By Adam Mann

Two trials of spacecraft have successfully unfurled their solar sails, and demonstrated that radiation from sunlight can power them through interplanetary space.

Solar sails use photons from the sun to propel spacecraft at high speeds. On January 20, the small lightweight spacecraft NanoSail-D deployed a 10-square-meter gleaming sail in low-Earth orbit.

Shortly after, on January 26, engineers at the Japan Aerospace Exploration Agency (JAXA) announced the extension of their solar-sail mission IKAROS. Launched in May 2010, IKAROS flew past Venus in December last year, and the mission will continue until March 2012.

"Both of these are very good technological advances and help give confidence to solar sailing," says Louis Friedman, executive director of the Planetary Society, a non-profit organization for space-based research based in Pasadena, Calif., that is hoping to launch its own solar sail by the end of the year.

Unlimited power

Solar sails are driven forwards when photons from the sun strike their large, thin sheets, imparting a small force. Unlike other propulsion methods, which require a spacecraft to carry fuel, the power source of a solar sail is essentially unlimited. Because they experience no friction in space, solar sails build up a great deal of thrust, and researchers hope that they could eventually be used to reach other planets or even other stars.

In contrast to IKAROS, NanoSail-D was designed to travel closer to home. Launched into low-Earth orbit, it will experience drag on the sail as it skims the planet's upper atmosphere. Within a few months, the spacecraft should slow sufficiently to re-enter and burn up. The technology could one day be attached to decommissioned satellites to slow them down and assist in de-orbiting, says Dean Alhorn, an engineer at NASA's Marshal Spaceflight Center in Huntsville, Ala., and principal investigator of the NanoSail-D mission.

With a sail area that extends to 200 square meters, IKAROS has made it to Venus, traveling closer to the sun in a way similar to that which allows a sailing ship to tack into the wind on Earth. Although an important milestone, the mission did not get to Venus under the power of a solar sail alone; it was launched in a trajectory that would have reached the planet with or without a solar sail, says John West, an engineer at NASA's Jet Propulsion Laboratory in Pasadena who has worked on solar-sail technology. It did, however, show that it received some acceleration from solar photons.

The missions have shown that the solar-sail technology works and provided engineers with a great deal of data, says Alhorn. "I think we understand solar sails and how they operate much better now."

Both space agencies are now considering the next steps for the technology. Alhorn is designing Feathersail, which would have a total sail area of 162 square meters and fly beyond low-Earth orbit. After IKAROS, JAXA plans to build a 50-meter-diameter sail that would launch in the late 2010s and cruise to Jupiter and the Trojan asteroids.

With a bit of luck, the next solar sail in space will be the Planetary Society's 32-square-meter LightSail-1, which would demonstrate that sunlight alone could propel and steer a spacecraft beyond Earth orbit. The mission should be ready by June, and the society is hoping that either NASA or the Russian Space Agency Roscosmos will carry the craft into orbit, says Friedman.

Lunar communication

As the technology matures, a possible use for solar sails is as a fleet that orbits between Earth and the sun, monitoring electromagnetic eruptions such as solar flares, says West. Current detection techniques provide a 30-minute warning, but a longer one could help to prevent damage to power grids on Earth and satellites in orbit, he adds. In addition, solar sails could be placed in high Earth orbit to aid in communication and imaging and be repositioned whenever necessary, says Alhorn.

A solar-sail satellite situated in orbit over the Moon's south pole could be used to relay communication to Earth from a lunar base. NASA's Constellation program, which planned a manned mission to the Moon and Mars, was considering such a proposal, says West, but the mission was canceled along with the rest of the program in February 2010. Still, the agency's Advanced Technology Development Center in Atlanta, Ga., remains interested in solar sails and was expected to release a call for proposals during the autumn of 2010, he says. When Congress voted in September to fund NASA under a continuing resolution, thereby preventing the agency from starting new projects, the proposal was delayed.

But if future solar-sail demonstrations succeed and electronic technology shrinks, allowing for even smaller and lighter probes, the propulsion method is certain to see some action, says West. "There's a niche for solar sails and it's there for the taking."