Penetration testing for the Optical Probe for Regolith Analysis (OPRA)

The Optical Probe for Regolith Analysis (OPRA) is a spike-shaped subsurface analytical probe that will be delivered to a planet, asteroid, or cometary body by a lander and/or rover. OPRA will be pushed down into the subsurface to record near infrared spectra as a function of depth down to maximum of 50 cm. Therefore, knowledge of the required penetration force to specific depths can be helpful in estimating the length of the probe. Test probes covering the anticipated diameter (2.5, 1.9, 1.2 and 0.9 cm diameter) and tip angle (T.A. = 30°, 60°, 90° and 120°) of OPRA were inserted mechanically into dry playground sand. The results showed that tip angle does not have a major effect, while probe diameter and density of the regolith are the most important parameters. Increasing probe diameter from 0.9 to 1.9 cm (i.e. a factor of 2) leads to an increase in penetration force from 200 to 1000 N (i.e. a factor of 5) at 20 cm depth. An increase in bulk density (B.D.) from 1550 to 1700 kg m⁻³ leads to an increase in penetration force from 10 to 200 N at 20 cm depth. Bearing capacity theory was used to explain the downward movement of the penetrometer through regolith and showed good agreement with the experimental results. This model was then used to take into account the effect of gravity on other planetary bodies. We observed a good agreement between the theoretical model and results from penetration testings on the Moon by the Apollo missions. Since the maximum allowed force for penetration is the weight of the lander/rover on their targeted planetary surface, our results put a strong constraint on the maximum reachable depth without endangering the whole mission.