Jupiter Observing Velocity Experiment (JOVE): Introduction to Wind Rider Solar Electric Propulsion Demonstrator and Science Objectives

The Wind Rider uses superconducting rings approximately 9 m in diameter to create a large-scale plasma bubble, tens of kilometers in diameter, that can interact with the solar wind or a planetary magnetosphere, producing significant drag force. The coils generate a rotating magnetic field (RMF) that inflates the plasma structure, which consists of a plume and wake as diagrammed in Figure 1.

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The Wind Rider spacecraft concept is based on the Plasma Magnet (Slough & Giersch 2005; Slough 2006, 2007) as a means of intercepting a propulsively useful, large area of solar wind momentum with modest driving coils. The principle is that a pair of coils, perpendicular to each other and mutually perpendicular to the wind axis, are driven by alternating currents 90° out of phase with each other, generating a RMF. The frequency of rotation is chosen to be well below the electron gyrofrequency of the freestream solar wind plasma and its associated magnetic field; therefore, the electrons are pulled around by the RMF a circulating current loop, the axis of which is aligned with the wind. Initially, the magnetic pressure in that current loop, driven by the RMF, is greater than the sum of the dynamic pressure of the solar wind and the magnetic pressure of the interplanetary magnetic field (IMF), so the loop expands—entraining additional electrons from the solar wind plasma as it does so—until the magnetic pressure of the current loop just balances the external sources of pressure. The frequency of rotation of the RMF must be above the ion gyrofrequency within the current loop perimeter so that the ions do not set up a canceling current by moving counter to the electron flow. The electron current, then, appears as a quasi-static magnetic field to the approaching solar wind ions, which do not respond to the oscillatory RMF component above the ion gyrofrequency, and the electron current loop deflects the solar wind much as would a conventional magsail (Andrews & Zubrin 1990) but of very large radius. The effective radius of the current loops is the radius of the driving coils times the ratio between the magnetic field strength of the driving coils and the magnetic field strength of the expanded sail in equilibrium with external sources of pressure—and that ratio can be very large (many orders of magnitude). The resulting structure in practical terms has a radius larger than 10 km.

The force generated by the Wind Rider device is limited by the input power from the spacecraft to the coils (overcoming losses in the superconductors) and the diameter of the coils; packaging the coils becomes impractical as they get larger. Operationally, the Wind Rider is limited by the solar wind speed and direction. Drag devices only work in the "downstream" direction. In the case of our Jupiter mission, this means launch must wait until Jupiter is in opposition. Finally, there are unknowns in how steerable the device will be.

The Wind Rider drag device has a lengthy history of experimental development on the ground. After Slough and Kirtley's initial development for NASA including demonstrations in a "solar wind tunnel," there were several attempts to develop an in-space demo for Earth orbit. Slough (2006), Kirtley (2012) A tech demo mission named MIDAS by Altair Space Machines Altius Space Machines (2014) was followed by an effort to dispense a Magnetoshell Aerobraking CubeSat (MAC) from the ISS by UW Master's student Kelly. Kelly (2018) There are scale-down challenges when using this approach for a spacecraft as small as a 3U Cubesat and inside the Earth's magnetosphere, which has a magnetosphere co-rotational velocity of only 1.5 km s⁻¹. Bagenal (2013) Slough's ground experiments indicate that the Wind Rider device works at a larger scale. Slough (2006) Outside the Earth's magnetosphere, Wind Rider can operate on the solar wind which exceeds 340 km s⁻¹, the regime we chose for Jupiter Observing Velocity Experiment (JOVE). The JOVE is a new design to test Wind Rider in interplanetary space via a 16U microsatellite.

JOVE is a 16U microsatellite that will use Wind Rider for a test flight past Jupiter orbit taking only 30 days. This provides a benchmark for achieving a 5 au distance with no gravity assist; JOVE will reach a velocity near 300 km s⁻¹ under Wind Rider power alone. JOVE has enough battery power to test magnetic braking near Callisto, but not to stop or enter orbit; this is a one-way trip (flyby). An upper stage provides escape velocity at mission start, deploying JOVE into a Cis-Lunar initial orbit. A rendering of the deployment steps is shown in Figure 2, where the two high-temperature superconducting (HTS) coils stretch out on a boom, followed by the solar panel arrays and sunshade (visible in Figure 3).

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Once the sunshade and solar panels are deployed, JOVE spends about 24 hr calibrating instruments before activating the RMF inside the two coils. It then departs from the initial Cis-Lunar orbit via Wind Rider toward the Jovian system. The concept of operations summary for JOVE is as follows:

• 1.  
  Q4 2023 or Q4 2024 launch
• 2.  
  Accelerate for 24 days to 300 km s⁻¹
• 3.  
  Pass Jupiter at closest approach on day 25
• 4.  
  Decelerate for 1 hr inside Jovian magnetosphere
• 5.  
  Mission ends on day 30.

A simple telemetry beacon will be operated by battery for most of the cruise phase, which might last beyond 5.2 au. Secondary objectives for the tech demo are to demonstrate steering, up to half a degree below the ecliptic plane; and deceleration in Jupiter's dawn eddy (to be discussed later). Without steering, JOVE crosses over the north pole of Jupiter as soon as 19 days after departing Cis-Lunar orbit. As mentioned previously, a sunshade to protect the superconducting coils is necessary at 1 au; while it is no longer necessary for thermal management at Jupiter (5.2 au), due to the much lower solar flux, it also serves as a thin sheet antenna providing S-band radio back to Earth. Thus, the "sunshade" is retained onboard for the full 30 days mission duration.

The electron cyclotron frequency is set by the solar wind parameters, but to match it, an EMF coil's inductance must be tuned. The shape and placement of the HTS strips impact the coil's inductance, at a large EMF coil diameter scale, leading to the choice of a flat conductor (not round wires). The principal current flows through the circuit, so it must be tuned without a matching capacitor in parallel. This also determines the maximum coil size. Likewise, the capacitance is twice the self-capacitance, due to a power supply match. Impedance can be matched using transformers or capacitors in series. In practice, JOVE's coils will use a 4 Hz frequency, which is slightly higher than the ion cyclotron frequency.

The larger the coil radius, the more it is limited by inductance (to reach the resonant frequency with the surrounding plasma), and then torque (buckling or structural issues). As for scaling down, the smaller the coil radius, the more it is limited by input power to achieve an equivalent drag force. For this paper, a trajectory seed code (discussed in Section 1.3) was used to optimize the coil radius, based on the internal losses (due to hysteresis) equaling the plasma losses (due to resistance). Finding this null derivative in the curve also causes the coil to occupy the design regime between the lower and upper bounds. JOVE's coils are optimized for use in the solar wind at 1 au.

The amount of lift to drag ratio is unknown and, to be conservative, is assumed negligible in the Mathcad trajectory seed code used to generate feasible trajectories. If that turns out to not be the case, based on experimental data from the technology demonstration telemetry, then further steering control would be enabled by the reaction wheel assembly offsetting the CG further from the radial direction of flight from the Sun.

A trajectory seed code was written in Mathcad 14 R020. It is comprised of six modules, the main three of which are discussed in further detail here.

1.3.1. Solar Wind Model

1.3.2. Wind Rider Model

The following are the core equations governing the Wind Rider subroutine in the trajectory seed code. v_w is wind speed in heliocentric coordinates, which is a function of distance r from the Sun and angle ϕ above the ecliptic plane. u_r is the radial velocity of the spacecraft. R_rw is the radius of the plasma plume, typically 60–100 km. The radius of the coils, R_coil, is 4.5 m on JOVE. Z_w is the effective charge of the solar wind.

Dynamic pressure of the solar wind, DynP:

$$\begin{eqnarray}&&{\mathrm{DynP}}_{\mathrm{sw}}=\displaystyle \frac{1}{2}{m}_{i}{n}_{p}{\left({v}_{w}(r,\phi )-{u}_{r}\right)}^{2}\end{eqnarray} \tag{ 1 }$$

where m_i is the average ion mass and n_p is the solar wind particle density.

Magnetic pressure, MagP, of the interplanetary magnetic field, B_imf:

$$\begin{eqnarray}&&{\mathrm{MagP}}_{\mathrm{sw}}=\displaystyle \frac{{B}_{\mathrm{imf}}{\left(r\right)}^{2}}{2{\mu }_{0}}.\end{eqnarray} \tag{ 2 }$$

Thrust from solar wind drag-force on Wave Rider, F_wr:

$$\begin{eqnarray}&&{F}_{\mathrm{wr}}={\mathrm{DynP}}_{\mathrm{sw}}\pi {R}_{\mathrm{wr}}^{2}.\end{eqnarray} \tag{ 3 }$$

The magnetic field equivalent inside the Wind Rider, B_wr, is computed from the dynamic pressure plus the magnetic pressure from the free-stream interplanetary field:

$$\begin{eqnarray}&&{B}_{\mathrm{wr}}=\sqrt{2{\mu }_{0}({\mathrm{DynP}}_{\mathrm{sw}}+{\mathrm{MagP}}_{\mathrm{sw}})}.\end{eqnarray} \tag{ 4 }$$

Magnetic field B_coil at the center of the Rotating Magnetic Field coil:

$$\begin{eqnarray}&&{B}_{\mathrm{coil}}={B}_{\mathrm{wr}}\displaystyle \frac{{R}_{\mathrm{wr}}}{{R}_{\mathrm{coil}}}.\end{eqnarray} \tag{ 5 }$$

Current in circular coil:

$$\begin{eqnarray}&&{I}_{\mathrm{coil}}=\displaystyle \frac{{B}_{\mathrm{coil}}\cdot 2{R}_{\mathrm{coil}}}{{N}_{\mathrm{turns}}{\mu }_{0}}.\end{eqnarray} \tag{ 6 }$$

The RMF frequency f must exceed the ion gyrofrequency within the electron current loop. The engineering constraints of the superconducting coils favor a low operating frequency to minimize hysteresis losses within the superconductors. We have limited the minimum operating frequency to 5 times the ion gyrofrequency measured within the near-spacecraft magnetic field. This may be excessively conservative and in-space characterization of how low the driving frequency can go before reducing the effective drag of the sail would be a desirable result of this JOVE mission

$$\begin{eqnarray}&&f=\displaystyle \frac{{Z}_{w}{e}_{0}{B}_{\mathrm{wr}}\cdot 5}{2\pi {m}_{\mathrm{ion}}}.\end{eqnarray} \tag{ 7 }$$

These core equations are based on the published work of Slough (2006). The input parameters for each of these equations come from the local solar wind plasma environment. To calculate the solar wind pressure correctly (to estimate the local drag force on the Wind Rider coils), it is important to not assume that the solar wind consists of only protons, but to use the average atomic mass of the actual solar wind composition. The same approach is used for the effective charge or average ion atomic number. This charge can affect the drag force calculations farther from the Sun, in the gradual transition from the foreshock (at 83 au) to the onset of interstellar space near the heliosphere boundary at 123 au. Also, the ion temperature in the solar wind versus distance from the Sun can fluctuate across the foreshock.

Electrons can be up to twice the local ion temperature, under fluctuating conditions even closer to the Sun, too. In addition, if the trajectory enters a magnetosphere of another planet, then the local environmental plasma parameters change in the simulation to address those differences in density, velocity and composition.

1.3.3. Magnetosphere Model

With Wind Rider, launch opportunities to each planet occur annually. As JOVE's target is Jupiter the launch window is one month before Jupiter is at opposition: 2023 November 2 or 2024 December 7. (The solar maxima occurs in 2025 July.) JOVE can be deployed from an apogee of 60,000 to 90,000 km Sun-side or into Cis-Lunar orbit. The mission duration is 30 days total to achieve the primary objective, including an optional 5 days period at the end, to transmit images (or until the radio stops telemetry due to lack of solar power). At the end of the mission, JOVE will attempt to send a photofrom a witness camera showing Jupiter. An illustration of JOVE passing Jupiter is shown in Figure 3.

A brief summary of the Wind Rider specifications, bus capabilities, navigation hardware and scientific instrumentation is provided in Table 1. This list is not intended to be exhaustive, as this is an introductory paper, but representative of the quality of components, many of which have flown on previous deep space exploration missions. The specifications were compiled based on minimum requirements for JOVE, but also for follow-on missions that would likely need slightly better or more accurate instruments for their scientific objectives. In particular, the active cooling system, a Brayton cooler, is oversized for that reason and to provide extra margin during the technical demonstration flight of JOVE.

We have used the trajectory seed model in Mathcad, based on ephemeris tables that include all planets, many minor planets and examples of interesting asteroids for exploration. The dynamics model uses the Sun, Earth and destination gravity wells, in this case Jupiter. As previously noted in Section 1.3.1, the solar wind plasma parameters vary over radial distance from the Sun. The code has a Wind Rider optimizer, based on available power. The trip estimator requires maximum power, initial mass and final destination, which for this paper is Jupiter. The optimizer also locates the closest approach launch window within the next decade. Although this paper focuses on Jupiter as a destination for future orbiter concepts, similar conclusions can be drawn for other gas giants, such as Saturn. The results for JOVE's flight path are plotted in Figure 6. The Deep Space Probe (DSP) in this case is JOVE.

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Future work includes further optimizing the JOVE's trajectory in professional simulation software, such as the Astrodynamics Workbench. A sample screenshot using the seed code's output parameters as input is shown in Figure 7.

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The Mathcad trajectory seed code also sizes the coils. The RMF coil radius (for 1 au) optimization depends on input power, which for JOVE comes from a solar panel array. The 9 m radius chosen for JOVE is a practical minimum for use in the Solar Wind at 1 au. However, that may not be the optimum at Jupiter, if attempting to decelerate inside the Jovian magnetosphere.

Jupiter's magnetosphere provides a dense plasma, which makes deceleration with a Wind Rider practical. Figure 8 shows an equatorial cross-section of Jupiter's magnetosphere in which the dawn eddy is in the red region. JOVE will pass through with a close approach of about 10 Jupiter radii, as shown by the arrow, moving from left to right. Passing through the dawn eddy, with velocities over −600 km s⁻¹ (in the opposite direction) will provide ample kinetic energy for braking. Orbital insertion is not required for mission success; JOVE need only show that it is possible to decelerate this way.

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Note that the co-rotational maximum velocity of the magnetosphere occurs for 12 hr, once every 30 hr (Chane et al. 2013). It is unlikely that JOVE can time this interval well on a first attempt. The batteries are sized to provide one hour of deceleration, which under ideal conditions might slow the spacecraft velocity by 5%. The data retrieved by JOVE during this process is invaluable scientific data for future mission designs, though. A much higher-powered spacecraft could slow to enter Jupiter orbit. If that process were repeated, the dawn eddy may enable up to 480 km s⁻¹ velocity back toward the Sun, which opens up rapid access to the inner planets or a sample-return trip to Earth.

Ultimately, a larger radius set of coils may be more optimal for deceleration inside the Jovian replacedplasmapausemagnetosphere, but more input data (from the JOVE mission itself) is necessary to optimize braking maneuvers over a 12 hr "stop" to Jupiter orbital insertion. For reference, a 40 m radius is the current estimated manufacturing limit. For now, JOVE intends to fly with a single set of 9 m coils for all phases of its tech demo mission.