Use of low-energy electron measurements to determine charging on lunar gateway

The electron electrostatic analyzer (EEA) is part of the heliophysics environmental and radiation measurement experiment suite (HERMES) on NASA's Gateway designed to measure electrons in the energy range from 1 eV to 18 keV. Unlike most science missions in which the spacecraft is designed around the needs of the science instruments, Gateway's primary purpose is to support human habitation in deep space. Thus, HERMES serves as a pathfinder to establish observational capabilities for science instruments, which opportunistically rideshare on crewed deep space platforms. Gateway platform effects such as spacecraft charging, photoelectron and secondary electron emission, and physical geometry are expected to impact the EEA low-energy electron measurements. We investigate how EEA measurements can provide useful information about the electrostatic potential on Gateway. We use the NASA/Air Force Spacecraft Charging Analyzer Program (Nascap-2k) to solve for the electrostatic potentials on Gateway under a variety of space environment conditions. At each condition, we simulate particles at the EEA instrument, track them backward to determine their origin, and create synthetic skymaps, which are collections of flux measurements as a function of elevation angle, azimuth angle, and energy. Finally, these skymaps are analyzed to estimate the electrostatic potential on each module of Gateway.