Solar Sailing

The world's space agencies are planning to explore space with the technology of sailing ships

Long ago, someone stood alone 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 alone 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 cloth to catch the Sun, and set sail.

The first paragraph: Already happened. The second: Any day now….

May the Solar Force be with You

To fully appreciate how spacecraft can deploy a solar sail to harness the power of sunlight, let's travel back in time for a brief history of solar sailing. 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, 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, the large balloon-like satellite Echo-1 (pictured) felt these solar pressure effects loudly and clearly. "Photon pressure played orbital soccer with the Echo-1 thin-film balloon in orbit.... The shards were flung far and wide by sunlight."

NASA Blazes the Trail

NASA had a more positive experience with solar sailing in 1974 when the Mariner 10 spacecraft ran low on attitude 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 Mariner's solar arrays into the Sun and used solar radiation pressure for attitude 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.

Also in the 1970s, Dr 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. Still the year-long work on preliminary design demonstrated that, indeed, solar sailing was a feasible spacecraft-propulsion technique.

But NASA's missions were overtaken by solar sails launched by Russia, Japan and India.  Read on to find out more....