Apophis close encounter : surface charging predictions in geostationary environment

Introduction: On 13th April 2029, S-type asteroid 99942 Apophis will approach Earth to a close enough distance to cross the Earth outer electron belt for a short period of time. The ESA RAMSES mission (Rapid Apophis Mission for Space Safety) [1] will perform a one of a kind rendez-vous with the irregular object of typically 300 to 400m in diameter. RAMSES mission scenario includes the deployment of two CubeSats that will aim to characterise the asteroid surface, internal structure and close environment, embarking a dust analyser, radar and plasma instrumentation.In this context, a unique opportunity arise to confront our current understanding of airless bodies regolith and dusty surfaces interactions with space plasmas. Effectively, the silicate based regolith surface will transition from a (comparatively) high velocity low temperature Solar Wind plasma (typically 400km/s bulk velocity, 1-10 particles / cm-3 and 10eV at 1AU) to a sequence of varying magnetospheric conditions when crossing the magnetosheath to reach tenuous hot geostationary (GEO) electrons and ions composed typically of several components including accelerated ions and electrons with several keVs to several 10s or keV in energy.While the GEO environment is typically considered as a threat to spacecraft, as responsible for strong negative charging of spacecraft surfaces downto several 10skeV negative with respect to the neutral plasma, it is not the case for a natural object such as Apophis. However, a number of phenomena are worth investigating :What charging level / surface potential(s) would Apophis develop at closest approach ? What consequences such potentials might have on surface dust properties during observations and especially due to potential upper layer mobilization ? What near object (dusty) plasma environment might result from the interaction of Apophis surface with the local plasma ? What is the impact of the local plasma on the near object dust population to be probed by RAMSES ? What level of differential charging between RAMSES and Apophis surface can be expected ? What impact could the differential charging between RAMSES and Apophis surface at closest approach have on target measurements such as plasma populations and the local asteroid dust population and the related scientific return ? To cover those questions, numerical simulations of Apophis and simplified RAMSES Cubesats surface charging and near surface plasma properties and dust transport in typical and extreme geostationary conditions will be presented and analysed.  References:[1] https://www.esa.int/Space_Safety/Planetary_Defence/Introducing_Ramses_ESA_s_mission_to_asteroid_Apophis