Flying on Sunshine

Once futuristic visions, solar sails now take off

When it comes to futuristic space travel, few concepts are more romantic than sailing on sunlight. Soar above Earth, unfurl a jib and tack your way through the solar system all the way to interstellar space.

After keeping scientists in suspense, NASA’s NanoSail-D (illustration shown) fanned out in space earlier this year. Solar sails like this one may be the future of space travel. MSFC/NASA

A CubeSat like this one has carried solar sails into space. Once there, the sails unfurl and begin sunjamming. Nakasuka lab/Univ. of Tokyo

DRIVING A SOLAR SAIL | A solar sail gets its momentum from particles of light hitting it and bouncing off. By tilting the sail so the photons hit at different angles, the sail can be made to fall in toward the sun (a shrinking orbit) or travel outward (an expanding orbit). Adapted by Janel Kiley

LightSail, designed by the Planetary Society, measures 5.6 meters on each side. Once launched, it will turn 90 degrees twice every 90 minutes as it circles Earth. Stellar Exploration

NanoSail-D, shown over Finland in February, can be seen streaking across the sky as it gradually falls out of orbit. Juha Peräsaar/Seinäjoki, Finland

UP, UP AND AWAY | A number of solar sail projects are planned to follow the recently launched IKAROS and NanoSail-D missions. While the more expensive missions better capture the grandeur of solar sailing, other projects are getting to space quicker with cheaper approaches.

Solar sails have been a mainstay of dreamers since Johannes Kepler, who speculated four centuries ago that ships would one day be powered by “heavenly air.” But sun sailing is no longer fanciful fodder for visionaries. Recent technological advances have moved solar sailing from science fiction to science fact.

Last year, Japan’s space agency launched the world’s first solar sail into interplanetary space; its metal-coated membrane unfurled and caught the light to begin sunjamming. And with help from tiny “nanosatellites” that allow scientists to pack folded-up sails in spacecraft no bigger than a loaf of bread, NASA this year sent its first sail skipping through Earth orbit.

Look overhead at the right time of night, and you can spot the gleaming streak of NASA’s NanoSail-D as it tumbles closer to Earth, mission accomplished. Within the next few months it will incinerate in the atmosphere in a bright flash.

In addition to the Japanese and U.S. efforts, the privately funded Planetary Society expects to launch its own sail next year, as does a satellite design team based at the University of Surrey in England.

Solar sail enthusiasts have waited decades to see such flights. And one day, they hope, solar sails will perform tasks other spacecraft cannot: hover above Earth’s poles to monitor climate change, flit near the sun to watch for solar storms, drag space junk out of orbit like a cosmic maid or even journey to a nearby star.

“As far as solar sails go, we are on the cusp of history,” says Dean Alhorn, an engineer at NASA’s Marshall Space Flight Center in Huntsville, Ala., who leads the NanoSail-D mission. “We are ready now with the technology to make these happen.”

Riding the wind

In principle, solar sailing could not be easier. Scottish physicist James Clerk Maxwell described in 1873 how light can exert pressure: A particle of light transfers up to nearly twice its momentum to an object it bounces off of.

Each individual transfer amounts to no more than a mosquito’s breath, but over time that breath accumulates to a steady wind that a spacecraft can ride just as a sailboat rides the wind on Earth. After 100 days, a solar sail could reach 14,000 kilometers per hour; after three years it could be zipping along at 240,000 kilometers per hour. At that rate it could get to Pluto in less than five years, rather than the nine years the plutonium-powered New Horizons spacecraft, now on its way, is taking. Solar sails are the tortoise to the hare of chemical rocketry.

Scientists have long wanted such a tortoise. In the 1920s Konstantin Tsiolkovsky, the founder of Soviet astronautics, and colleague Fridrikh Tsander separately wrote of the idea of using solar radiation pressure to accelerate sails. After a few decades on the back burner, the idea took off in the ’50s and ’60s, with engineers drafting up grandiose designs and Arthur C. Clarke plotting a solar sail race in his short story “The Wind from the Sun.” By 1976 engineers at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., were dreaming of sending a massive solar sail to fly alongside Comet Halley as it passed close to Earth the next decade.

Without the need to carry fuel, solar sails promised to be a cheaper way to explore Earth and its environs. They could also make visits, such as hovering above the North Pole, that traditional spacecraft can’t because of the dictates of gravity. But solar sails lost the funding battle to other alternative propulsion systems — at least in the United States. By the early 1990s a few other sporadic attempts, including a plan for a solar sail race to Mars, also fell apart.

Now, tiny satellites may be saving the big dreams of some would-be solar sailors. One of the hottest things in satellite technology today is the CubeSat, a box just 10 centimeters on a side that weighs about 1 kilogram. Such boxes can be mixed and matched in “nanosatellite” combinations of up to three cubes yet still be launched using a shared deployment system. CubeSats are thus relatively cheap and easy to work with, so researchers have used them to carry a variety of science experiments. A small solar sail, thinner than a trash bag and weighing just grams, turns out to be nearly the perfect payload to fly on a CubeSat.

“When we first thought of solar sails back in the ’70s and ’80s, it was these huge structures, a mile or half a mile on a side,” says Louis Friedman, cofounder of the Planetary Society, headquartered in Pasadena. “That was kind of unimaginable. Now that we’re talking about things 5 meters or 10 meters on a side, you realize that a lot of people might be able to build them and use them a little more practically.”

Two flights and failure

Existing solar sail designs fall into two main categories: ones that deploy rigid booms to hold the sail taut, like a sailboat’s mast, and ones that spin to blossom sail blades outward using centrifugal force. The main challenges are to unfold the whisker-thin sail in space without ripping, and to direct the sail to move in the right direction.

Thanks to hefty government funding, Japan’s space agency, JAXA, was the first to conquer both challenges. It built a large square sail, too big to fit in a CubeSat, and launched it on board a probe headed to Venus. In June of last year, the probe released the still-folded solar sail, named IKAROS after the mythological boy who flew too close to the sun and melted the wax anchoring his wings. In the great space acronym tradition, the name also stands for Interplanetary Kite-craft Accelerated by Radiation Of the Sun.

On cue, IKAROS unfolded its sail, 20 meters across diagonally, and made its way toward Venus, flying past that planet in December. By turning on and off an innovative set of liquid crystals, project engineers showed they could change the sail’s reflectivity and thus direct its motion.

JAXA has extended the mission to March 2012 so engineers can test some more risky flight maneuvers. “I’m just so in admiration of them,” says Friedman. The agency is also working on a much larger solar sail, 50 meters across, with hopes of launching it around 2020 to set sail for Jupiter and distant asteroids. Eventually, JAXA wants to develop novel hybrid propulsion systems, combining solar sails with ion drives to enable long trips through the solar system.

With far less money to spend than Japan, the first U.S. solar sail is far smaller, cheaper and less ambitious. Like so many projects in the solar sail world, the currently orbiting NanoSail-D was born from the ashes of much grander ideas.

NASA’s solar sail research program has waxed and waned over the years depending on funding. In the middle of the last decade, the agency conducted its biggest experiment to date, when it tested two 20-meter-by-20-meter solar sails at a research facility in Ohio. Then NASA began funneling most of its money into the Constellation program to return astronauts to the moon, and the big solar sail project foundered.

“When that ended we had a lot of hardware and only a little budget,” says Les Johnson, deputy manager at the Advanced Concepts Office at the Marshall center. “And that’s where NanoSail-D was born.”

A few people began working on a CubeSat-based design informally called LunchSat, “because the only time people had to work on it was during lunch,” Alhorn says. But suddenly a chance to launch arose, and the team hurriedly built two kite-shaped sails, 3 meters on a side, that could tuck inside a nanosatellite. The first NanoSail-D launched in 2008 on a Falcon 1 rocket provided by the private company SpaceX, but the rocket never made it into orbit. A second option arose the next year, and the spare NanoSail-D launched successfully in November 2010 aboard a Minotaur rocket.

But then disaster struck. NanoSail-D didn’t emerge and unfurl when it was supposed to. Mission managers had given it up as lost when in January the sail apparently decided to deploy itself on its own schedule. “Somehow it freed itself,” says Alhorn. “We all have theories of what stuck it and why it came loose, but there’s no conclusive evidence.”

NanoSail-D unfurled itself and since then has been orbiting Earth, the first solar sail NASA has deployed in space. The craft is drifting gradually lower in altitude, and Alhorn estimates it will burn up sometime before next January.

Ordinarily, putting a solar sail into Earth orbit is harder than sending one to interplanetary space, simply because the sail has to keep readjusting its trajectory. Although NanoSail-D has succeeded in showing how a solar sail would deploy, it isn’t actually controlling its position. Because it may be tumbling along under atmospheric drag instead of solar radiation pressure, some purists insist it isn’t a true solar sail. Alhorn is now working on a concept for a larger sail with a novel kind of attitude control, which sets the sail’s orientation with panels feathered up to 90 degrees.

Closest in concept to the original grand dreams about solar sailing, yet freighted with the memory of a recent failure, is the LightSail project of the Planetary Society. Friedman, its architect, has seen pretty much everything in the world of solar sailing; he worked on the original Halley proposal in the 1970s and spearheaded the society’s drive to fly a privately funded sail in the early 2000s. That effort, paid for mainly by an entertainment company led by Carl Sagan’s widow, ended with a splash in 2005 when the Russian rocket it was supposed to ride from a nuclear submarine failed to reach orbit.

After licking his wounds, Friedman decided to work with NanoSail-D in its initial stages. That restored his enthusiasm and inspired LightSail. “We got so interested in the design that we said we’ll go further: We’ll instrument the craft and build in attitude control and a telemetry system,” says Friedman. Thanks to CubeSats, the sail could be built for less money than the society’s last, failed attempt.

LightSail’s design calls for the main CubeSat bus to unfold four rectangles covered with solar panels, then unfurl blades of Mylar film to form a kite 5.6 meters on a side. It will have cameras to photograph itself, accelerometers to measure solar pressure and a motor to help keep it pointed on course. As it goes around the Earth, the sail will have to turn 90 degrees twice every 90 minutes.

And this time, just in case, the society is building two copies: Twin LightSails are in the final stages of construction at Stellar Exploration in San Luis Obispo, Calif.

Finding a ride is next. To get above 825 kilometers in altitude, where solar radiation pressure begins to dominate over atmospheric drag, LightSail needs a launch vehicle that goes higher than most CubeSat launches. The project is now waiting for that lift, Friedman says.

Never one to give up dreaming, Friedman envisions two other LightSails to come. LightSail-2 would aim to do a longer flight in a higher Earth orbit, and LightSail-3 would fly to the gravitationally stable L1 Lagrangian point between the Earth and sun.

Future seas

Over the next few years, a handful of other solar sails under development may see the light of space, each proving in its own way that sailing on sunshine is possible. In England, a consortium from the University of Surrey and its industry partner Astrium is building two prototypes for yet another CubeSat-based solar sail 5 meters on a side, called CubeSail. Engineers have constructed one sail that relies on booms of metal tape that unroll like party poppers, and a second that uses rigid carbon fiber booms that unfold directly. The team will test both in the laboratory and by December decide which design to fly, says project leader Vaios Lappas of the University of Surrey. He expects CubeSail to launch in early 2012.

Lappas’ team is also working on a larger European Union–funded project, called DEORBIT SAIL, for launch in 2014, and an inflatable sail for launch that year or the next. As its name suggests, DEORBIT SAIL’s main objective is to get decommissioned space junk out of orbit. Though garbage cleanup may sound like a pedestrian task for a glorious solar sail, such applications may be what gets sails built and flown in the years to come, Lappas says. Tens of thousands of large pieces of spent rockets and other trash drift dangerously in low-Earth orbit, threatening collisions with pricey working satellites. Some countries are beginning to require spacecraft designers to install a way to deorbit satellites after their useful life has ended.

One cheap and lightweight way would be to stick a solar sail on board, which could unfurl at the end of the mission and gently guide the craft down to incineration. Or a sail could go pick up the trash directly: “We want to develop a system where we can take our deorbiting system to pieces of space debris, dock with them and bring them to the atmosphere and let them burn up,” Lappas says.

Yet another approach to solar sails is taking shape in a clean room in an Illinois laboratory. Researchers there have designed a sail that would unfurl from bobbins into a giant space ribbon 250 meters long, says Victoria Coverstone, an aerospace engineer at the University of Illinois at Urbana-Champaign. This project, also dubbed Cube Sail, is basically ready to fly, she says, if the team can find money for a launch and to upgrade the Mylar film that makes up the sail. The Illinois group next aims to test a spinning deployment of sail blades, on the way to an ambitiously large spinning sail whose rotating blades could measure up to 5 or even 10 kilometers long.

Meanwhile, the German space agency DLR and the European Space Agency are planning their own series of solar sails dubbed Gossamer. The first of these would launch a 25-square-meter sail into Earth orbit in 2014, followed by bigger ones over the next several years.

How all these new projects come together may shape the future of solar sailing for decades. “I think there’s a lot that will happen in the next two to three years that could essentially define how solar sails take off from Earth and go into space,” Lappas says.

In the longer term, solar sails will move forward only if the scientific community promotes them for missions where no other propulsion technology can do the job, says Colin McInnes, director of the Advanced Space Concepts Laboratory at the University of Strathclyde in Glasgow, Scotland. It may seem a practical end to a romantic concept, but “in the long term that’s how it’s going to advance,” he says. “The advocates of solar sailing have to identify what the really compelling science or operational missions are where solar sailing outcompetes other propulsion technologies. It’s not going to advance just because it’s such a neat idea.”

Alexandra Witze is a contributing correspondent for Science News. Based in Boulder, Colo., Witze specializes in earth, planetary and astronomical sciences.

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