Japanese Solar Sail Headed for Venus and Beyond

An ambitious solar sail mission designed by Japan is poised for launch tomorrow could become the first successful mission powered solely by sunlight, but that's not all. The spacecraft is also aimed at Venus and beyond, and could pave the way for a future hybrid space engine.

The solar sail will hitch a ride aboard an H-2A rocket slated for launch on Monday (Tuesday local time) from Japan's Tanegashima Space Center. That rocket carries the main mission of the Japan Aerospace Exploration Agency (JAXA), the Venus Climate Orbiter called Akatsuki — which means "Dawn" in Japanese.

But only Akatsuki has a planned meet-up with Venus, even though the sail — called Ikaros (Interplanetary Kite-craft Accelerated by Radiation Of the Sun) — will also launch along the same trajectory toward the mysterious planet.

"This will be the world's first solar powered sail craft employing both photon propulsion and thin film solar power generation during its interplanetary cruise," said a JAXA mission website.

Venus would mark just a six-month pit stop for the solar sail during a three-year trek toward the far side of the sun.

"To me it's a very bold activity to be conducting a technology test like this on an interplanetary mission," said Louis Friedman, an executive director of the Planetary Society in Pasadena, Calif. "I think it shows a lot of foresight on their part."

Past solar sail demonstrations have fallen short of achieving actual solar-propelled spaceflight, but that certainly has not stopped JAXA from planning an ambitious technological debut. Even Ikaros itself represents just a stepping stone to a "hybrid" space engine that incorporates solar sail technology, mission planners have said.

Space hybrid vehicle

The kite-shaped Ikaros relies upon the pressure of sunlight for propulsion, but it also carries thin film solar cells built within its sail. Such cells could generate electricity from the same sunlight pushing the solar sail along.

That won't do much good by itself for a solar sail without an engine. But JAXA hopes that the power-gathering demonstration could eventually lead to spacecraft with ion-propulsion engines that draw electricity from solar cells and also take advantage of solar sail propulsion — a hybrid propulsion system.

"They want to ultimately have a solar electric [ion propulsion] and solar sail vehicle that would be used for outer planetary missions," Friedman told SPACE.com.

Yet the history of solar sail tests presents a sobering reminder of the troubles that can arise. The California-based Planetary Society attempted to fly its Cosmos-1 solar sail in 2005, but lost their prototype because of a Russian rocket malfunction. NASA's NanoSail-D was also lost in the third failed flight of SpaceX's Falcon 1 rocket in 2008.

A British shoebox-sized mission slated for launch next year might also test solar sail propulsion, but would mainly test the sails as brakes for taking defunct satellites down.

Japan did deploy a solar sail from a sounding rocket in 2004, but did not actually attempt to demonstrate controlled flight. If that represented the dry run, then Ikaros comes as the real deal.

True solar sailing

Ikaros is designed to unfurl its sail during its first stage by taking advantage of its spinning momentum, and then actively deploying the rest of the way during a second stage.

"The membrane is deployed, and kept flat, by its spinning motion," the JASA mission website stated. "Four masses are attached to the four tips of the membrane in order to facilitate deployment."

The Planetary Society still has ambitions to someday launch a solar sail mission into deep space, but its first planned solar sail test would involve a much smaller spacecraft than Ikaros, which stretches almost 66 feet (20 meters) at the diagonal of its square sail.

A refitted NASA solar sail might weigh a little less than 10 pounds (4.5 kg) compared to the 700-pound (315 kg) Ikaros.

The Planetary Society would aim first for launch to low-Earth orbit, before eventually launching a second mission that lasted perhaps weeks. Only the third mission would try for interplanetary traveler status, Freidman said.

For now, Friedman and the Planetary Society will share technological information and results from the JAXA mission, and keep an eye on their own hopes for the future.

"We wish we were first, of course, but it doesn't matter," Friedman said. "It's about advancing solar sail technology."

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