Long ago, someone stood on a sandy shore and gazed longingly out at the seemingly endless expanse of ocean, over a horizon suffused softly with ocean mist, musing "I wonder, what's out there?" Then, they fashioned a boat, rigged it with a large cloth to catch the wind, and set sail.

Not quite so long ago, someone stood on a sandy shore and gazed longingly up at the seemingly endless expanse of space, suffused softly with sparkling stars, musing "I wonder, what's out there?" Then, they fashioned a spacecraft, rigged it with a large reflective sheet to catch the Sun, and set sail.

Well… not quite yet, but maybe sometime soon if space propulsion research continues at its current pace. Two space missions are planned for the near future, which both aim to deploy a solar sail to harness the power of sunlight.

Upcoming missions

U.S. space agency NASA's NanoSail-D is a small solar sail slated for launch in the very near future, possibly even later this month.

Another craft, The Planetary Society's Cosmos 2, does not yet have a specific launch target date. However, its goal is to make "a controlled flight under sunlight pressure," says Louis Freedman, president of the society, which is based in Pasadena, California.

Solar sails are giant, flat sheets of very thin, reflective material – 40-to-100 times thinner than a piece of writing paper – supported by an arrangement of lightweight booms or masts.

The sails reflect sunlight, which provide the force needed to push a spacecraft through space, without using any fuel. This sunlight pressure also provides enough thrust to allow such manoeuvres as hovering at a fixed point in space and rotating the vehicle's position in orbit – moves that would require a significant amount of fuel for conventional rocket systems.

Even though these sails are only beginning to be practically realised, the concept behind them has a long history.

Solar breeze

Almost 400 years ago, German astronomer Johannes Kepler observed comet tails being blown by what he thought to be a solar "breeze." This observation inspired him to suggest that "ships and sails proper for heavenly air should be fashioned" to glide through space.

Little did Kepler know, the best way to propel a solar sail is not by means of solar wind, but rather by the force of sunlight itself.

In 1873, Scottish physicist James Clerk Maxwell first demonstrated that sunlight exerts a small amount of pressure as photons bounce off a reflective surface. This kind of pressure is the basis of all modern solar sail designs.

In 1960, a NASA communications satellite, Echo-1, felt these solar pressure effects loud and clear. The satellite was a metallic balloon that was eventually ruptured by the force of photons, which were knocking it around in its orbit.

NASA had a more positive experience with solar sailing in 1974 when the Mariner 10 spacecraft ran low on altitude control gas. Because Mariner 10 was on a mission to Mercury, there was plenty of sunlight around and this gave mission controllers an idea: they angled the craft's solar arrays into the Sun and used solar radiation pressure for attitude (orientation relative to the direction of motion) control.

It worked. Though Mariner 10 was not a solar sail mission, and though the radiation pressure it used was incredibly small, this ingenious use of Mariner's solar arrays did demonstrate the principle of solar sailing.

Five-kilometre-wide bright spot

Also in the 1970s, Louis Friedman – then at NASA's Jet Propulsion Laboratory – led a project to try the first solar sail flight. Halley's Comet was to make its closest approach to Earth in 1986, and NASA conceived the exciting idea of propelling a probe via solar sail to rendezvous with the comet. Eventually, the project was scrapped, but it demonstrated that the concept was feasible.

In 1993, the Russian Space Agency launched a 20-metre diameter, spinning mirror called Znamya 2, hoping to beam solar power back to the ground.

"Some call Znamya 2 a sail because it was made of a large, lightweight reflector and unfurled like a solar sail might be unfurled," says Les Johnson of the NASA Marshall Space Flight Centre in Huntsville, Alabama. Johnson is co-author of the soon-to-be-published book Solar Sails: A Novel Approach to Interplanetary Travel.

"In fact, if I were asked to demonstrate solar sail technology and was constrained to deploy it from a large spacecraft, I might design a 'sail' like Znamya," he says.

The foil reflector unfurled and when illuminated produced a five-kilometre-wide bright spot which crossed Europe from France to Russia at a speed of some eight km/h. Unable to control its own flight, however, the mirror burned up in the atmosphere over Canada. Russia's proto-sail program was abandoned in 1999 after a larger, follow-up mission failed to deploy properly.

Light pressure

Solar sails were an accessory on India's INSAT 2A and 3A communications satellites, circa 1992 and 2003. The satellites were powered by a four-panel solar array on one side. A solar sail was mounted on the north side of each satellite to offset the torque resulting from solar pressure on the array.

In 2004, the Japanese deployed solar sail materials sub-orbitally from a sounding rocket. Although it was not a demonstration of a free-flying solar sail that could be used for deep-space exploration, the deployment was nevertheless "a valuable milestone" remarks Friedman, who appreciates the challenges of deploying gossamer sheets from fast-moving spacecraft.

To date, no solar sail has been successfully deployed in space as a primary means of propulsion.

The Planetary Society hoped to demonstrate the technology with its Cosmos 1 mission in 2005 (see image). "Cosmos 1 was a fully developed solar sail spacecraft intended to fly only under the influence of solar pressure for control of the spacecraft's orbit," says Friedman.

"If all had gone as planned, the U.S.-based Planetary Society, working with Russia, would have been the first to fly a fully functional, though performance-limited, solar sail in space," agreed Johnson. "It would have been the first spin-stabilized, free-flying solar sail to fly in space."

Cosmos 1, however, was lost when the launch vehicle failed.

Meanwhile, NASA also continued to dabble in solar sailing. Between 2001 and 2005, the Agency developed two different 20-metre solar sails and tested them on the ground in vacuum conditions.

"These sail designs are robust enough for deployment in a one atmosphere, one gravity environment and are scalable to much larger solar sails – perhaps as much as 150 metres on one side," write Johnson and his co-author Giovanni Vulpetti in their upcoming book. They estimate that a NASA flight test is possible by 2010.

Low Earth orbit

Now, NASA's smaller NanoSail-D is shooting for the immediate future, and space.

Researchers led by Edward E. Montgomery's at the Marshall Space Flight Centre and Elwood Agasid at NASA's Ames Research Centre in California are working towards deploying the sail "any day now," says Montgomery. It will travel to orbit onboard a SpaceX Falcon 1 rocket, to be launched from the island of Omelek in the Pacific Ocean.

NanoSail-D will feel two kinds of pressure: aerodynamic drag from the wispy top of Earth's atmosphere and the pressure of sunlight. Montgomery's team hopes to measure both types of pressure as the sail circles Earth.

"Our primary objective is to demonstrate successful deployment of a lightweight solar sail structure in low Earth orbit," says Montgomery.

Johnson cautions: "If – and it's a big if – they can measure the solar pressure, they will have demonstrated [it] as a primary means of orbital manoeuvring. [But] they'll have to show conclusively the effects of solar pressure, with a convincingly high signal-to-noise ratio."

Montgomery acknowledges that challenge. "The orbit available to us in this launch opportunity is so low, it may not allow us to stay in orbit long enough for solar pressure effects to accumulate to a measurable degree," he says. "We are going to have to look closely at the flight data to see if we can make that determination."

"Practical interstellar flight"

Unlike the NASA initiative, Cosmos 2 is a privately funded mission, a partnership of The Planetary Society and Carl Sagan-linked, U.S. media company, Cosmos Studios. Work has begun at the Russian Space Research Institute on some Cosmos 2 spacecraft hardware and they are also studying possible launch configurations on a reliable launch vehicle.

If successful, NanoSail-D and Cosmos 2 could profoundly affect the future of science and exploration missions. "Success would be huge for the future of space exploration," says Montgomery.

"Solar sailing is the only means known to achieve practical interstellar flight," says Friedman. "It is our hope that the first solar sail flight will spur the development of solar sail technology so that this dream can be made real."

Each effort "is a stepping stone," in the great visionary Carl Sagan's words, along "the shore of the cosmic ocean," leading us closer to sailing among the stars.

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