional possibility is to mount occasionally a stand-alone Langmuir probe at the edge of the Gateway external robotic manipulator, so as to use this robotic arm in order to investigate the properties of the plasma sheath, forming around the different Gateway modules, at various locations.
Langmuir probes benefit from the heritage of instruments that have flown on several space missions, including the RPWS instrument onboard Cassini (Gurnett et al., 2004), ISL onboard the DEMETER satellite (Lebreton et al., 2006), DSLP onboard the PROBA-2 satellite, etc.

cWAVE: Waves Radio Instrument
cWAVE is an electromagnetic waves instrument for the study of terrestrial AKR (auroral kilometric radiation) emissions, occurring in the auroral region. It would then take advantage of the Moon occultation method, which was first implemented by the Radio Astronomy Explorer-2 mission (Kaiser and Alexander, 1976). An additional objective is the study of the radio emissions emitted by accelerated particles in the solar corona and the solar wind.
The proposed cWAVE instrument will measure the AC electric field (one component: fast electric waveform at 16.5 MHz, decimated electric waveform at 64.5 kHz) and the AC magnetic field (one component: magnetic waveform at 64.5 kHz). These three products will be delivered as waveform (event mode) and / or as averaged spectra (survey mode, with onboard FFT computation).
As for the cMAGF and the cLPEF instruments, the cWAVE sensor needs to be placed at the tip of a dedicated retractable boom.
Radio waves instruments benefit from the heritage of instruments that have flown on several space missions, including STAFF onboard Cluster (Cornilleau-Wehrlin et al., 2003), RPWS instrument onboard Cassini (Gurnett et al., 2004), etc.

Retractable booms
As indicated above, the mounting of the sensors of the combined cMAGF and cLPEF instruments and of the cWAVE instrument requires a total of three retractable booms, ~3 m each. These booms need indeed to be stowed at the beginning and at the end of the mission to allow the instruments to stay within the allocated envelope and to be transferred by the Airlock. We propose to use the compact deployable and retractable boom that has been developed by Oxford Space Systems: the Astrotube Boom. It can be deployed to up to 3 m and is TRL 9.

Instruments not included in the conceptual design study
The above-described model payload instruments (cf. also Table 5) cover satisfactorily the instrumentation requirements, as defined by the topical team (cf . Table 4). However, there are two instruments that were not included in this conceptual design study: the MeV ENA Imager and the Soft X-ray Imager.
The MeV ENA Imager was not included due to the absence in Europe, in our knowledge, of a developed instrument or protype, for ENAs at these very high energies. However, as described above, the proposed cGCRD instrument (Mini.PAN) will be able to detect few-MeV hydrogen ENAs, separating them from similar energy protons and providing 1-pixel images of this population.
For X-ray imaging, a soft X-ray imager with a wide field-of-view, using lobster-eye optics and a position-sensitive MCP detector operating at the 0.1 -2 keV X-ray bandpass has been considered (cMXRI instrument). This instrument is based on the DXL/STORM soft X-ray imager protype flown This is a provisional file, not the final typeset article onboard a sounding rocket mission (Collier et al., 2015). However, the size of this instrument (78 cm length) appears to be incompatible with the Gateway interfaces for mounting external payloads. This suggests that the X-ray imager could not be accommodated as part of this instruments package. However, it is recommended to propose cMXRI as a payload for the Large European Lander for the Moon.

Instrument accommodation
The CAD model for the instrument conceptual design and their accommodation on the Gateway was established in cooperation with the CNES PASO (Plateau d'Architecture des Systèmes Orbitaux), with the help of its concurrent engineering facilities (CIC : Centre d'Ingénierie Concourante), and particularly by using the IDM-CIC (Integrated Design Model) and IDM-View tools (https://idm.virtual-it.fr/).
The instrument accommodation on the Gateway modules has to fulfil several requirements: − In-situ measurement low-energy plasma instruments have to be placed on areas with low electrostatic charging (cf. section 4.2). − Pointing requirements for instruments with a field-of-view (cf. Table 5). − Unobstructed field-of-view for these instruments. − cGCRD, which has a strong permanent magnet perturbing the low-energy plasma measurements, should not be placed close to these instruments. − Instrument grouping, when possible, to form self-contained "instrument suites", with instruments mounted on a common platform, minimizing interfaces with the Gateway and using a single SORI (external Small ORU Interface) for attachment on the Gateway.
The instrument accommodation configuration we propose, and is compatible with the above requirements, has the instruments grouped on one main and one secondary platform. Each of these two platforms, of the order of 0.8 m × 0.8 m, is mounted externally on a SORI attachment and is double sided, i.e. has instruments mounted on both sides of the platform. This accommodation is of course notional and could be subject to modifications, depending on Gateway engineering and programmatic constraints.
Both platforms are on the Logistics Module and they are mounted on two diametrically opposite positions, on the +X side (Main Instrument Platform) and on the -X side (Secondary Instrument Platform) of it, cf. Figure 16.
The two sides of the Main Platform are shown in Figure 17. With its positioning on the +X side of the Logistics Module, the Main Platform provides an unobstructed view to the solar wind arrival direction (Figures 13C and 13D) and takes advantage of a favorable electrostatic environment (Figure 15). It is thus well suited for mounting the solar wind instruments (cSWFC and cSWIS), shown in Figure 18 and Figure 19 respectively.
The Main Instrument Platform hosts also: − The magnetospheric particle instruments cMESP and cMISP, shown in Figure 18 and Figure 19 respectively. − The energetic particle detector cENPD, shown in Figure 20, which is mounted on the -Y edge of the platform. cENPD has two oppositely directed FOVs, one along the +Z axis and one along the -Z axis. In this way, during periapsis passes one of the FOVs looks in the zenith direction, to monitor the pristine energetic particles precipitating towards the Moon's surface, whereas the other looks in the nadir direction, to monitor the albedo energetic particles that are the result of the interaction of the precipitating energetic particles with the lunar regolith (cf. section 4.5). Moreover, the +Z / -Z orientation of the two detector heads allows avoiding direct sunlight entering the detectors (Sun is in the +X direction). − The two "compact" remote sensing instruments cMENA and cLENA, shown in Figure 18 and Figure 19 respectively. cMENA uses a dedicated azimuthal (1-axis) articulation. The cLENA orientation gives access, during the periapsis passes, to the Moon surface and plasma environment. − The two booms of the fluxgate magnetometer package (cMAGF), which as described in section 4.3.1 consists of three different types of units. The main type is two pieces of boom-mounted dual fluxgate magnetometers (one sensor at each of the two ~3 m boom tips and one at the boom root). These two retractable booms are mounted on the Main Instrument Platform -Z side (Figure 18). The boom tip mounted cMAGF sensors are integrated together with Langmuir probes (cLPEF instrument). These units will provide the main measurements. In order to monitor the perturbations from the station close to the source in more detail, several (~5+) single magnetometer sensors will be also mounted on various places around the station (not shown). − The cWAVE instrument, also mounted on a retractable boom, which is on the Main Instrument Platform +Z side (Figure 19).
The two sides of the Secondary Platform are shown in Figure 21. This platform, mounted on the -X side of the Logistics Module, is permanently in the shadow. In this way there is no direct sunlight that could interfere with the measurements of the two instruments mounted on it. On each of its two sides there is a remote sensing instrument: cUVIS on the one side and cHENA on the other side of the Secondary Platform. Each of these two instruments in mounted on a dedicated azimuthal (1-axis) articulation.
The cGCRD instrument, due to the containment of a strong magnet (0.4 Tesla) that would deviate charged particles to be measured by the other instruments if in close vicinity with them, is not mounted on any of the two instrument platforms. It is instead mounted as a "standalone" instrument on the SORI attachment of the +Z side of the Logistics Module (Figure 22). Its FOV, looking radially out, near periapsis gives access to the albedo energetic particles that are the result of the interaction of the precipitating galactic cosmic ray particles with the lunar regolith. During the remaining part of the orbit (most of the time) it points to the open sky.

Instruments fields-of-view simulation
The appropriate orientation of the fields-of-view (FOVs) of the remote sensing and of the highenergy particle instruments, as projected on the sky and on the celestial objects, was analyzed by simulating the evolution of the FOVs along the Gateway orbit. This simulation was performed in cooperation with the CNES PASO and by using the VTS software tool (https://logiciels.cnes.fr/en/content/vts).
The FOVs of the two oppositely directed sensor heads of the cENPD instrument, near a periapsis pass, are shown in Figure 23. As shown in this figure, one of the two sensor heads is oriented towards the local zenith, and has an unobstructed view to the pristine energetic particles flux (purple FOV), whereas the other is oriented towards the nadir and its FOV is dominated by the albedo energetic particles from the Moon (yellow FOV). Both populations (pristine and albedo high-energy particles) are thus covered by the cENPD instrument detection capabilities.
For the cGCRD instrument, which is a single sensor head GCR detector, the FOV near a periapsis pass is shown in Figure 24, left panel (light blue FOV). As shown, near periapsis it is dominated by the albedo GCR particles from the Moon. However, during most of the remaining orbit (right panel) it has an unobstructed view to the open sky and gives then access to the pristine GCR environment.
The field-of-regard (FOR) of the cMENA instrument, i.e. the total accessible FOV taking into account the rotation of the 1-axis articulation on which the instrument is mounted, at a given point of the orbit, is shown in Figure 25A. The azimuthal rotation mechanism gives to the instrument access to a very large "ribbon" of the sky, which includes the Earth environment and the Moon environment. The pointing of the instrument to any of these two principal targets, using the flexibility provided by the 1-axis articulation, can then be programmed as a function of the scientific target of each observation session.
In Figure 25B is the FOV of the cLENA instrument, close to periapsis, as projected on the sky (no articulation for this instrument). As shown in this figure, the way the instrument is mounted on the Gateway gives access, during the periapsis passes, to the Moon surface and to its exosphere and plasma environment.
The FOR of the cUVIS instrument is shown in Figure 25C. As shown in this figure, the dedicated articulation allows also for this instrument to point to targets as the Earth space environment (plasmasphere, exosphere), the Moon space environment (exosphere), or targets in the open sky. The narrow width of the instantaneous FOV of this instrument (0.1°), in combination with the articulation, allows also performing altitude profile scans of the lunar exosphere.

Conclusion
The Moon is a unique location to study the deep space plasma environment. The Lunar Orbital Platform -Gateway, that will be assembled and operated in the vicinity of the Moon starting from the mid-2020s, is a crewed station that offers new opportunities for fundamental and applied scientific research in the field of space plasma physics. These have multi-disciplinary dimensions, and they include: − Studying the lunar space environment and its interaction with the solar wind and the terrestrial magnetotail plasma. − Terrestrial space weather: monitoring, through remote sensing techniques, the response of the terrestrial magnetosphere and exosphere to solar activity events. − Planetary space weather: monitoring, through in-situ measurements and through remote sensing, the response of the lunar space environment to solar activity events. − Radiation physics: characterizing the lunar high-energy particles environment, including energy and mass spectrometry of these populations and their variability, particularly in view of the Artemis human missions to the Moon and the associated radiation risks. − Studying the heavy ion escape from the terrestrial ionosphere, through in-situ measurements of the downtail streaming ions, and the role of this escape in the long-term evolution of the composition of the terrestrial atmosphere (and its habitability). − Studying the lunar regolith -bounded exosphere -interplanetary space environment as a complex interacting multi-scale system, and as an archetype of the interaction of an unmagnetized planetary body with the solar wind. − Studying the mini-magnetospheres that form above the "swirls" on the Moon, and which constitute probably the smallest magnetospheres in our solar system.
− Understanding the surface electric fields that develop on the Moon as a part of a complex and interacting plasma environment, and their role in electrostatic lunar dust levitation. − Planetology: understanding the composition of the lunar regolith, and its hydration, through the spectrometry of the albedo energetic particles.
In preparation of the scientific payload of the Lunar Orbiter Platform -Gateway we first formed a topical team, under the auspices of ESA, to prepare and to support the definition of payload studies in the field of space plasma physics. This allowed to identify the scientific objectives that can be addressed from onboard the Lunar Orbital Platform -Gateway, the physical parameters needed to be measured in order to address these objectives, and the corresponding instrumentation required to perform these in-situ measurements and remote-sensing observations.
We then undertook for ESA a conceptual design study for a "Space Plasma Physics Payload Package onboard the Gateway" (SP4GATEWAY), addressing the objectives identified by the topical team while remaining compatible with the technical requirements. This conceptual design has considered, as baseline, a typical "Gateway Phase 2" configuration.
As a first part of this conceptual design study, we simulated the interaction between the Gateway and its plasma environment, for the case where the Gateway is in the solar wind and also for the case where the Gateway is in the terrestrial magnetotail. This allowed to identify the Gateway modules on which the perturbation of the natural plasma environment by the Gateway will be minimal, and are thus best-suited for placing there the in-situ measurement plasma instruments.
We then defined a model payload consisting of a suite of instruments, for in-situ measurements and for remote-sensing observations, corresponding to the requirements. These measurement instruments are largely based, either on existing flight-proven instruments, adapted here for the lunar plasma environment, or on tested and validated laboratory prototypes. The main characteristics of these instruments have been defined and CAD conceptual instrument designs elaborated. The instruments' measurement characteristics will however have to be refined during a follow-on Phase A study.
The next step was the study for accommodating this model payload on the Gateway modules, taking into account the various constraints, and in particular the surface and volume charging of the various Gateway modules, their exposure to the ambient plasma and the pointing and field-of-view requirements of the instruments. This resulted in an integrated CAD design, including the Gateway and the instruments, which were grouped into two platforms mounted on two sides of the Logistics Module.
The fields-of-view of the remote sensing instruments and of the high-energy particle instruments, as projected on the sky and on the celestial objects, were then analyzed by simulating their evolution along the Gateway orbit. This allowed to verify the appropriate orientation of the fields-of-view and the coverage of the observational scientific targets.
Following this conceptual design study for a Space Plasma Physics Payload Package onboard the Gateway, it results that the Gateway is very well-suited for space plasma physics research and it allows to address a series of relevant scientific objectives.