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A concept illustration of the Ikaros spacecraft.

Ikaros and its solar sail are seen in an artist's rendering.

Image courtesy JAXA

Julian Ryall in Tokyo

for National Geographic News

Updated May 21, 2010

NOTE: The original May 18 launch of JAXA's Akatsuki Venus Climate Orbiter—and therefore the Ikaros mission—was scrubbed due to weather concerns. Akatsuki and Ikaros launched successfully from Japan on May 21.

The Japan Aerospace Exploration Agency (JAXA) has launched the first spacecraft that will speed across the solar system using a hybrid solar sail—one propelled partly by solar pressure, partly by traditional solar power.

Dubbed Ikaros—for Interplanetary Kite-Craft Accelerated by Radiation of the Sun—the experimental craft launched from the Tanegashima Space Center at 5:58 p.m. ET on May 20 (6:58 a.m. on May 21, local time).

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. (Related blog: "Tiny Solar Sail Pitched to Clean Up Space Junk.")

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. (Explore a time line of space travel milestones.)

"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.

(Related: "Can Earth Be Powered by Energy Beamed From Moon?")

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.

Solar Sail Headed for Venus and Beyond

The first month of the Ikaros mission will be spent deploying the sail and carrying out initial checks, Tsuda said.

"As soon as the sail has deployed, the craft will be able to start solar sailing," Tsuda said. "Over the six-month scheduled duration of the mission, we believe it will reach a velocity of a hundred meters [328 feet] per second."

Flying along the same path as the Akatsuki spacecraft, Ikaros will be headed toward Venus. But Tsuda and his team hope the solar sail-powered craft will continue even farther, flying for as long as possible.

Instruments on board Ikaros will send back data on the basic state of the core spacecraft, how much power it is generating versus how much it's using, and the status of the sail. Six cameras aboard the craft will help the team monitor how the sail deploys and how it fares during its trip.

The growing distance between the spacecraft and Earth will make communication increasingly difficult. Still, Tsuda's team hopes to be able to operate the vehicle and collect data for at least a year.

After that, lessons learned from Ikaros will be applied to its planned successor, a craft equipped with a 164-foot-wide (50-meter-wide) solar-power sail that will be launched toward Jupiter around 2020.

Solar Sails No Good for Astronauts?

For the Ikaros mission, JAXA has been working closely with the California-based Planetary Society, which aims to get its own solar sail—LightSail-1—into space before the end of 2011. (Related: "Cosmos 1 Solar Sail Spacecraft Fails, Space Agency Says.")

That mission will be "very different" from Iakros, as it will carry a smaller spacecraft, said Louis Friedman, executive director of the Planetary Society.

"But it will be capable of higher accelerations, and it's a step toward developing the very lightweight, pure solar sailing missions of the future."

NASA and the European Space Agency have suspended research into solar sails due to budgetary constraints, although Russia continues to work on a concept vehicle.

Friedman said the technology is crucial for the next generation of space travel. (Related: "Space-Elevator Contests Lure With Big Money, NASA Glory.")

"It is the only known technology which may someday enable interstellar flight," he said, if light from onboard lasers could one day replace sunlight as the main propellant.

JAXA's Tsuda noted, though, that current solar sail technology is not suitable for manned missions. With the added weight of people and supplies, it would simply take too long for the craft to reach acceleration speed.

But Tsuda believes that Ikaros will open up new frontiers in robotic space exploration.

"On the 2020 mission, we hope to be able to go to the Jupiter system and the concentrated belt of asteroids that exist nearby that are known as the Trojan asteroid region," Tsuda said.

"That part of our solar system has never been visited by a man-made craft, and we want to be the first to reach it."

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