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63 might be recreated in software. He was then contacted by a colleague who told him NASA was looking for ways to explore under the ice of moons such as Europa and Enceladus. “There are eight bodies in the Solar System that have been identified as ‘ocean worlds’,” he remarks. “The first issue would not be how to drill through ice – we already have solutions for that – but to navigate the huge oceans beneath the ice; you can’t carry out scripted missions like they are doing on Mars now with the Curiosity rover. Under ocean surfaces, there’s no external navigation aiding, no GNSS and no prior maps to use for mission planning.” Stone and his colleagues therefore began planning how a vehicle might survive and autonomously explore at the 100 km depth of Europa’s ocean floor. Europa is the first target for off-world ocean exploration, and the pressure on its floor is roughly the same as that in the Challenger Deep, Earth’s deepest known subsea point. From 2003 onwards (with NASA funding), he and his team developed a form of 3D simultaneous localisation and mapping (SLAM) that would consume as little power as would be needed for accurately calculating the vehicle’s pose (its combined position and orientations) and velocity through its surroundings, to find the best balance between the limits on onboard energy storage and the power needed elsewhere. The first AUV to incorporate this system – and the first to use real-time 3D SLAM as the basis of its guidance, navigation and control – was Stone’s DEPTHX (DEep Phreatic THermal eXplorer) in 2007. That year it successfully guided itself down an unexplored hydrothermal vent in northern Mexico, and returned by riding the vent upwards without getting lost or hitting the walls. It also discovered four new phylla of bacteria, of which fewer than 100 were known at that time. Many of the same technologies have been incorporated into the Sunfish AUV, which also has an endurance of up to 10 hours between recharges, a thruster architecture enabling six degrees of freedom, and a nose designed for grabbing onto a vertical docking bar to enable charging or data uploads. “Our navigation system is robust enough to allow the Sunfish to closely inspect offshore metallic structures such as wind towers and oil rigs for maintenance or damage without suffering attenuations in navigation abilities. Its autonomy makes it far faster and less labour-intensive than the conventional ROVs used in such inspections, so aside from its off-world applications there’s huge commercial potential for this system on Earth,” Stone adds. Hull structure The AUV’s shape and structure depart significantly from the traditional torpedo- shape of most modern commercial and defence AUVs, with the design having sprung largely from two requirements. The first is that it had to be neutrally buoyant in water, not only in its neutral position, as with many AUVs, but in all rotational and pitching axes. That means it will remain in any given orientation without needing thruster power to perform dynamic station-keeping, assuming no external forces are acting on it. “Most important, we did that without installing any syntactic foam inside the hull – something normally every AUV has – but how we achieved that needs to stay proprietary,” Stone says. Second, the AUV needed to Stone Aerospace Sunfish AUV | Digest Unmanned Systems Technology | December/January 2021 The Sunfish’s hull is made from carbon fibre panels and, uniquely, contains no syntactic foam for its buoyancy Its autonomy means there’s huge commercial potential for the system on Earth, aside from its off-world applications