Sailing on sunshine- learning from the past

by Alexandra Witze, Science News/Sail-World Cruising on 27 Aug 2011 One of the oldest technologies to be developed by the human race emerges again. It was sailing vessels that sailed on the wind into the unknown to discover our oceans and the lands beyond them, and now it is sailing vessels which are sailing on sunshine into the unknown to discover the mysteries of space. Alexandra Witze here writes of 'unfurling your jib and tacking your way into space' as a growing trend among space scientists.

When the privately funded Pasadena-based Planetary Society launches its sailing vessel into space next year, it will be the third such vessel to make a space voyage, and if the satellite design team at the University of Surrey get theirs 'off the ground' that will be the fourth.

After keeping scientists in suspense, NASA’s NanoSail-D fanned out in space this year (and is now sailing an orbit around the earth). 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.

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.


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.'

How does it work?
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).

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 promise to be a cheaper way to explore Earth and its environs. They can 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. 'That was kind of unimaginable. Now we’re talking about things 5 meters or 10 meters on a side, so a lot of people might be able to build them and use them a little more practically.'

For more information about general science and astronomy, go to http://www.sciencenews.org!Science_News