PyRTX: a software package for non-gravitational accelerations on deep space probes

The success of deep space missions, particularly those with scientific objectives involving the measurement of classical and relativistic gravity, relies on accurately determining the spacecraft's orbit. Every space platform experiences non-gravitational accelerations, such as atmospheric drag and radiation pressure. The inherent difficulty in precisely modeling these perturbations can lead to significant errors in orbit determination, introducing biases in scientific measurements and potentially leading to incorrect conclusions. Accurately representing these effects is crucial for reconstructing spacecraft orbits and ensuring robust scientific results. This is especially important given that state-of-the-art spacecraft tracking systems have achieved unprecedented precision. The quality of observable quantities must be matched by new, high-fidelity physical-numerical models of spacecraft dynamics. The classical plate model used in orbit determination software tools is inadequate for accounting for secondary effects such as self-shadowing and multiple reflections of incoming radiation, which can significantly influence the overall acceleration. Improved numerical models would greatly benefit a wide range of ongoing and future missions, especially ESA-JAXA’s BepiColombo to Mercury and NASA’s VERITAS to Venus. Due to their proximity to the Sun, these spacecraft experience strong accelerations from radiation pressure (both solar and planetary). For VERITAS, whose nearly circular orbit has an altitude as low as 180 km, atmospheric drag from the upper layers of Venus' atmosphere is also significant. This work introduces a novel Python software package, named pyRTX, which utilizes ray-tracing techniques for high-fidelity computations of non-gravitational accelerations. Following an overview of the code, we discuss a validation case in a real mission scenario. We demonstrate that the new ray-tracing-based code is a valuable tool for planetary geodesy and geophysics investigations, offering enhanced modeling of spacecraft dynamics for precise orbit determination.