Setting sail,in space

“Sailing on light is the only technology that can someday take us to the stars,” said Louis Friedman,director of the Planetary Society,the worldwide organisation of space enthusiasts.

Even as the National Aeronautics and Space Administration continues to flounder in a search for its future,Friedman announced that the Planetary Society,with help from an anonymous donor,would be taking baby steps toward a future worthy of science fiction. Over the next three years,the society will build and fly a series of solar-sail spacecraft dubbed LightSails,first in orbit around the Earth and eventually into deeper space. The voyages are an outgrowth of a long collaboration between the society and Cosmos Studios of Ithaca,New York,headed by Ann Druyan,a film producer and wife of the late astronomer and author Carl Sagan.

“Sails are just a marvellous way of getting around the universe,” said Freeman Dyson,of the Institute for Advanced Study in Princeton,New Jersey,and a longtime student of the future,“but it takes a long time to imagine them becoming practical.”

The solar sail receives its driving force from the simple fact that light carries not just energy but also momentum—a story told by every comet tail,which consists of dust blown by sunlight from a comet’s core. The force on a solar sail is gentle,if not feeble,but unlike a rocket,which fires for a few minutes at most,it is constant. Over days and years a big enough sail,say a mile on a side,could reach speeds of hundreds of thousands of miles an hour,fast enough to traverse the solar system in five years. Riding the beam from a powerful laser,a sail could even make the journey to another star system in 100 years,that is to say,a human lifespan.

Whether humans could ever take these trips depends on just how starry-eyed one’s view of the future is. Friedman said it would take too long and involve too much exposure to radiation to sail humans to a place like Mars. He said the only passengers on an interstellar voyage—even after 200 years of additional technological development—were likely to be robots or perhaps our genomes encoded on a chip.

In principle,a solar sail can do anything a regular sail can do,like tacking. Unlike other spacecraft,it can act as an antigravity machine,using solar pressure to balance the Sun’s gravity and thus hover anyplace in space. And it does not have to carry tonnes of rocket fuel.

“We ought to be doing things that are romantic,” said Friedman,adding that nobody knew yet how to build sails big and thin enough for serious travel. At one time or another,many of NASA’s laboratories have studied solar sails. Scientists at the Jet Propulsion Laboratory even once investigated sending a solar sail to rendezvous and ride along with Halley’s Comet during its pass in 1986. But efforts by the agency have dried up as it searches for dollars to keep the human spaceflight programme going,said Donna Shirley,a retired JPL engineer and former chairwoman of the NASA Institute for Advanced Concepts.

Japan continues to have a programme,and test solar sails have been deployed from satellites or rockets,but no one has ever gotten as far as trying to sail them anywhere. An effort by the Planetary Society and the Russian Academy of Sciences to launch a sail about 100 feet on a side,known as Cosmos-1,from a Russian missile submarine in June 2005 ended with what Druyan called “our beautiful spacecraft” at the bottom of the Barents Sea.

Druyan and Friedman were beating the bushes for money for a Cosmos-2,when NASA asked if the society wanted to take over a smaller project known as the Nanosail. These are only 18 feet on a side and designed to increase atmospheric drag and thus help satellites out of orbit.

And so LightSail was born. Its sail,adapted from the Nanosail project,is made of aluminized Mylar about one-quarter the thickness of a trash bag. The body of the spacecraft will consist of three miniature satellites known as CubeSats,4 inches on a side,which were first developed by students at Stanford and now can be bought on the Web,among other places. One of the cubes will hold electronics and the other two will carry folded-up sails,Friedman said.

Assembled like blocks,the whole thing weighs less than 5 kg,or about 11 pounds. The LightSail missions will be spread about a year apart,starting around the end of 2010. Friedman said the first flight,LightSail-1,would be a success if the sail could be controlled for even a small part of an orbit and it showed any sign of being accelerated by sunlight. The next flight will feature a larger sail and will last several days. For the third flight,Friedman and his colleagues intend to set sail out of Earth orbit with a package of scientific instruments to monitor the output of the sun and provide early warning of magnetic storms. The plan is to set up camp at a point where the gravity of the Earth and sun balance each other—called L1,about 900,000 miles from the Earth—a popular place for conventional scientific satellites. That,he acknowledges,will require a small rocket,like the attitude control jets on the shuttle,to move out of Earth orbit,perhaps frustrating to a purist. But then again,most sailboats do have a motor for tooling around in the harbour,which is how Friedman describes being in Earth orbit. Because the direction of the sun keeps changing,he said,you keep “tacking around in the harbour when what you want to do is get out on the ocean.”

The ocean,he said,awaits.