Gone but not forgotten: An artist's impression of The Planetary Society's Cosmos 1 in orbit. The solar sail-equipped craft was lost during launch in 2005, but a replacement is already under development.
Credit: The Planetary Society
Under development: A four quadrant, 20-metre-across solar sail is fully deployed during testing in a space simulation chamber at NASA's Glenn Research Centre in Ohio, USA.
Credit: NASA
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.
