Thermal Drilling and Anchoring on Icy Planetary Bodies

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Halperin, Adam Hugh
Sedwick, Raymond J
Europa is of scientific interest because it is made primarily of water, may have subsurface liquid oceans, and has active cryovolcanoes. Despite the cryogenic surface temperatures, ambient vacuum pressure, and high levels of radiation, potential missions to its surface or its subsurface oceans are topics of current research. Exploring Europa’s surface poses an interesting challenge because of these ambient conditions. Furthermore, any regions of crevasses, ridges, penitentes, cryovolcano formations, and other extreme terrains would be inaccessible to existing rover designs. The “thermal pick” proposed here is a novel system to enable a rover to traverse even the most extreme Europan terrains. It is a dual-function system that first uses a thermal drilling process to burrow into Europa’s surface ice and then serves as an anchor, supporting the mobility of the rover to which it is attached. Thermal drilling provides high reliability but can be energetically costly. An intermittent thermal drilling approach was developed that dramatically reduces the primary drivers of energy cost for thermal drilling in cryogenic ices. Since thermal drilling can cause thermally induced ice fracturing, operational conditions that minimize the likelihood of ice failure modes were established. Three test rigs, two end effectors, and over ten thermal picks were prototyped and tested. Testing with dry ice at atmospheric pressure and testing with cryogenic water ice in a vacuum chamber provided an understanding of thermal drilling in the sublimation and melting regimes, respectively. This testing demonstrated efficiencies of up to 90% relative to ideal sublimation with dry ice and efficiencies of up to 50% relative to melting with cryogenic water ice under vacuum. For safe mobility in the toughest icy terrains, a single anchor should be capable of supporting an entire rover’s weight. Anchoring strengths in excess of 130N were demonstrated, which is the full weight of 100kg on Europa. This 130N anchoring force was supported by even the weakest anchor tested, suggesting potentially greater anchoring loads can be supported.