Surface charging and outgassing during the Juice lunar gravity assist

We use Spacecraft Plasma Interaction Software (SPIS) simulations to study the interaction between the Juice spacecraft and its environment during the recent lunar gravity assist (LGA). We also use SPIS to study a suspected outgassing event that occurred shortly after the spacecraft crossed the lunar terminator.

On August 19, 2024, Juice performed its first gravity assist as it passed the Moon.  During the LGA the Moon was situated in the lobe region of Earth’s magnetotail. Present in the near vicinity of the Moon were also the two Artemis spacecraft P1 and P2. Despite a distance of around 14,000 km between the two spacecraft, the Artemis P1 and P2 observations of the magnetic field, and plasma density, velocity, and temperature are in excellent agreement. Given that Juice was located in between 4,000 and 7,000 km from Artemis P2, it is reasonable to consider the Artemis observations as a reliable representation of the space environment encountered by Juice during the LGA. 

The Artemis observations are used as input to simulate the interaction between the Juice spacecraft and its environment during the LGA, excluding the period when the spacecraft crossed the lunar wake. 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. Due to the tenuous plasma environment in the magnetotail lobe and the spacecraft’s close proximity to the Sun, the surface charging is predominantly driven by the emission of photoelectrons from the spacecraft. We also simulate the suspected outgassing event observed shortly after the lunar terminator crossing, and discuss its impact on the particle and field measurements obtained by Juice.