by Dauna Coulter
A solar sail is a spacecraft without a rocket engine that is pushed directly by light particles from the Sun, with the sunlight reflecting off its giant sails. Composed of a gossamer material, when unfurled in the vacuum of space, the sail feels the pressure of sunlight and is propelled by that pressure to carry spacecraft among the stars.
Long ago, someone stood alone on a sandy shore and gazed longingly out at the seemingly endless expanse of ocean, musing, I wonder, whats out there? Then, they fashioned a boat, rigged it with a large cloth to catch the wind, and set sail to exciting new adventures and lands unknown.
Not as long ago, someone also 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, whats out there? They then 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
Two very special missions, one in the past and one in the future, were designed to deploy a solar sail to harness the power of sunlight. NASAs NanoSail-D was a small solar sail that fell victim to a failed launch on August 2nd, 2008. The Planetary Societys Cosmos-2 does not yet have a specific launch target date and its goal is to make a controlled flight under sunlight pressure. 1 To fully appreciate these two missions, lets travel back in time for a brief history of solar sailing.
Sailing Into History
Almost 400 years ago, German astronomer, Johannes Kepler, observed comet tails being blown by what he thought to be a solar breeze.2 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, Echo-1 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.3
NASA had a more positive experience with solar sailing in 1974 when the Mariner-10 spacecraft ran low on attitude control gas. As Mariner-10 was on a mission to Mercury, there was plenty of sunlight around and this gave mission controllers an idea: They angled Mariners 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 Mariners solar arrays did demonstrate the principle of solar sailing. Also in the 1970s, Dr. Louis Friedman, then at NASAs Jet Propulsion Laboratory, led a project to try the first solar sail flight. Halleys 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.4
In 1993, the Russian Space Agency launched a 20-meter 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 Center, co-author of the 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. The foil reflector unfurled and, when illuminated, produced a spot of light, which crossed Europe from France to Russia. Unable to control its own flight, however, the mirror burned up in the atmosphere over Canada. Russias proto-sail program was abandoned in 1999 after a larger, follow-up mission (Znamya-2.5) failed to deploy properly.
Solar sails were an accessory on Indias INSAT-2A and -3A communications satellites, circa 1992 and 2003. The satellites were powered by a 4-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, Japan deployed a big, think film for solar sail in space. The S-310 rocket carried aloft two kinds of deploying schemes of films with 7.5 micrometers thickness. A clover type deployment (see image left) started at 100 seconds after liftoff at 122 km altitude. A fan type deployment was started at 169 km altitude at 230 seconds after liftoff, following the jettison of the clover type system. Both deployments and subsequent experiments were successful, and the rocket splashed down into the sea approximately 400 seconds after liftoft.
Although this flight 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 (see image right) mission in 2005. Cosmos 1 was a fully developed solar sail spacecraft intended to fly only under the influence of solar pressure for control of the spacecrafts orbit, says Friedman, now the director of the Planetary Society.
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, says Johnson. It would have been the first spin-stabilized, free-flying solar sail to fly in space.5 Cosmos-1, however, was lost when the launch vehicle failed.
Meanwhile, NASA has also continued to dabble in solar sailing. Between 2001 and 2005, the Agency developed two different 20-meter solar sails (fabricated by ATK Space Systems and LGarde, Inc., respectively) 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 meters on one side. A NASA flight test is possible by the year 2010.6
Our primary objective is to demonstrate successful deployment of a lightweight solar sail structure in low Earth orbit, says Montgomery. The NanoSail-D would have felt two kinds of pressure: (1) aerodynamic drag from the wispy top of Earths atmosphere and (2) the pressure of sunlight. Unfortunately, Montgomerys teams hope of measuring both types of pressure as the sail circles Earth did not come to fruition.
What of Cosmos-2? The mission is a privately funded project, a partnership of The Planetary Society and Cosmos Studios. Work has begun at the Russian Space Research Institute on some Cosmos-2 spacecraft hardware. They are also studying possible launch configurations on a reliable launch vehicle.7
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, as stated by the great visionary Carl Sagans, along the shore of the cosmic ocean, 8 leading us closer to sailing among the stars. Future attempts will surely take us the rest of the way.
Twas all so pretty a sail it seemed
As if it could not be,
And some folks thought twas a dream theyd dreamed
Of sailing that beautiful sea.9
1,7 New developments on the road to Cosmos 2 by Dr. L. Friedman
2 Measuring Up to a Solar Sail- NASA feature story
3 Solar-sail mission reflects past and future - MSNBC
4 The History of Solar Sailing by Dr. L. Friedman
5,6 From Solar Sails: A Novel Approach to Interplanetary Travel, by Giovanni Vulpetti, Les Johnson (Author), Gregory L. Matloff.
8 From Cosmos, by Carl Sagan, page 5.
9 Excerpt from Winken, Blinken, and Nod, by Eugene Field, 19th century poet.
About the author
Dauna Coulter is an avid runner and writer with Schafer Corporation and supports NASAs Marshall Space Flight Centers Office of Strategic Analysis and Communications.