Rover wheel tribocharging in lunar shadowed regions: deriving a speed limit for charge accumulation

• Present a model of wheel-regolith tribocharging and corresponding plasma dissipation for the nightside lunar environment. • Equilibrium potentials can be estimated by comparing primary tribocharging source term to the primary plasma dissipation term. • Detailed analysis of wheel charging within a simulated polar crater plasma starved environment is presented. A model is presented that describes the tribocharging of a conductive lunar rover wheel as it moves through the lunar regolith, and which includes the remediating dissipative effects of the surrounding plasma environment. The model is applicable to lunar shadowed regions such as within polar craters and the lunar nightside, where there is no photoemission from exposed surfaces and plasma flux levels are low. The model is in the form of first order differential equations that can be solved to obtain the rover wheel potential as a function of time for a given rover velocity. Unique features of the model include the derivation and use of a tribocharging current to the wheel that varies as a function of wheel velocity, wheel size parameters, and the wheel-regolith triboelectric charge exchange. Analysis of the wheel tribocharging sources and plasma losses show that there are threshold wheel velocities which, if exceeded, triboelectric charging will exceed plasma dissipation. This effect will allow charge to accumulate on the wheel. This threshold velocity to initiate charge accumulation varies directly with the surround compensating plasma flux in the vicinity of the wheel. In a simulated polar crater plasma environment, the threshold wheel velocities are small (<< 1 m/s). The model results give rise to a number of recommendations including that rover entry into polar craters should be in a sunward direction (to remain within the deflected solar wind proton flow) and that rover wheels should be conductively coupled to the rover body to increase the plasma current collection area. We find that within plasma depleted polar craters, the wheel velocities should remain below 2 x 10 -3 m/s to ensure that wheel plasma dissipation is comparable to regolith tribocharge buildup.