The effects of reduced-gravity on planetary rover mobility

Abstract

One of the major challenges faced by planetary exploration rovers today is the negotiation of difficult terrain, such as fine granular regolith commonly found on the Moon and Mars. Current testing methods on Earth fail to account for the effect of reduced gravity on the soil itself. This work characterizes the effects of reduced gravity on wheel–soil interactions between an ExoMars rover wheel prototype and a martian soil simulant aboard parabolic flights producing effective martian and lunar gravitational accelerations. These experiments are the first to collect wheel–soil interaction imagery and force/torque sensor data alongside wheel sinkage data. Results from reduced-gravity flights are compared with on-ground experiments with all parameters equal, including wheel load, such that the only difference between the experiments is the effect of gravity on the soil itself. In lunar gravity, a statistically significant average reduction in traction of 20% is observed compared with 1 g, and in martian gravity an average traction reduction of 5–10% is observed. Subsurface soil imaging shows that soil mobilization increases as gravity decreases, suggesting a deterioration in soil strength, which could be the cause of the reduction in traction. Statistically significant increases in wheel sinkage in both martian and lunar gravity provide additional evidence for decreased soil strength. All of these observations (decreased traction, increased soil mobilization, and increased sinkage) hinder a rover’s ability to drive, and should be considered when interpreting results from reduced-load mobility tests conducted on Earth.