M 5 — Mars Magnetospheric Multipoint Measurement Mission: A multi-spacecraft plasma physics mission to Mars

Mars, lacking an intrinsic dynamo, is an ideal laboratory to comparatively study induced magnetospheres, which can be found in other terrestrial bodies as well as comets. Additionally, Mars is of particular interest to further exploration due to its loss of habitability by atmospheric escape and possible future human exploration. In this context, we propose the Mars Magnetospheric Multipoint Measurement Mission (M 5 ), a multi-spacecraft mission to study the dynamics and energy transport of the Martian induced magnetosphere comprehensively. Particular focus is dedicated to the largely unexplored magnetotail region, where signatures of magnetic reconnection have been found. Furthermore, a reliable knowledge of the upstream solar wind conditions is needed to study the dynamics of the Martian magnetosphere, especially the di ﬀ erent dayside boundary regions but also for energy transport phenomena like the current system and plasma waves. This will aid the study of atmospheric escape processes of planets with induced magnetospheres. In order to resolve the three-dimensional structures varying both in time and space, multi-point measurements are required. Thus, M 5 is a ﬁve spacecraft mission, with one solar wind monitor orbiting Mars in a circular orbit at 5 Martian radii


Introduction
Among the planets in the solar system, Earth, Mercury, and the gas giants possess a global intrinsic magnetic field due to an active internal dynamo process.This is the dominant driver in the deflection and thermalization of the solar wind plasma.The region where the solar wind dynamic is influenced by the planet's magnetic field is called the magnetosphere.However, other planets such as Mars (Dubinin & Fraenz, 2015) and large solar system bodies like the Moon do not show such a dynamo and therefore lack a global intrinsic magnetic field.These bodies can still have local intrinsic magnetic fields -Mars possesses strong magnetic anomalies (crustal fields) of up to 300 nT (Mittelholz et al., 2017) at its surface -but in general, the large scale interaction with the solar wind of such systems is much different.For Mars, the direct interaction with the upper-atmosphere generates the so called induced magnetosphere (Sánchez-Cano et al., 2021).The different regions of the Martian magnetosphere are presented in Figure 1.Referring to the numbers in the figure, the Interplanetary Magnetic Field (IMF, 2) draped around the planet interacts with the solar wind (1), forming a bow shock (BS, 3) and a magnetic pileup boundary (MPB, 4), resembling the magnetopause at Earth, as dayside boundary regions (Trotignon et al., 2006) above the ionosphere (5).On the nightside, there is the magnetotail with its two lobes (7) that are separated by a plasma sheet (8), directed in opposite directions (Eastwood et al., 2008).Due to the induced character of the magnetosphere, the average sub-solar bow shock distance (3) at 0.63 planetary radii from the surface (Trotignon et al., 2006) is much shorter than compared to e. g.Earth at about 13 Earth radii.The weak crustal fields (6) of Mars only reach up to 0.38 Martian radii (Crider, 2004).
It is believed that Mars used to be more Earth-like, with a wetter and warmer climate.For this to have been the case, the atmosphere must have been denser than at present (Jakosky et al., 2017).Today, this is no longer the case, and in order to answer the question of how Mars became less habitable, we must investigate how the atmosphere was lost over time.The absence of a strong intrinsic magnetic field and the resulting weaker protection of the Martian system from solar wind and high solar activity events like coronal mass ejections (CMEs) results in the loss of the Martian atmosphere (Jakosky et al., 2015a).Therefore, the lack of a dynamo to power an intrinsic magnetic field is thought to be a strong driver in the evolution of habitability of Mars.In order to understand how the atmosphere evolved in the past, a detailed picture of the whole Mar-tian plasma environment is needed.With this, past habitability of Mars could be investigated as well as giving implications of the possible fate of terrestrial planets' atmospheres once they lose their active dynamo.
Additionally, the knowledge of space weather at Mars is an important driver for future exploration of Mars.The high variability of the Martian magnetosphere means that events like solar storms are potential threats to spacecraft and space infrastructure flying even within the induced magnetosphere (Hassler et al., 2018), with possible catastrophic consequences (Marusek, 2007).Moreover, astronaut safety in the future manned exploration of Mars could be jeopardized if the conditions at Mars are not known in detail (Cucinotta et al., 2013).Therefore, near-continuous observations of the solar wind conditions at Mars are needed in order to both determine the average and extreme space weather conditions and determine their influence on the Martian magnetospheric system.Furthermore, a dedicated Martian solar wind observatory not only extends the "orchestra" of solar wind monitors, but also could aid in the study of the evolution of solar transient structures like solar storms.
The Martian induced magnetosphere offers the opportunity to study such a system in greater detail within manageable reach of current in-situ space exploration capabilities.Not only is it a representative example of a solar system non-dynamo magnetosphere for large bodies (like Venus or the Moon), but also relevant to studies of comets and active asteroids (Götz et al., 2019).Furthermore, if unique characteristic properties of such magnetospheric systems are identified, these could have implications for the characterization of exoplanetary plasma environments (Airapetian et al., 2020).
It is known that the Martian induced magnetotail is variable depending on the solar wind conditions.The modifications in the IMF, which induce a reorientation of the tail (DiBraccio et al., 2017) are characteristic of this variability.In order to separate temporal and spatial variations of these moving or flapping structures in the tail, simultaneous multi-point measurements are needed.Despite comprehensive studies of the Martian environment of previous missions, the far tail region has never been characterized in detail by in-situ measurements.A current open question is whether magnetic reconnection of the IMF occurs in the far tail at Mars, and if so, to what extent.
Magnetic reconnection is a fundamental plasma process where magnetic energy is converted to kinetic energy.It has been studied at Earth with formation missions like Cluster and the Magnetospheric Multiscale (MMS) mission.Similar processes occur on other magnetized and unmagnetized planets.On Mars, both measurements (Harada et al., 2015;Wang et al., 2021), and simulations (Ma et al., 2018) suggest that reconnection occurs on the nightside, playing a role in the dynamics of the magnetotail influencing ion flow velocities with possible effects on atmospheric escape.
Reconnection is not the only physical process of interest that takes place in the magnetotail.The magnetotail is one of the main paths for planetary ions to escape from the Martian atmosphere (Lin et al., 2021).Therefore, a mapping of the properties of the Martian magnetotail complements ongoing studies of this important process and will allow a more complete assessment of balancing terms of atmosphere system in-and outflow.This is vital for the understanding of how habitability of Mars has changed over time.
Moving from the Martian nightside to the dayside, crucial features of the induced magnetosphere are the BS and MPB.MAVEN (e.g.Jakosky et al., 2015b) has studied this region, showing a strong variation of the position of both BS and MPB (Matsunaga et al., 2017).However, a systematic characterization of their variability depending on solar wind conditions is lacking.Knowledge of the dependency of the system's shortterm evolution on solar wind conditions -especially for solar high-energy events -is imperative for spacecraft and astronaut safety.
Energy transfer and transport, especially on global and ionscales, is another important aspect of the characterization of the Martian magnetospheric system, which will help in understanding the complete picture of the evolution of the atmosphere.One of the ways to transport energy is by currents.A yearaverage picture of the Martian current system has been acquired by MAVEN (Ramstad et al., 2020), but a detailed, time-varying characterization is lacking.To measure the instantaneous current, a tetrahedral multi-spacecraft configuration is needed, in which methods such as the curlometer technique can be used, as it has been done at Earth for Cluster (Dunlop et al., 2021).This would allow the measuring of transient currents, which are lost in the process of averaging.Furthermore, by having a solar wind monitor, the response of the currents to changing solar wind conditions can be investigated.
Another way of transferring energy is through plasma waves, which are important to study due to their ability to accelerate and scatter particles, which can lead to the escape of particles from the atmosphere.Many waves around Mars have been identified, such as Whistler waves, Proton Cyclotron waves and Magnetosonic waves (Yadav, 2021;Brain et al., 2002).Other waves such as Ion Acoustic waves and Lower Hybrid waves are predicted to exist in the Mars ionosphere, but have yet to be detected (Yadav, 2021).The detection of the latter could explain some of the loss of particles from Mars outer ionosphere through particle acceleration.In order to fully charac- Table 1: Scientific questions and objectives of the M 5 mission.The specific regions, that are referred to by the scientific objectives are given by numbers in parenthesis, corresponding to the regions specified in Figure 1.

Primary scientific question
Primary scientific objectives Q1: How do the Martian magnetospheric system's structure and dynamics depend on solar wind conditions?
O1.1 (1, 3, 4): What are the dynamics and orientation of boundary regions, with particular interest for their dependence upon solar wind conditions?O1.2 (1, 7, 8): What is the structure of the Martian magnetotail on different scales, with particular interest for its dependence upon solar wind conditions?O1.3 (1, 3, 4, 5, 7, 8): What is the dynamical structure of the current system in the Martian magnetosphere, with particular interest for its dependence upon solar wind conditions?Q2: How is energy transported within the Martian magnetospheric system on ion scales and above?O2.1 (7, 8): Is magnetic reconnection observed in the magnetosphere tail, and if so, where and how?O2.2 (3, 4): What are the direction and temporal evolution of low frequency plasma waves?Secondary scientific question Secondary scientific objectives Q3: How does the solar wind propagate through the solar system?O3.1 (1): What are the temporal variations of the upstream solar wind conditions at Mars? Q4: Excluding magnetic reconnection, are there other processes driving the energy transport at the Martian magnetotail?
O4.1 (7, 8): Are other energy transport processes observed at the Martian magnetotail that exhibit signatures different to magnetic reconnection?terize these waves, temporal and spatial variations would need to be resolved and separated, which requires a tetrahedron formation of spacecraft (Karlsson et al., 2004;Narita et al., 2010).
In order to allow for the separation of spatial and temporal variations of 3D plasma structures, again a four-spacecraft tetrahedron constellation is needed.This has been demonstrated by the Cluster mission at Earth (Escoubet et al., 2021).This mission allows the characterization of the time variation of the dayside boundaries and simultaneously determine their 3D spatial extent.Additionally, currents on above-ion-scales were detected by Cluster using the curlometer technique (Dunlop et al., 2021), as well as waves and turbulence with the wave-telescope technique (Narita et al., 2022) which are techniques only possible using four-point measurements.Another example of a multi-spacecraft mission dedicated to magnetospheric research at Earth is the THEMIS mission, launched in 2007.The five THEMIS satellites designed to study space weather phenomena were also able to study Earth's boundary regions, with for example Haaland et al. (2019) characterising the current sheet thickness, motion and current density of the magnetopause.Finally, MMS is a four-spacecraft plasma research mission dedicated to characterizing reconnection.All this shows the success and need for a four-spacecraft constellation to study a planetary magnetospheric system comprehensively.
In the last decades, multiple missions have targeted Mars, tackling diverse science topics like the search for water and bio-signatures and the exploration of Mars' surface.The ongoing missions Mars Express (Chicarro et al., 2004) and MAVEN (Jakosky et al., 2015b) are focused mostly on atmospheric composition, evolution and circulation.Therefore they are also equipped with plasma instrument suites, however are limited as for example Mars Express lacks a magnetometer.Additionally, the scientific output on the Martian magnetosphere is limited due to the lack of additional orbiters which would allow the observation of temporal and spatial variations.Moreover, there is currently no dedicated solar wind monitor at Mars, which is needed to investigate the variability of the magnetosphere depending on solar wind conditions.The upcoming mission Escape and Plasma Acceleration and Dynamics Explorers (Es-caPADE) -scheduled to launch in 2025 -will study the flow of both energy and ions in and out of the Martian atmosphere (Lillis et al., 2022).Mars Magnetosphere ATmosphere Ionosphere and Surface SciencE (M-MATISSE) is a mission currently being studied for the ESA M7 call aiming to characterise the region between the Martian upper atmosphere and the outer magnetosphere, and to study how surface processes are affected by space weather (Sanchez-Cano et al., 2022).
Despite the considerable number of Martian exploration missions, there has been a paucity of plasma physics-focused missions in the past.Furthermore, both of the future dedicated plasma missions lack the capabilities to produce a complete and detailed picture of the structures and energy transport with both temporal and spatial dependencies in the whole Martian induced magnetospheric system as well as providing this information with dependency on precise upstream solar wind conditions.
All in all, the change of the magnetosphere with solar wind conditions and how energy is transferred across different scales -both spatially and temporally -remain to be fully understood.Additionally, the Martian magnetotail is still largely unexplored.This is reflected in the Voyage 2050 Senior Committee Report (Voyage 2050 Senior Committee, 2021), which was written to identify key science areas for ESA's science program during the period 2035-2050.Relevant key areas are "Magnetospheric Systems" (3.1.1)and "Plasma Cross-scale Cou-pling" (3.1.2).They state that, "important questions such as 'How is energy and matter transported in induced magnetospheres' still need to be answered by studying entire magnetospheres as complex systems".In this context, we propose the Mars Magnetospheric Multipoint Measurement Mission, hereafter M 5 , a 5-spacecraft mission to study the different regions of the Martian magnetosphere comprehensively, by using a four-spacecraft tetrahedron formation for in-situ measurements while monitoring the solar wind with an additional spacecraft.
This paper is organized as follows: in Section 2, the Scientific Objectives and Questions, derived from the above shown open research areas are given.With that, measurement requirements for different physical quantities to be measured at Mars are specified.Subsequently, the mission profile is described in Section 3, with the required scientific payload following in Section 4. In Section 5, all technical aspects of the proposed mission are assessed in detail.Finally, programmatics are addressed in Section 6 followed by a general conclusion (Section 7).

Scientific Questions and Measurement Requirements
In order to structure the different regions and physical phenomena and make them more approachable from an instrument point of view, we define a broad scientific theme for the M 5 mission: "To understand how the variable solar wind conditions influence the dynamics and energy transport of the Martian induced magnetosphere".
From that, two primary scientific questions are derived, which are then segmented into scientific objectives.This hierarchy is shown in Table 1, including reference to the regions of interest shown in Figure 1.
The first primary scientific question (Q1) focuses on the dependency of the Martian magnetosphere on solar wind conditions.The second question (Q2) relates to energy transport in the Martian magnetosphere.In addition to these two primary scientific questions, M 5 will be able to tackle two other secondary scientific questions.The third question (Q3) concentrates on the propagation of the solar wind in the solar system.The fourth question (Q4) is related to the possibility that reconnection in the Martian magnetotail is not the only process driving energy transport.
The respective scientific objectives allow for the definition of measurement requirements by using a traceability matrix.Table 2 shows the required measurement quantities for instruments on each spacecraft respectively, both on the Solar Wind Observatory (SWO) and the four Magnetospheric Formation Orbiters (MFO) constituting a tetrahedron constellation.The requirements were derived from each of the measurement regions, physical quantities, timing constraints, and specific measurement needs (e.g.range and accuracy) in question.The typical parameters that are expected to be observed by the M 5 missions are derived by previous in-situ measurements (Nilsson et al., 2012;Holmberg et al., 2019;Ergun et al., 2021).
The requirements for magnetic field, ion distribution functions, electron distributions functions, and electric field measurements are detailed in Table 3, Table 4, Table 5, and Table 6 respectively.Based on the measurement requirements, corresponding heritage instruments or instrument options have been selected and are presented in Section 4.

Mission Profile
To answer the science questions and objectives stated in Table 1, the M 5 mission requires a tetrahedral formation of four spacecraft.This allows the resolution of both spatial and temporal variations, as well as a three-dimensional mapping of the boundary regions, even when the location, velocity, and orientation of the boundary are unknown.This will result for example in the ability to take into account nonuniform conditions such as ripples and reformation, as has been done with Cluster.The same applies to the largely unexplored magnetotail.In addition, such a constellation enables the mapping of currents in the magnetosphere, using the curlometer technique (Dunlop et al., 1988) to derive currents from magnetic field measurements.Furthermore, it will be used for measurements of wave direction and time dependency using the wave telescope technique (Motschmann et al., 1996).Finally, multiple spacecraft are needed to determine origin regions of magnetic reconnection by observing ion outflow.Spacecraft separation distances on and above ion scales are required to observe all the mentioned phenomena.Ion scales at Mars range from the proton gyroradius in the near tail on the magnitude of 100 km, to around 750 km maximum in the magnetosheath (Nilsson et al., 2012).In addition, an active solar wind monitor is needed to provide necessary simultaneous information about the solar wind conditions.
Therefore, we propose a five spacecraft mission.Four identical MFOs will be placed in an elliptic orbit in a tetrahedral cartwheel helix formation (subsection 5.4).Their goal is to investigate the Martian magnetotail and the boundary regions, addressing all primary science objectives of the mission.The fifth spacecraft, the Solar Wind Observatory (SWO), targets a circular orbit around Mars.The SWO will characterize the solar wind properties around Mars during the whole Martian year, thus addressing the secondary science question Q3, which supports addressing the primary science question Q1.As a result of the chosen orbit the SWO will spend a part of its orbit in the magnetotail, covering a region similar to the one explored by MAVEN.Furthermore, it acts as a data relay for the MFOs to Earth. Figure 2 shows the both the SWO and one MFO in their final configuration at Mars.

Payloads
In this section, we provide an overview of the proposed instruments for the M 5 mission, in terms of the heritage instruments they are based on.O1.1   The magnetometers proposed for the mission are 3-axis fluxgate magnetometers with heritage from THEMIS (Auster et al., 2008).Each spacecraft will carry a pair of these magnetometers mounted on different locations of a deployable boom stretching 5 m in length.One magnetometer will be located at the tip of the boom, whereas the other one halfway up the boom.This configuration allows for effective magnetic interference mitigation, as described in Section 5.6.9.

Ion Spectrometers
The mission will utilize electrostatic analysers to measure the ion energy distribution function.The instrument placed on the SWO will be used as an ion energy spectrometer.A heritage  (Owen, C. J. et al., 2020).
In contrast, the instrument on each of the MFOs will use magnets to act as a mass over charge spectrometer.As heritage, the Ion Composition Analyser (ICA) instrument from Rosetta (Nilsson et al., 2007) is considered a viable option.The ion mass spectrometer will measure the 3D distribution function of the ions to study how the particles interact with the solar wind.

Electrostatic electron analyser
In order to measure the electron composition of the plasma environment, an electrostatic electron analyser will be em-  (Owen, C. J. et al., 2020).The solar wind electron analyser will measure the effects from the electron impact ionization from the solar wind as it encounters the Martian atmosphere.

Electric field instrument
In order to measure the 3D electric field vector of the plasma environment, each MFO will have an electric field instrument using 6 booms (4 wire booms, 2 telescopic booms).In addition, two orthogonal probes will have Langmuir probe capabilities.This will be used to measure the temperature and density of the plasma.The instrument proposed for the described purpose is the electric-field and wave instrument (EFW) that has heritage from ESA's Cluster mission (Gustafsson et al., 1997).

Mission Design
In the following, we will detail the technical aspects of the mission.

Margin Philosophy
The margin philosophy adopted for the mission design is based on recommendations detailed by ESA (ESA, 2014).The applicable sections of the margin philosophy have been considered for all system budgets including mass, ∆V, propellant, data, and link budgets, as well as the power and thermal budgets.

Ground Segment
For ground segment communications section, the ESA Deep Space Antennas network, which include the antennas located in Cebreros (Spain), Malargüe (Argentina) and New Norcia (Australia) will be used.Science operations will take place at the European Space Astronomy Centre (ESAC), close to Madrid.

Launch & Propellant
The M 5 mission is designed to be launched using an Ariane 64 launcher from Kourou, French Guiana.Figure 3 presents the M 5 mission spacecraft in the launch configuration inside the Ariane 64 fairing.After the launch, the five spacecraft will utilize thrusters with MMH/N204 bipropellant in order to perform the orbital and attitude maneuvers needed to reach and maintain the required orbits, stabilization, and attitude of the spacecraft.Helium pressurizing is used in order to maintain the operating pressures.Heritage thrusters from the ExoMars orbiter with a bi-propellant propulsion system (Pavón et al., 2012)

Orbits & Maneuvers
After launch, the five spacecraft will fly in a heliocentric elliptic transfer orbit to Mars.The approach trajectory along with the final orbits of the spacecraft and the transfer orbits needed to reach them are illustrated in Figure 4. Initially the four MFOs are stacked on top of the SWO.In this transit configuration the spacecraft will perform a Deep Space Correction Maneuver (DCM) before reaching Mars' sphere of influence, arriving at a periapsis of 1.8R m with an inclination of 150 • .Next, the four MFOs detach from the SWO and from each other.
After detaching, the SWO performs an Orbit Insertion Maneuver (OI) at the periapsis to increase the apoapsis to 5R m .Once at the apoapsis, the SWO circularize its orbit with an Increasing Periapsis Maneuver (IPM), entering its nominal orbit (5R m × 5R m ).The four MFOs, in contrast, perform an Increasing Apoapsis Maneuver (IAM) at the periapsis of the insertion orbit.Their target orbit has an apoapsis equal to 6R m and they will orbit in a cartwheel helix formation configuration that is reached using Formation Configuration Maneuvers (FCM).The mission trajectory can be seen in Figure 4.
The choice of orbit for the MFOs (6R m × 1.8R m ) satisfies the scientific requirement of orbiting in the magnetotail.In particular, because of the J 2 effect, the time spent in the tail region is increased by a factor of five to 280 days.A schematic of the orbit propagation can be seen in Figure 5, and the simulated temporal evolution of the orbits is shown in Figure 6.The ∆V required to perform the required orbital and attitude maneuvers and the propellant mass burned during the thrusts are presented in Table 7.

Orbit & Attitude Maintenance
In addition to propellant required for the ∆V to reach the required Martian orbits, propellant is budgeted for orbit main-tenance and attitude control over the mission lifetime.Orbit Trim Maneuvers (OTMs) are required to maintain and fine tune the orbits.The propellant mass required for OTMs of each spacecraft is estimated based on the experience gained from the MAVEN mission (Jesick et al., 2017).Attitude Control Maneuvers (ACMs) augment the use of reaction wheels to adjust or maintain the attitude of the spacecraft.ACMs include periodical thruster firings for offloading torques from the reaction wheels to keep them out of saturation.The propellant allocated for OTMs and ACMs is 21.1 kg for the SWO and 7.4 kg for each MFO.Attitude control details and requirements are presented in subsubsection 5.6.8.

Space Segment
The space segment of the mission consists of the SWO and the four MFOs, which differ in design due to varying payloads and functionalities.The following subsections cover the space segment in more detail.

Structure & Spacecraft Design
The primary structure of both types of spacecraft consists of a 1.214 m cylindrical core that encloses the propellant tanks, made of titanium (Ti6AI4V STA).Exterior panels are attached to the central core.An aluminium honeycomb sandwich structure with graphite composite face sheets is used for all the primary structure elements, providing enough stiffness to sustain the launch loads and induced vibrations.The panels sections are joined with bonded composite L-brackets.The general dimensions of the SWO spacecraft are 2.3 m × 2.3 m × 1.8 m and the whole structure has a preliminary mass of 240 kg.The material structure and structure layout is widely used in space missions (Yasaka & Onoda, 2003).This provides a high TRL, and heritage e.g. from the Dawn (Thomas et al., 2011) and MAVEN (Jakosky et al., 2015b) spacecraft.
In the bottom part of the spacecraft, a central cylinder is used to ensure precise attachment to the payload adaptor.On the top part of the spacecraft, an attachment and locking mechanism is used.The MFOs are stacked on top of each other using the aforementioned locking mechanism which will be designed in further mission design phases.An exploded view of the SWO with major subsystems is presented in Figure 8.

Mass Budget
To calculate the mission mass budget, the mass of each subsystem was derived based on estimates and data on existing subsystems.A margin of 5 % to 20 % was added to the calculated mass of each subsystem.Moreover, an additional overall system margin of 20 % was added to the sum of subsystem masses to obtain the final dry mass estimate of the system.The total wet mass of the system was obtained by adding up the dry mass and the required propellant mass with margins.The margin philosophy is explained in subsection 5.1.The mass budget that shows the masses of each spacecraft and the total system mass is presented in Table 8.The SWO and MFO will operate in seven different main state modes presented in Figure 7.The different state modes are designed for different phases of the mission.At the beginning of the mission, during launch and part of the transit, the system will stay in Safe Mode.This is a low power mode where as many subsystems as possible are turned off, and special safety measures are taken to ensure they will not turn on unexpectedly in any critical phase at the start of the mission.In addition, unintended separation of the spacecraft from each other should be strictly prevented.
From Safe Mode the system will proceed to Commissioning Mode, where e.g.solar panels are deployed in order to start power generation and health checks are performed on the instruments.Sun Safe Mode is entered after commissioning for the duration of the transit.It ensures that the system generates power, but payloads stay powered down or in a low power mode.Orbital Control Mode is entered as the spacecraft arrives at Mars.This mode enables orbital maneuvering utilizing the thrusters of the spacecraft.The mode is critical for reaching the desired orbits of the spacecraft, and performing small corrective maneuvers later on during the mission.
When the required orbits are reached, the spacecraft can proceed to start the science phase of the mission by operating in Science Mode.In this mode the spacecraft are designed to operate all of their instruments in order to collect data.The time spent in Science mode should naturally be maximized.At specific events during the mission, e.g.boundary crossings, the so-called Burst Mode can be initiated to enable short periods of increased data acquisition rates for the instruments.
For transmitting the generated data, each spacecraft can enter Downlink Mode.For the MFOs this enables data transmission to the SWO.Furthermore, the SWO is able to downlink the self-generated data and the data received from the MFOs to the ground station on Earth.Receiving is activated in most state modes to enable commands to be sent to the spacecraft.The only exceptions are Safe Mode and Sun Safe Mode during transit, where only the SWO is receiving, as the spacecraft are still attached together.
In the following sections, Safe Mode and Sun Safe Mode can together be referred to as "safe modes", whereas "nominal modes" refer to all other operating modes.

Power Budget
The power budget of the spacecraft has been designed by assuming end-of-life conditions for different parts of the power system.This means that e.g. the degradation of solar cells and batteries over the mission lifetime has been accounted for when sizing the system.The total power consumption of the SWO in nominal state modes at the Red Planet will range from a maximum of 440 W (Downlink Mode) to 240 W (other nominal modes).The power generated by the SWO's solar panels in the Sun will be 400 W at Mars.In contrast, the total power consumption of the MFO will vary between 250 W (Downlink Mode) and 150 W (other nominal modes).The power generated in the Sun by an MFO at Mars will be 250 W.
All nominal state modes of a spacecraft, except Downlink Mode, consume the same amount of power.This results from sufficient heat dissipation being the restricting factor that determines the lower limit for power consumption.The reason for the higher power consumption of Downlink Mode is that in addition to the heat required to maintain the thermal balance of the satellite, some power is also radiated away from the satellite in transmission.Furthermore, for the SWO, Downlink Mode is considered in two separate submodes: transmitting to Earth, or transmitting to the MFOs.When transmitting to the MFOs, the SWO can use its payloads without compromising the thermal or power budget.
In the safe modes, Safe Mode and Sun Safe Mode, the power consumption can potentially be lower than in nominal state modes.For example, during transit in Sun Safe Mode, the spacecraft are closer to the Sun than they are at Mars, and the required heating power produced by the spacecraft is lower.Additionally, if the power balance of a spacecraft would become compromised during nominal operations at Mars, the Sun Safe Mode can be initiated in order to save power while waiting for the batteries to recharge.The power consumption of different state modes is illustrated in Figure 9.
In the safe modes, the main factor limiting how low the power consumption can be decreased is the requirement to maintain the thermal balance of the spacecraft on a level that does not harm the spacecraft or their subsystems.The required  power can be minimized, if the most temperature sensitive components are placed close to each other, and they are thermally well isolated from the environment.However, the tentative thermal modelling of the spacecraft does not enable detailed estimations of the power consumption in the safe modes during different mission phases.The detailed analysis of the power consumption in the safe modes will be performed in later mission design phases.
The eclipse time of the SWO and the MFOs will cover approximately 10 % of their orbital periods.The designed solar array power generation capacity is sufficient to charge the batteries of both types of spacecraft between eclipses while staying in nominal operation modes.Without accounting for Downlink Mode, power is produced with a margin of approximately 50 % compared to the other nominal state modes.Accounting for the higher power consumption of Downlink Mode reduces the margin signifcantly, but battery capacity is sized to enable the downlink sessions required during the mission (see subsubsection 5.6.6).The batteries used for the SWO and each MFO are 3000 Wh and 1500 Wh silver-cadmium batteries respectively.If, for any reason, the power balance of any of the spacecraft would become compromised, the Sun Safe Mode can be initiated in order to save power while waiting for the batteries to recharge.

Thermal Budget
For thermal modelling of the spacecraft, a coarse overall spacecraft thermal mathematical model (TMM) was utilized.The tentative modelling shows that to stay inside the estimated nominal operating temperature range with margins (−20 °C to 60 °C), the SWO and each MFO require a continuous average heat dissipation of 240 W and 150 W respectively.As subsystem heat dissipation alone does not reach the required level, heaters are used to generate the required total heat.In addition, multi-layer insulation (MLI) is considered for thermal insulation of the spacecraft.No active cooling is required to maintain the spacecraft temperature according to this estimate, provided sufficient heat transfer within the spacecraft to even out internal thermal gradients.At later system design phases, a more sophisticated thermal control scheme could be devised to optimize the power consumption and thermal stability of the spacecraft.As of now, the feasibility of the thermal budget has been demonstrated by assuming simple constant thermal dissipation power.
As all power produced by the subsystems on-board the spacecraft (except power radiated from the antennas in Down- .link Mode) is assumed to be dissipated as heat in the spacecraft, the total heat dissipation budgets are equal to the power budgets in each operating mode (except Downlink Mode).In Downlink Mode, the heat dissipation of the SWO is 200 W lower than the power consumption.Similarly, the heat dissipation of a MFO is 100 W lower than its power consumption in Downlink Mode.

Telemetry Budget & Telecommand
In addition to performing scientific measurements, the SWO serves as a communication relay between the MFO formation and the ground segment on Earth.For this purpose, the SWO carries a high gain dish antenna (HGA) with a diameter of 2.5 m.The X-band is chosen for the data link between Earth and Mars, similarly as has been done for instance on the Mars Reconnaissance Orbiter (Graf et al., 2005a).The strict pointing requirement of the HGA (< 0.3°) is achieved by pointing the antenna semi-independently from the spacecraft body.To enable communications between the SWO and the MFOs, each of the five spacecraft carries a low gain dipole antenna (LGA) that poses no strict pointing requirements.Communication be-tween the MFOs and the SWO will use the S-band frequency range, which was shown by link calculations to be suitable for the intersatellite link.
The link budget of the mission is heavily dependent on the mutual distances between the spacecraft, as well as the distance of the SWO from Earth.The simulated best and worst case distances, as well as the average distance over time, are presented in Table 9.The corresponding link budgets are detailed in Table 10.The significant variance in downlink rates is attributed to differences in free-space path loss (FSPL) that depends on the distance between the transmitter and receiver.FSPL grows rapidly as distance d between the transmitter and the receiver increases (FSPL ∝ d 2 ), and leads to signal attenuation.

Min.
Max. Mean SWO/Earth 5.7 × 10 7 km 3.2 × 10 8 km 1.5 × 10 8 km MFO/SWO 1.2 × 10 3 km 3.7 × 10 4 km 2.0 × 10 4 km A majority of the proposed scientific heritage instruments (see section 4) enforce lossless compression on their measurement data or stream continuously low resolution data while storing high resolution data to be transmitted only on demand.The maximum estimated total data volume produced by the instruments is presented in Table 11.The data rate estimations are designed to account for both nominal Science Mode operations and higher data rate Burst Mode measurements.A significant margin of 50 % has been added to the tentative estimations that are based on data rates specified for the proposed heritage instruments.
Table 12 shows estimated downlink times for the amount of data produced during an average 24 h period of mission operations.The downlink times are estimated between the different spacecraft, as well as between the SWO and the ground station network.The SWO achieves downlink times of 3.4 h even in the worst case scenario, corresponding to a total of 15 % of operation time on average.This enables downlinking all data produced by the SWO and the MFOs to Earth with good margin during the whole mission duration, independent from the mutual distance of Earth and Mars.
The MFOs, in contrast, require optimized downlink schedules to be able to transmit all science data to the SWO, as the worst case and mean downlink rates are too slow for efficient data transfer, but the best case downlink rate is excellent.The downlink sessions should be scheduled to take place when the distance between the MFOs and the SWO is close to minimum to ensure the downlink time is minimized.As the orbital periods of the SWO and the MFOs are 18.6 h and 12.8 h respec-tively, the spacecraft will undergo a sufficiently close encounter roughly every 38 h.The amount of on-board data storage is sufficient to store the data produced over significantly longer periods of time than the time between adjacent downlink time slots (see section subsubsection 5.6.7).Thus, not all downlink opportunities have to be utilized.Downlink opportunities can occasionally be skipped, e.g. if the opportunities happen to occur during particularly interesting measurement possibilities, such as magnetotail border crossings or exceptional solar wind conditions.
The uplink times from the SWO to the MFOs or from Earth to the SWO will be short, since the transmitted data volumes are minor, as only short commands need to be transmitted in these directions.In addition, the uplink data rate from Earth is relatively high during the whole mission lifetime.The radiation hardened RAD-750 onboard computer (OBC) proposed for the mission has heritage from several missions such as the Mars Reconnaissance Orbiter (Graf et al., 2005b) as well as the Curiosity (Welch et al., 2013) and Perseverance (Abcouwer et al., 2021) rovers.As the SWO poses a major single point failure risk for the mission, the spacecraft is equipped with two redundant OBCs.The four MFOs are each equipped with a single RAD-750 OBC.
The onboard data storage required on each MFO is estimated to be 10 GB, whereas the SWO will carry 26 GB of memory.The combined total data storage is designed to be sufficient for storing the total data produced by all spacecraft over an average 60 day period.This is possible, as an MFO can store the data produced by itself over 30 days, whereas the SWO can store the data produced by each MFO over 30 days, as well as the data produced by itself over 60 days.The amount of data storage contains significant margin to enable flexible downlink scheduling, especially from the MFOs to the SWO (see subsubsection 5.6.6).

Attitude Determination & Control
For attitude determination, each spacecraft will use two star trackers.The SWO carries four reaction wheels for standard attitude and pointing control and a total of twelve thrusters: one main thruster for orbital insertions and major orbital maneuvers accompanied by eleven smaller thrusters for attitude control and minor orbital maneuvers.Each of the spin stabilized MFOs will also carry twelve thrusters in a similar configuration.
The high gain antenna of the SWO requires a pointing to Earth with < 0.3°error for downlink mode.The HGA can be pointed semi-independently from the rest of the SWO spacecraft body.The low gain dipole antennas of all the spacecraft are required to maintain an alignment with the normal of the orbital plane with < 30°of error in order to obtain a data link between the SWO and the MFOs.
During science mode operations, the solar wind observing instruments of the SWO require a pointing accuracy of < 10°t owards the incoming solar wind.The MFOs are required to spin in orbit in order to extend their wire booms.The measurements do not impose any pointing requirements on the MFOs.

Electromagnetic Interference Considerations
As accurate and high resolution measurements of the Martian magnetosphere are key to the scientific goals of the mission, strict magnetic cleanliness of the spacecraft will be necessary to prevent unwanted interference from impacting measurements.
To limit the influence of spacecraft-induced magnetic fields on the measurements, all fluxgate magnetometers are placed on 5 m long booms.Additionally, each spacecraft has two magnetometers on the same boom to allow for noise mitigation.The primary scientific magnetometer is placed on the tip of the boom, whereas the second one, closer to the spacecraft body, acts as an auxiliary magnetometer that assists in identifying and removing potential magnetic interference by the spacecraft from the data.This approach has previously been employed e.g. on the Cluster mission (Balogh et al., 1997).
Electromagnetic interference must be considered also from a communications perspective to ensure the spacecraft are not producing interference on their communication frequencies in the S-and X-bands.

End-of-life & Planetary Protection
ESA missions are required to abide by planetary protection standards.M 5 would be classed as a Category III mission by the relevant planetary protection standard (European Cooperation for Space Standardization, 2019).Therefore, this mission will inventorise and retain samples of organic materials used in the spacecraft, comply with bioburden requirements, and assemble the spacecraft in a cleanroom of ISO class 8 or above.

Cost Estimate & Descoping Options
We expect M 5 to be classified as an L-class mission according to the Cosmic Vision strategy of ESA.We have not made detailed cost estimates, but we expect that meeting the cost limit of MEUR 1000 will be challenging.One area for cost reduction, which is not required but may be desirable, is the possibility of collaborating with international partners.
Given the significant cost of the mission, descoping options are possible at the cost of reducing the scientific objectives.From the MFOs, one or more spacecraft could be descoped to lower mass and cost.However, this would significantly hinder the fulfillment of the science objectives, as a 4 spacecraft formation is needed to achieve most science objectives, namely O1.1, O1.2, O1.3, O2.2 (see Table 2).A reduction to 3 spacecraft would reduce the 3D picture to a 2D picture, meaning that boundary orientation and movement could no longer be separated.In addition, the curlometer and wave telescope techniques would only give good scientific return in a limited number of cases.A further reduction to 2 spacecraft would make answering of the science questions even more challenging, reducing the data to a 1D picture.
As given by the traceability of the instrument requirements in Table 2, there are possibilities to descope instruments.In particular, the absence of electric antennas on the MFOs would result in a limited loss of scientific objectives.

Mission Readiness & Risk Analysis
All mission components have Technology Readiness Level (TRL) ≥ 6, so there are no significant technological risks to the mission.Some significant operational risks have been identified for the mission.One risk would be if either the communication with the SWO or with one (or more) of the MFO would be lost (resulting in the loss of some science objectives).In the case of losing the SWO, it may be possible to use MRO as a relay instead.Another risk would be a failed launch, as well as an error in the orbit insertion, both of which could result in a total loss of the mission.An error in the alignment of the MFO tetrahedron is also be a possible risk.The solar panels or the electric antennas not deploying would cause major difficulties for the mission.

Outreach
Outreach is a key aspect for scientific space missions.As a scientific community there is a responsibility to inform taxpayers about how their money is being spent on research.Furthermore, outreach is a key driver for inspiring and encouraging young people to consider careers in STEM.M 5 would be accompanied by a varied and ambitious outreach program, consisting of social media accounts, online and in-person events throughout ESA member states, and open-source educational materials for use in schools.One example would be an artsand-crafts activity where children can make their own tetrahedron.

Conclusion
Through detailed preliminary analysis, we show the feasibility of a multi-spacecraft mission to Mars, aiming to extend and complement our understanding of the Martian induced magnetosphere.This understanding will further extend our comprehension of induced magnetospheric systems generally, and of their interaction with the solar wind.Atmospheres are important for the presence of life, and the escape of the Martian one will be better understood by the quantitative characterization of the magnetotail and of the processes taking place there.In order to study these regions and phenomena on different scales, and in order to separate spatial and temporal variations without having to use imperfect a priori information, a threedimensional picture of the bow shock, magnetic pile-up boundary as well as the magnetotail are achieved thanks to a four spacecraft configuration.The remaining spacecraft will complement the fleet of solar wind observatories in our solar system, crucial in order to provide better data for space weather applications.We show the feasibility of these objectives through detailed analyses of the orbital dynamics, formation requirements, and budget constraints such as mass, power and communication.We give an overview of spacecraft design incorporating all critical systems, and show the availability of heritage instruments sufficient to achieve the desired science objectives.The presented ambitious but feasible mission concept shows that a comprehensive study of the Martian magnetospheric system is possible, which is imperative for future human exploration of Mars.We show that M 5 would greatly advance our understanding of atmospheric escape, and give a crucial reference point for comparative studies of other solar system and exoplanetary induced magnetospheres.

Fig. 1 :
Fig. 1: Overview of the Martian induced magnetosphere.The Interplanetary Magnetic Field (IMF) is draped around the planet, forming boundary regions and a highly dynamical magnetotail that is yet to be studied in detail.The numbers indicate the different plasma zones addressed in the text.1. Solar wind, 2. IMF, 3. Sub-solar point of the bow shock, 4. Sub-solar point of the magnetic pile-up boundary, 5. Ionosphere, 6. Crustal field, 7. Lobes of the magnetotail, 8. Plasma sheet of the magnetotail.
a) The Solar Wind Observatory.(b) A Magnetospheric Formation Orbiter.

Fig. 2 :
Fig. 2: Three-dimensional rendering of the two spacecraft types forming the M 5 mission.
. For Instructional Use Only.

Fig. 4 :
Fig.4: Mission trajectory close to Mars.The approach trajectory of the five spacecraft is shown in green.At the end of the approach trajectory, the four MFOs separate from the SWO and each other.The MFOs raise their apoapsis to obtain their final orbit shown in red.The SWO, in contrast, raises its apoapsis twice: first to reach the transfer orbit in blue, and then to obtain its final circular orbit in orange.The figure is a screen capture from the STK simulation software.

Fig. 5 :
Fig.5: Final orbit configuration of SWO and MFO; the orbit of the MFOs will apparently move relative to Martian reference frame, moreover the orbit precesses due to the oblateness of the planet (J 2 ) as described in subsection 5.4 .

Fig. 6 :
Fig. 6: Orbits propagated for 100 days.The orbit of the SWO is shown in orange, and the orbit of the MFOs in red.J 2 perturbations will move the RAAN of the MFOs' orbit over time at a constant rate of 0.22 • per day.The figure is a screen capture from the STK simulation software.

Fig. 7 :
Fig. 7: State Mode Diagram.Arrows depict the possible transitions between different modes.In general, any state mode is accessible directly from any other state mode.The exceptions are Safe Mode and Commissioning Mode, which are not used after they have been completed at the early phases of the mission.Sun Safe Mode acts as the contingency mode after launch.

Fig. 8 :
Fig. 8: Expanded view of the Solar Wind Observatory and all major subsystems.

Fig. 9 :
Fig.9: Power consumption in different state modes of the SWO and an MFO.Note the different scale of the vertical axis for Downlink Mode.In addition, note that in Safe Mode and Sun Safe Mode, the total power consumption may be lower than the total shown in the figure.The uncertain part is illustrated with a lighter box surrounded by a dashed line.The power budget is presented in detail in subsubsection 5.6.4

Table 2 :
Scientific objective addressed by each instrument used by the M 5 mission.A big dot stands for the Solar Wind Orbiter (SWO) and a small dot • for an Magnetospheric Formation Orbiter (MFO).

Table 3 :
Magnetic field measurement requirements

Table 4 :
Ion moments measurement requirements

Table 5 :
Electron moments measurement requirements

Table 8 :
Final mass budget

Table 9 :
Mutual distances during the mission.

Table 10 :
Link budget as achievable downlink/uplink data rates.

Table 11 :
Maximum combined instrument data rate averaged over an orbit.

Table 12 :
Downlink times over an average 24 h period.