Science News - September 10, 2011

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

Light’s push

Sail
Force
SOURCE: JEROME L. WRIGHT/SPACE SAILING, CALTECH

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.”

A CubeSat like this one has carried
solar sails into space. Once there, the
sails unfurl and begin sunjamming.

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

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.

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).