Asked by Stewart Wilkinson
The next big advance in space propulsion looks like it could be the solar sail. Scientists have considered spacecraft being ‘blown’ on gusts of the solar wind now for many years, but only recently have experiments on sails in space been carried out. The Sun emits a wind of charged particles that, combined with the pressure of its brilliant sunlight, can push a thin reflective sail (light can push an object because photons have momentum that can be transferred to the sail). It’s only a tiny push, at first, but this builds up over time. Solar sails are made from extremely thin and very reflective material such as carbon fibre or aluminium just millionths of a metre in thickness.
In 2010, the Japanese space agency launched IKAROS, which was deployed en route to Venus and achieved a velocity of 360 kilometres per hour (224 miles per hour) over half a year. NASA’s NanoSail-D had a 9.3-square-metre (100-square-foot) sail in low Earth orbit and next year it plans to launch the Sunjammer mission, with a 1,200-square-metre (13,000-square- foot) sail, in collaboration with the UK Space Agency. But how can a solar sail fly towards the Sun when the sunlight is pushing it away? If its velocity ‘vector’ is at a right angle to the Sun, then by orientating the sail correctly you can get an extra push and increase or decrease the sail’s velocity by thrusting in the same direction, or against it.
Image Credit: NASA
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