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98 PS | Interstellar travel W hile unmanned systems typically take on the dull, dirty and dangerous tasks on and around Earth, in outer space they do jobs that many people would love to do but simply can’t (writes Peter Donaldson). Interstellar exploration is certainly the most extreme of these because the propulsion technology we have that’s suited to manned spacecraft can’t get a crew to even the nearest stars within a human lifetime. For tiny unmanned probes with masses of only a few grams, however, it’s a different story. Take the proposed StarChip craft for example. Propelled in large numbers by a proposed gigawatt-class ground- based laser array after launch on a conventional rocket, they could achieve up to 20% of the speed of light (0.2 c) with just 10 minutes of laser propulsion and reach the nearest star to the Sun, Proxima Centauri, in about 20 years, while reports of their arrival would take about 4.5 years to reach Earth. That is the idea driving Breakthrough Starshot, a $100 million proof-of- concept research and engineering effort announced in April 2016 by Israeli- Russian entrepreneur, venture capitalist and physicist Yuri Milner, and Professor Stephen Hawking. The first test hardware – in the form of 3.5 x 3.5 cm, 4 g Sprite precursors of the StarChip craft – are already in space. They were launched in June 2017 aboard OHB System’s Max Valier and Venta satellites to test their electronics in orbit and demonstrate their novel radio comms architecture, which are reportedly performing as designed. Launched from a carrier satellite 1000 at a time to allow for attrition along the way, the proposed StarChip craft would be equipped with up to four tiny cameras of at least 2 MP each, four photon thrusters for manoeuvring nearer their destination, a radio and antenna system, a central computer and a solar panel. They would get their power from a plutonium-238 or americium-241 nuclear battery, and they’d all have a protective coating to ward off collisions with dust particles and erosion by atomic particles encountered along the way. Each craft would also have a lightsail measuring about 4 x 4 m. Made of nanomaterials, they would be no more than a few hundred atoms thick and have gram-scale mass. Once in space, each sail would be targeted by a 100 GW, multi-square- kilometre phased laser array on the ground to provide just a few newtons of thrust – a small amount but enough to accelerate the craft to a significant fraction of the speed of light. Among the technical challenges are maintaining the lightsail’s stability and pointing accuracy in the beam, and its structural integrity during the acceleration phase. Then there are the issues of focusing the laser on each craft in turn through the Earth’s atmosphere and keeping it on target, along with phase control of the laser array, power generation and storage at the laser site – it might need its own powerplant – and, naturally, the cost of the array. The target star has an approximately Earth-sized planet, known as Proxima Centauri b, orbiting within the habitable ‘Goldilocks’ zone where it is neither too hot nor too cold for liquid water to exist, and precise determination of this exoplanet’s orbital position will be another challenge. Dirty and dangerous this mission might be, but the first flyby of a planet beyond the Solar System could never be called dull. Now, here’s a thing “ ” The proposed StarChip could achieve up to 20% of the speed of light with just 10 minutes of laser propulsion February/March 2019 | Unmanned Systems Technology