Juice spacecraft forming wake in solar wind

Juice was launched on 14 April 2023, and has spent the last year making one full orbit around the Sun. During this time it has been immersed in an ever-changing environment of particles, photons and magnetic fields, spewed from our Sun.

“This environment is interacting with the spacecraft and charging up the surface of the spacecraft, altering Juice’s local environment, which is potentially a big problem for the instruments focused on measuring the spacecraft’s natural surroundings,” explains study author Mika Holmberg of the School of Cosmic Physics at Dunsink Observatory in Ireland, part of the Dublin Institute for Advanced Studies.

“The main science objectives of Juice is to study Jupiter and its space environment, with a special focus on the habitability of Jupiter’s moons Europa, Ganymede and Callisto, which are all believed to have vast saltwater oceans under their thick ice surfaces. But in order to study the habitability of these moons we need accurate and detailed measurements. Accordingly, any perturbations need to be fully characterised and corrected for.”

The lorry-sized Juice bristles with a total of ten cameras, probes and antennas. Its two most sensitive instruments to surface charging are its Radio and Plasma Wave Investigation, RPWI, package and its Particle Environment Package, PEP, which will sample the plasma environment surrounding Jupiter, whose powerful magnetic field is surpassed in size only by that of the Sun’s own equivalent field.

Guided by initial solar wind measurements acquired by Juice three months after launch, the study team employed ESA-funded software called the Spacecraft Plasma Interaction Software, SPIS, to model the consequences for Juice.

Mika – who was previously an ESA Research Fellow at the Agency’s Space Environment and Effects section – explains: “SPIS is typically used during the design phase of ESA missions, to simulate the risk of damaging electrostatic buildup – just like terrestrial static buildups on metal surfaces during very dry conditions – which can result in damaging discharges. But the software can also be employed to study the impact on spacecraft measurements of the interaction between a spacecraft and its environment.’

The study found that the main spacecraft body and solar panels experiences charging up to around six volts, with its High Gain Antenna (which is covered in a different surface material and is acting as a Sun shield while JUICE is in the inner Solar System) could reach a potential of eight volts, while Juice’s always-in-shadow radiators, employed to dump waste heat, can reach up to negative 36 volts.

This surface charging distorts incoming particle trajectories and alters the energy of surrounding particles, especially in the case of low-energy particles. The particle environment around the spacecraft is altered, and an ion wake is formed behind Juice, reaching more than 65 m behind it. The characteristics of this wake need to be detailed so it will not be mistaken for a natural plasma structure when the particle instruments are performing their measurements within it.

The asymmetric shape of the spacecraft, which also incorporates materials with differing material properties, plus the ion wake, will also give rise to an asymmetric surrounding environment, which will need to be taken account of when it comes to electrical field measurements, which generally assume symmetry in these effects.

Finally, this interaction is causing the spacecraft itself to emit particles. For Juice, this is creating a dense cloud of electrons around the spacecraft, with a density of more than 130 times that of the local solar wind. These electrons originating from the spacecraft will also be detected by the electron analysers aboard Juice and will need to be separated from solar wind particle observations.

Mika adds: “The results from this study will help the Juice particle and field observation teams to interpret their measurements and start developing correction techniques for the perturbations caused by the spacecraft-environment interaction. This will be essential when it comes to maximising the scientific outcome of the mission, assessing the habitability of Jupiter’s moons.”