Modelling of an asteroid photoelectron sheath and implications for a sample return mission

Electrostatic forces from asteroidal photoelectron emission both redistribute regolith and modulate the surface particle size distribution. A 2D particle-in-cell code has been developed to investigate the possibility of direct measurement of the electrical environment near the surface of an asteroid during a sample return mission. The spacecraft is expected to reach equilibrium with the surface photoelectron layer in ≲1ms, which is rapid compared to the descent timescale, permitting the simulation equilibria to be assumed representative of the dynamic spacecraft environment. Typical signals on a set of spacecraft-mounted sampling electrodes during descent are presented. The distorting effect of the spacecraft significantly modifies the potential on any sensing electrodes by introducing an equipotential body; creating a shadow; and photoemitting itself. Optimal locations for mounting the electrodes to measure the asteroid's surface electric field are suggested, where the potential differences measured are expected to be both readily detectable and representative of the local electrostatic environment. The high electron concentration near the surface is likely to ensure that the spacecraft and surface potential are similar at touchdown, in the absence of other charging effects, such as those introduced by the sampling mechanism. This study was carried out for an asteroid at 3AU, but enhanced photoemission will increase the likelihood of electrostatic regolith disturbance by the spacecraft shadow during descent to an object nearer the Sun.