Spacecraft surface charging and outgassing during the JUICE lunar gravity assist

We use Spacecraft Plasma Interaction Software (SPIS) simulations to study surface charging of and outgassing from the Jupiter Icy Moons Explorer (JUICE) spacecraft during the JUICE lunar gravity assist (LGA). The results of this study will be of great importance for future observations of the plasma environments of Jupiter and its icy moons.On August 19, 2024, JUICE performed its first gravity assist maneuver at the Moon, which was located in Earth’s south magnetotail lobe region. During LGA, the two ARTEMIS spacecraft, P1 and P2, were orbiting the Moon. Despite being separated by approximately 14,000 km, the ARTEMIS P1 and P2 observations of the magnetic field and plasma parameters (density, velocity, and temperature) are in excellent agreement. As JUICE was only 4,000 to 7,000 km from ARTEMIS P2, these observations are considered representative of the space environment encountered by JUICE during the LGA. The ARTEMIS P1 and P2 observations are therefore used as input for the surface charging and outgassing simulations.SPIS is used to simulate the interaction between the JUICE spacecraft and its environment during the LGA (excluding the lunar wake crossing). Our simulation results show that the spacecraft bus reaches a potential of approximately 11 V, while non-conductive surfaces, such as the radiators and the high-gain antenna, reach potentials ranging from - 2 to 14 V. These surface potentials affect both cold plasma and electric field measurements. Due to the tenuous plasma environment in the magnetotail lobe and the spacecraft being located at 1 AU, hence relatively close to the Sun, the surface charging is predominantly driven by the emission of photoelectrons from the spacecraft. During LGA, shortly after JUICE crossed the lunar terminator, a suspected outgassing event occurred. Several independent JUICE observations are consistent with an outgassing event, including the detection of water molecules moving away from the spacecraft, a sudden increase in the local plasma density accompanied by a decrease in the spacecraft potential, and an unforeseen excess torque acting on the spacecraft. SPIS is also used to simulate the suspected outgassing event and to assess its impact on the spacecraft charging and the JUICE particle and field measurements.