ICARUS'S wings melted when he flew too close to the sun. Here's hoping a similar fate doesn't befall his namesake, the solar sail due to be unfurled by Japan's aerospace exploration agency (JAXA) next week. If all goes to plan, it will be the first spacecraft fully propelled by sunlight.
Solar sails like IKAROS, short for Interplanetary Kite-craft Accelerated by Radiation Of the Sun, aim to move forward by harnessing the momentum of photons colliding with it. The idea may be decades old, but solar sails have remained largely untested. Several sails have been unfurled in space to test deployment, and spacecraft like NASA's Mercury probe, Messenger, have used the pressure of sunlight to alter trajectories. But no spacecraft has used a sail as its primary means of propulsion.
Made of polyimide resin, IKAROS's sail measures 20 metres from corner to corner, but is just 0.0075 millimetres thick. To survive the launch and the trip into space, the gossamer sail will be folded accordion-style, then wrapped around the centre of the spacecraft.
To unfurl its sail, IKAROS will spin some 25 times per minute. The spacecraft's rotation will be used to extend four "arms" of folded material, and the rest of the sail will follow (see diagram) . On 18 May, an H-IIA rocket will carry IKAROS into space along with its main payload, Japan's new Venus orbiter (see "Venus orbiter to fly close to super-rotating wind").
By piggybacking on the Venus launch, IKAROS will be able to get out of Earth orbit, where testing should be relatively simple. Solar sails that are tested in Earth's orbit must adjust their orientation with the sun regularly to build energy, says Bruce Betts of The Planetary Society in Pasadena, California, which hopes to launch its own sail, LightSail-1, into orbit as early as next year, paving the way for an eventual interplanetary mission. "They're doing it the way we would like to do it," Betts says. "Interplanetary space is what solar sails are really designed for."
IKAROS's trip will probably last six months at the longest, says JAXA's Junichiro Kawaguchi. But it could pave the way for more missions. The spacecraft will carry thin-film solar cells on its sail to show that it can also generate power. If all goes well, the demonstration could lead to a "hybrid", sun-driven mission to Jupiter.