Ikaros is hitching a ride into space aboard an H-IIA rocket, piggybacking with JAXA's Akatsuki Venus Climate Orbiter mission. Once in space, the cylindrical, 677-pound (307-kilogram) craft will separate from the rocket and spin itself to unfurl its roughly 46-foot-wide (14-meter-wide) solar sail. First proposed in the 1920s, solar sails are large reflective membranes that allow a spacecraft to be pushed by radiation pressure from sunlight, negating the need for heavy onboard fuel. "It's the space equivalent of a yacht sailing on the sea," said Yuichi Tsuda, deputy project manager for Ikaros. Like wind filling a boat's sails, particles of light—or photons—streaming from the sun bounce onto a mirrorlike aluminized solar sail. As each photon strikes, its momentum is transmitted to the spacecraft, which begins to gather speed in the almost frictionless environment of space. A solar sail can eventually reach speeds five to ten times greater than a rocket powered by conventional fuels. Ikaros is considered a hybrid, because the sail's membrane—itself just 0.0075 millimeters thick—sports thin-film solar cells for generating electricity, which will be used to power high-efficiency ion-propulsion engines, Tsuda said.

(Source: National Geographic/ JAXA)