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Magnetic levitation can provide manoeuvring power for an autonomous craft (Courtesy of ETH Zurich) Finally, sensors that track the position of the sun are used to determine the Rover’s absolute attitude on the Martian surface and the direction to Earth. At the destination, the rover’s sampling device will autonomously drill to the required depth of up to 2 m to collect samples that are then analysed on board. The US will also return to Mars with a rover in 2020 based on the existing Curiosity vehicle, and China will also be sending a mission to the planet with its own rover. There are however even more ambitious plans to explore Titan, one of Saturn’s moons. Titan is covered in lakes of liquid hydrocarbon, so NASA is proposing an autonomous submarine to explore the planet. The submarine will autonomously carry out detailed scientific investigations under the surface of the largest lake, Kraken Mare, which measures 100 km long and is estimated to be up to 300 m deep. NASA researchers acknowledge though that no-one has yet envisioned what such a craft might look like, how it would operate or if it could be built, so the agency is developing a semi- autonomous planetary submersible that could be extended to other planetary oceans. The Titan mission is planned for around 2038. New drive technologies Longer missions with autonomous systems could also have a new propulsion system. Testing has started at NASA’s Marshall Space Flight Center in Alabama on a concept for a propellant- less system called the Heliopause Electrostatic Rapid Transport System (HERTS) E-Sail. The idea is that, extending outward from the centre of the craft, between ten and 20 electrically charged, bare aluminium wires would produce a large, circular ‘sail’ that would electrostatically repel the fast-moving (400-750 km/s) protons of the solar wind. The momentum exchange produced as the protons are repelled by the positively charged wires would create the spacecraft’s thrust. Each wire is just 1 mm wide but 20 km long. As the spacecraft slowly rotates at one revolution per hour, centrifugal forces will stretch the tethers into position. The testing, which is taking place in the centre’s High Intensity Solar Environment Test system, is designed to examine the rate of proton and electron collisions with a positively charged wire. In a controlled plasma chamber simulating plasma in a space, the team is using a stainless steel wire to mimic the aluminium wire. The engineers are measuring the deflections of protons from the charged wire in the chamber to improve modelling data that will be scaled up and applied to future development of E-Sail technology. The tests are also measuring the number of electrons attracted to the wire. “The same concerns don’t apply to the protons in the solar wind,” says Bruce Wiegmann, an engineer in NASA’s Advanced Concepts Office and principal investigator for the HERTS E-Sail. “With the continuous flow of protons, and the increased area, the E-Sail will continue to accelerate to 16-20 AU [one AU is the distance from the Sun to the Earth] – at least three times farther than a solar sail. This will create much higher speeds. “Our investigation has shown that an interstellar probe mission propelled by an E-Sail could travel to the heliopause [the theoretical boundary where the Sun’s solar wind is stopped by the interstellar medium] in just under ten years,” he says. “This could revolutionise the scientific returns of these types of missions.” If the results from plasma testing, 64 October/November 2016 | Unmanned Systems Technology A comparison of the current space launchers (Courtesy of Blue Origin)