Estimation of the Strength of the Lunar Soil by the Depth of the Lunar Rover Wheel Tracks

The paper describes the results of measuring the depth of the track left by the wheels of Lunokhod-1 and -2; the depth was used to estimate the compressive strength of the upper layer of the lunar soil. The measurements were taken at 13 locations along the Lunokhod-1 path and 13 locations along the Lunokhod-2 path. The values turned out to be close: 9-49 mm (average 24 mm) for Lunokhod-1 and 12-32 mm (average 21 mm) for Lunokhod-2. It is natural to expect that the stronger the soil, the shallower the track depth, and the calculations given in the paper confirmed this. The measurement technique took into account the specifics of the wheels of the lunar rovers: they had metal mesh rims with high lugs. The range of strength values turned out to be 9.8-23.4 kPa (average 15.0 kPa) along the Lunokhod-1 path and 13.5-22.4 kPa (average 15.6 kPa) along the Lunokhod-2 path. Since the wheel rim designs of the Soviet and Chinese lunar rovers are identical, it was possible to use the developed technique for calculating the strength of the lunar soil along the path of Yutu and Yutu-2, invoking the literature data on the depth of the wheel track for Yutu (2.5-9.3 mm) and Yutu-2 (2.3-7.8 mm) and taking into account their lower mass than that of the Soviet lunar rovers. The results of estimating the strength of soil along the path of these vehicles are 10.0-19.4 kPa and 10.9-20.2 kPa, respectively. In this study we have also performed the theoretical and computational analysis of the interdependence of measurements of soil strength with a cone-blade stamp of the onboard passability assessment device (PrOP) of the Soviet lunar rovers and arbitrary lunar rover wheel stamps. A linear relationship has been established between the measurements and calculations of the strength of the lunar soil by these two methods, and the causes for the differences have been explained.