Understanding collisionless shocks is vital to the understanding of our Universe, from the heating and deflection of bulk flows to the acceleration of cosmic rays. Moreover, collisionless shocks directly influence our own terrestrial space environment, e.g., solar wind-magnetosphere interactions.

Vital questions regarding collisionless shocks remain unanswered:

1. What is the partition of energy across collisionless shocks?

Shocks are spatial discontinuities that form when a supersonic flow encounters an obstacle. If the medium travels faster than the speed of communication, the medium has no time to smoothly adjust its trajectory. A shock forms ahead of the obstacle and slows the supersonic flow to subsonic speeds in order for the medium to travel past. In high density media, the shock structure and evolution are governed by particle collisions. In space, however, the majority of shocks are collisionless, making the dominant mechanisms of energy conversion difficult to define.

The most relevant collisionless shock to humans, and the one most often measured in situ, is the terrestrial bow shock. The terrestrial bow shock is also significantly more straightforward to observe relative to Interplanetary (IP) shocks in the solar wind, as it remains in the same spatial position relative to Earth (to within a few Earth rad ii). Therefore, we derive the majority of our knowledge of collisionless shock dynamics from the terrestrial bow shock.

The solar wind inputs primarily bulk proton ram energy upstream of the bow shock. However, the shock outputs energy in several different forms. These include, but are not limited to, electron, proton and heavy ion acceleration and heating, together with Poynting flux and turbulent fluctuations. Previous missions together with numerical simulations have provided invaluable insight to the overall structure and behavior of the terrestrial bow shock [e.g. (Burgess, 2015)]. Past missions that have observed the terrestrial bow shock include MMS, THEMIS, Cluster, Wind, AMPTE, and ISEE. They have confirmed that the shock can exist as a nonstationary discontinuity. It can act as a “breathing barrier” between the solar wind and the terrestrial magnetosphere, changing in response to varying upstream conditions.The spatial scale, energy conversion processes, and output of the shock are most heavily dependent on the orientation of the Interplanetary Magnetic Field (IMF) relative to the shock normal vector ( n ̂ ) and the fast magnetosonic Mach number (M _f ). Shocks are generally categorized as either Quasi-perpendicular (Q _⊥) or Quasi-parallel (Q _‖) depending on whether the angle between the IMF and shock normal (θ _Bn ) is greater than or less than 45°. The terrestrial bow shock also tends to grow more turbulent in nature as the Mach number (M _f ) increases and as the plasma β decreases.

Figure 1 summarizes the complexity of the global structure of the terrestrial bow shock. At Q _⊥ shocks (i.e., toward the top of the figure), particle motion in the shock-normal direction is restricted by the Lorentz force to within one gyroradius in the upstream direction. Thus, Q _⊥ shocks tend to have short coherent transition regions, with quasi-static magnetic and electric fields making the largest contributions to the bulk particle dynamics.

Q _‖ shocks (bottom of Figure 1) permit particle traversals in both directions across the shock, including well into the upstream region. Such shocks exhibit an extended transition region and are dominated by strongly varying particle sub-populations, particle reflection with corresponding kinetic instabilities and turbulence, and particle acceleration. They can also be populated with foreshock transient events such as hot flow anomalies (Schwartz et al., 2018) and foreshock bubbles (Turner et al., 2013), which can locally generate their own shocks and foreshocks (Wilson et al., 2013; Wilson et al., 2016) in poorly understood ways.

While it is known that collisionless shocks perform energy conversion, specifically to process the bulk flow kinetic energy density (Wilson et al., 2014a; Wilson et al., 2014b; Chen et al., 2018; Goodrich et al., 2018), the details of this energy conversion and output remain unclear. The kinetic-scale processes that perform this energy conversion are not well known or well observed within the terrestrial bow shock. Moreover, it is not clear what the resulting energy budget is once the plasma traverses the shock or how it varies for different shock conditions. In this section, we describe the scientific motivation of MAKOS science questions and the measurements necessary to address them.

Figure 4 shows an abridged version of the MAKOS Science Traceability Matrix with the mapping to the science questions listed above. The MAKOS mission design, instrumentation suite, cost/risk analysis, and enhancing technological developments are summarized in “Investigation Description”.

The full knowledge of shock micro-processes will more firmly establish our knowledge of fundamental plasma processes. This will further enable collaboration with the laboratory plasma community, as they develop and experiment with similar scale and mechanisms. This will also enable greater collaboration with the astrophysical community, as they observe astrophysical shocks via remote sensing. The observed radiation from these shocks stem from the post-energy conversion process. By acquiring an accurate knowledge of energy partitioning resulting from collisionless shocks, we will establish clearer connections to the processes and implications in shocks beyond our in-situ capabilities.

The baseline MAKOS mission concept comprises four spacecraft (S/C) with varying spatial separations at ion-kinetic to MHD scales in high-altitude, slightly elliptical (23.1 × 18.0 R_E) five-to-one (5:1) lunar resonance orbits (LROs) with oppositely oriented lines of apsides that maximize the number of bow shock crossings, even when apogee is on the nightside. Each of the two orbits has two S/C with separations on the order of ∼100—1,000 km to obtain the required simultaneous upstream, downstream, and transition layer observations at shocks, including multipoint observations at ion-kinetic scales through every shock transition layer crossing. The separations between the S/C on the different orbits range from ∼5 to 12 R_E. This provides year-round crossings of the bow shock with simultaneous multipoint separations ranging from ion kinetic (100—1,000 km; each pair) to MHD (several R_E; the pair of pairs) scales, as well as prolong ed dwell time throughout the year in the solar wind, enabling opportunities to also study interplanetary (IP) shocks and for MAKOS to simultaneously probe electron- and ion-kinetic plus MHD-scale processes during every single bow shock and IP shock crossing (>1,000 expected during MAKOS’ 2-year prime mission).

MAKOS requires each S/C to carry a comprehensive science payload of particles, fields, and waves instruments specifically tailored to measure the in-situ processes at play in collisionless shocks. The need to resolve microphysical phenomena in and around each shock and to fully characterize the plasma populations upstream and downstream of each shock drives a mission requirement that the complete three-dimensional thermal and suprathermal electron and ion velocity distributions be sampled at very high temporal resolution (<1 s). This is achieved in the notional mission design by carrying multiple dedicated sensors targeting each species and energy range on a rapidly-spinning (10 RPM baseline) S/C. Furthermore, the need to resolve the evolution of the solar wind ion beam necessitates a dedicated detector, that is, constantly pointed into the solar wind - i.e., along a S/C spin vector anti-aligned with the solar wind flow direction.

The MAKOS science payload will consist of a full particle and fields instrument suite totalling eight detectors. These instruments are illustrated on one of the MAKOS spacecraft in Figure 6. The resource demands of the MAKOS science payload are summarized in Table 1 and instrument details are listed below.

The MAKOS observatories comprise the MAKOS payload integrated with a spin-stabilized S/C using a single-string hardware architecture with functional and selective redundancy included for critical areas. The architectural approach achieves a mission success of >90% over its two-year mission lifetime as demonstrated by the NASA CYGNSS eight-observatory mission (Rose et al., 2013), which has operated for over 5 years without failure.

The simple operational nature of the MAKOS instruments and science profile allows significant autonomous on-board control of the observatory during all normal science and communication operations without need for daily on-board command sequences. Observatory initialization and science operations use five sub-modes: rate damping, nutation damping, Sun acquisition/precession control, spin-rate control, and science. After initial damping of launch vehicle separation rates is complete, the observatory transitions to Sun acquisition using Sun sensors and a sky-searching algorithm to locate the Sun vector. The observatory then uses reaction control thrusters to point the S/C solar arrays at the Sun using the rate and Sun sensors. The star trackers are initialized followed by spin-up of the S/C to its operational spin rate of 10 rpm. The vehicle spin axis then precesses to align with the local solar wind vector for science operations.

The MAKOS observatory design (Figure 7 and Table 2) is mission-specific to meet science requirements and instrument accommodations. Physical accommodation of the MAKOS instruments and spin stabilization implementation drives the observatory’s structure and thermal design. Fixed solar arrays, located on the Sun-oriented face of the observatory provide electrical power for the S/C. The LRO enables use of a simple direct energy transfer architecture for battery charging with the batteries sized to accommodate full science operations during solar eclipse periods. Primary attitude knowledge is star tracker-based augmented with rate sensors for stability and nutation determination.

Sun sensors are included for emergency operations. Observatory orientation, spin-rate and precession are all controlled using an on-board cold-gas SF6-based reaction control subsystem. Observatory positional knowledge is based on GPS receivers augmented by on-board optical navigation during GPS outages. Communication is provided by an X-band transponder and low-gain patch antennas to provide communications without interrupting science operations. On-board timing requirements are driven by science data synchronization within the constellation relative to measurement of the solar wind and electric field waveforms. Specific S/C performance characteristics are provided in Figure 2 and Table 2. Observatory magnetic and electrostatic cleanliness is key to the MAKOS instruments meeting science Level 1 requirements. MAKOS uses mature electromagnetic requirements consistent with previous missions (e.g., MMS, Cluster, THEMIS) to develop a magnetically and electrostatically clean observatory.

Expected science data generated is 53.5 GB/orbit. Baseline on-board data storage provides 188 GB or 3.5 orbits of science data storage to allow for recovery from downlink anomalies. The baseline reference communication uses a 14 Mbps X-band RF link that, with 14% overhead for CCSDS, requires ∼9.6 h to downlink science data from 1 orbit. Significantly improved data rates would be available to reduce downlink durations and/or increase data downlink quantities if the optical communications are realized prior to MAKOS implementation.

Telemetry is a major driver of the notional MAKOS mission design as the science requires very high data rates for observatory science telemetry at and around each collisionless shock crossing. Furthermore, MAKOS should also capture the highest rate data from any interplanetary shocks encountered upstream of the bow shock. Despite this, the MAKOS concept of operations (CONOPS) (Figure 8) is simple by design and consists of collecting science data (telemetry) from each of the four identical observatories during the two-year prime science mission. Each observatory will record telemetry in one of two science modes: 1) high-rate and 2) low-rate. Even under extreme solar wind driving conditions, the bow shock is consistently located outside of the average (i.e., typical) magnetopause location. Thus, the average magnetopause location offers an opportune surface to use for routine orbit-to-orbit operations and systematically toggling the MAKOS S/C between high- (i.e., along the orbit beyond the average magnetopause location) and low-rate (i.e., along the orbit within the average magnetopause location) modes. Using the average magnetopause location and the orbit predicts to schedule onboard science telemetry mode changes, each MAKOS observatory shall switch from low-to high-rate data collection when it transits from the magnetopause into the magnetosheath (i.e., outbound model magnetopause crossings), and each observatory will switch from high-to low-rate data collection when it transits from the magnetosheath into the magnetosphere (i.e., inbound magnetopause crossings).

The MAKOS payload generates data at either 807 kbps (low-rate) or 20.875 Mbps (high-rate) to achieve the temporal resolutions required for each observable. Acquiring high-rate data only when the S/C are sunward of the average magnetopause requires high-rate telemetry being recorded for ∼60 h (46%) of each 5.46-day orbit.

However, all 611 GB of science data per orbit cannot be transmitted to ground each orbit because of limitations of the communications subsystem and ground network. To ensure that all collisionless shock transits are captured during the prime mission, MAKOS will employ a “scientist-in-the-loop” (SITL) strategy similar to that used by MMS (Fuselier et al., 2016). A trained MAKOS science and data expert (i.e., SITL) will review a special low-rate data product produced onboard and telemetered to ground each orbit to make prioritized selections of which periods of the high- and low-rate data shall be telemetered to the ground. Shock crossings will be prioritized, and data from and around each shock crossing will be telemetered to the ground to ensure prime science closure. The expected SITL-selected high-rate data volume averages 5.9 GB (∼1% of recorded data) per S/C per orbit; combined with the low-rate data generated each orbit (47.6 GB per S/C), this yields 53.5 GB of data to be telemetered to ground from each MAKOS S/C each orbit (9% of 595 GB total recorded data). Over the two-year prime mission, all four MAKOS S/C will telemeter 28.6 TB of total 327 TB scientific data recorded.

The four-observatory configuration will require $651M (FY22) funding as a current best estimate. Recognizing that this is a preliminary concept study, conservative reserves are applied to all cost elements: 50% for all Phase B-D work and 25% for Phase E-F. This brings the baseline estimate to $964M (with NASA’s addition of a Phase A study and Launch Services to complete the funding). Additional development costs are not included, as baseline instruments and supporting hardware were chosen to be at TRL 6 prior to Phase A.

The MAKOS risk assessment combined with cost to identify key risks is likely to drive significant variances if not managed. A four-observatory constellation, each carrying eight instruments, is within the overall experience base of the institutional partners, but the need to deliver multiple flight units raises the criticality of certain common development issues. Figure 9 shows the top-level schedule with major milestones. Table 3 lists the top three identified risks and potential mitigations.