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46 Insight | Space systems smaller, SWaP-optimised orbital vehicles. The system avoids needing a neutraliser by applying RF voltages to continuously accelerate the ions from the plasma (the discharged propellant), while also causing electrons to exit during a period of each RF cycle. As ions and electrons come from the same plasma source, a neutraliser is unnecessary, reducing the cost and complexity of the overall system. Also, since electrical power from a satellite’s solar panels would be used mainly for generating RF power, the system does not require any high- voltage DC amplifiers. Satellite propulsion systems normally integrate several of these in the neutraliser and gas discharge channels, but by forgoing them, the spacecraft’s power processing unit can be made far smaller and lighter, enabling more SWaP-optimised craft that can carry heavier payloads. ThrustMe’s design currently uses xenon gas, but the company is looking at iodine as a solid propellant but exhibiting high vapour pressure. That would eliminate the need for costly high-pressure storage systems and complex flow control valves. Meanwhile, Californian start-up Momentus has signed a launch partnership agreement with German company ECM Space. The latter will provide its launch services for the first in-orbit test of the former’s water-based plasma propulsion technology in early 2019. Momentus’ thrusters could provide a more sustainable fuel source for asteroid mining and other space-based industries. Its solution claims to use solar energy to power microwave generators that will heat the water before it is ejected as a plasma from an exhaust nozzle to propel the spacecraft. The company is designing a few different systems based around this concept, with its Zeal thruster producing up to 30 W for CubeSat-sized craft, and its Vigor thruster aiming for at least 100 W of power and a specific impulse of 350- 500 s in small spacecraft main engines (or as attitude thrusters on future, larger propulsion systems). Asteroids Asteroid-based research and economic development could depend on findings from missions such as the Osiris-Rex (Origins, Spectral Interpretation, Resource Identification, Security – Regolith Explorer) spacecraft, which is scheduled to arrive at asteroid Bennu this December. Currently using its thrusters to match the velocity of this near-Earth asteroid, the craft will deploy various sensors to gather data on Bennu for more than a year before approaching it to gather a sample of material for study back on Earth. The sensors include a camera suite consisting of PolyCam, a 20 cm telescope that has imaged Bennu from 2 million km away and will provide high- resolution images of its surface, a sensor for mapping Bennu’s surface in four colours, and a sensor for recording the sample collection. Also, a Lidar scanner from the Canadian Space Agency will assist in mapping the surface and measuring the distance between the asteroid and the spacecraft. Three onboard spectrometers will gather mineral, organic and temperature information as well. The craft has a total of 28 thrusters, powered by 1230 kg of fuel loaded before launch. “For further power, two solar panels generate 1226-3000 W, depending on the spacecraft’s distance from the Sun,” says Sandy Freund, the Osiris-Rex mission support manager at Lockheed Martin Space Systems, which built the space vehicle. As it nears the asteroid, Osiris-Rex can deploy its Touch-And-Go Sample Acquisition Mechanism (TAGSAM), an articulated robotic arm 11 ft long with a sampler head, provided by Lockheed Martin Space. “Think of it like a real arm – it has a shoulder, elbow and wrist,” Freund says. “TAGSAM will release a burst of nitrogen gas, stirring up asteroid regolith from the surface, directing it all into a collector filter on the sample head. There are also contact pads on the bottom of the TAGSAM head, which will collect surface dust on contact. It will collect between 60 and 2000 g of material,” she adds. The spacecraft is equipped with enough nitrogen gas for three sampling attempts, after which it will embark on its return path to Earth. Osiris-Rex is expected to arrive in orbit around September 2023, when it will deploy the samples in a return capsule from Lockheed Martin Space Systems. October/November 2018 | Unmanned Systems Technology Osiris-Rex will travel to near-Earth asteroid Bennu and conduct a range of imaging surveys before returning to Earth with a sample of Bennu’s material (Courtesy of NASA)