SOLAR SAIL

A spacecraft propulsion device that employs the pressure of light from the sun.

Like so many ideas concerning space travel, this one has been in existence for decades, mostly in science-fiction stories, though it is now being thought about more seriously as new materials become available.

The idea is that a spacecraft would unfurl a huge but incredibly thin solar sail, perhaps a kilometre in diameter. The pressure of sunlight on the sail — radiation pressure — would be tiny, but it would be there. A craft massing several tonnes could accelerate to more than a kilometre per second within days, and then go on accelerating so long as it remained relatively close to the sun. It is suggested that an initial shove could be given by giant ground-based lasers (as described in Larry Niven’s novel The Mote in God’s Eye). It would even be possible to tack the craft by angling the sail. A combination of lasers and a solar sail could make it possible to send a craft anywhere within the Solar System.

The name was popularised by Arthur C Clarke, in his short story Sunjammer of 1964 (reprinted as the title story of The Wind from the Sun in 1972), though the concept in science fiction goes back at least as far as Cordwainer Smith’s The Lady who Sailed the Soul of 1960. In factual speculation it is even older: the Russian aeronautics pioneer Konstantin Tsiolkovsky and his colleague Fridrickh Tsander wrote in 1924 of “using tremendous mirrors of very thin sheets” and “using the pressure of sunlight to attain cosmic velocities”. The term itself seems to have been coined in the late 1950s by the American engineer Richard Garwin.

Ed Gabris, a senior engineer at NASA, wrote: “Solar sailing is more than a science fiction fantasy. NASA used solar sailing to increase the experiment time for the Mercury Mariner spaceprobe in 1974-75. The ‘sail’ was the spacecraft’s solar panels. And by controlling the attitude of the spacecraft and the angle of the solar panels to the sun, the operations team was able to cause the spacecraft to visit Mercury several times more than would have been possible with the on-board liquid propulsion system”.

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