The Dragonfly Surface Thermal Properties Sensor (STHERM): Design and Testing

A Surface Thermal Properties Sensor, STHERM, is part of the Dragonfly Geophysics and Meteorology (DraGMet) sensor suite. Dragonfly, NASA's 4th New Frontiers mission, is a rotorcraft lander to explore Saturn's moon, Titan. The STHERM instrument, one in each skid of the lander, is designed to determine the thermal properties of the surface at each of the approximate 50 landing sites. In addition to its science function, STHERM data provides situational awareness and will influence the decision whether to use the sampling drill at each site.This paper describes the design of the STHERM sensor, a mathematical model that allows the extraction of thermal properties from the response, and the experiments performed to determine mechanical stability as well as performance for the STHERM sensor.STHERM consists of two 1.2 cm Cu-Be blades, 11 mm apart, that can be heated via resistors while their temperature is monitored. These thermal probes are pressed into the ground on a spring-loaded structure mounted vertically through the skid. In order to verify that the sensor will survive a minimum of 50 landings at Titan conditions, we have developed a test setup, based on a UR5 robotic arm. The setup allows performance assessments of the sensor prototypes held in a 3D-printed skid mock-up with the expected landing force, including an approximately 1.5-foot slide on simulated ground material.The testbed also allows thermal measurements on various simulated surfaces to investigate how well-posed the sensor is on different textures and how accurately the thermal properties can be determined. Measurements in the Titan Pressure Environment Chamber (TPEC) at the Johns Hopkins University Applied Physics Lab (APL) were performed to determine the thermal performance and sensitivity of the instrument with Titan environment conditions, at -180°C and 1.5 bar.