Solar Sailing in Space

For the first time, NASA is preparing to send into orbit a small satellite that can be propelled by solar sails. When light particles from the sun strike the surface of the sail, the energy is transferred to it, providing a propulsive force that moves the satellite through space.

NASA’s goal is to test the complex deployment mechanism of the 10-square-meter sails, says Dean Alhorn, an engineer at NASA’s Marshall Space Flight Center, in Huntsville, AL, and the lead engineer on the project. “A successful flight will not only make for a unique historical event, but will show that we have a reliable mechanism to deploy a solar sail in space for future missions,” says Alhorn.

The satellite, called NanoSail-D, is scheduled to launch from Omelek Island, in the Pacific Ocean, on July 29 onboard the Falcon 1 rocket developed by Space Exploration Technologies (SpaceX), of Hawthorn, CA. The NanoSail-D satellite’s main frame is only 30 centimeters long and weighs nine pounds. Its solar sail is made of a custom polymer that is thinner than a piece of paper and coated with aluminum to reflect the photons. “It looks like Saran Wrap with a metalized surface but is stronger and suited for the space environment,” says Alhorn.

In theory, a solar sail could be used as propulsion for round-trip missions in the solar system. In friction-free space, the tiny propulsive force of photons could conceivably get a craft up to about 100 miles per hour in a day and nearly 100,000 miles per hour in three years. Changing the sail’s angle can change the craft’s trajectory. “There is a lot of potential for solar-sail propulsion once we show that this can be deployed in space,” says Alhorn. “Already we are working on ways to maneuver the sails, and we can theorize better designs that are based on proven technology.”

The concept of solar sailing was invented in the 1920s by two Russian scientists, and it has been the subject of a few projects over the years, says Louis Friedman, the executive director of the Planetary Society, a public space organization based in Pasadena, CA. The Russians deployed a large reflective sheet of material outside their Mir space station in 1992, and the Japanese did something similar in 2004, but neither was used for solar-sail propulsion.

The most recent effort was led by Friedman, whose team from the Planetary Society and from Cosmos Studios, a media company based in Ithaca, NY, actually built a solar-sail-powered spacecraft in Russia called Cosmos 1. It did not launch because of rocket failures. Now, Friedman is working on a satellite called Cosmos 2 that is similar to NASA’s design but uses inexpensive Mylar, a basic plastic material. “Mylar is easy to get, is manufactured in large quantities, and is adequate for short flights,” says Friedman. “If you want to do an interplanetary mission, which is part of NASA’s future plans, you would need something longer lasting and more ultraviolet resistant, so you would use a more exotic material.”

But the most complex part may be deploying sails after a spacecraft has been launched out of the earth’s atmosphere. Once the NASA satellite is aloft, a computer will command a heater to burn a high-strength fishing line to open four spring-hinged panels, exposing the solar sail. Fifteen seconds later, another so-called burn-wire system will cause four booms to unfold. The booms will pull the solar sail off a center spindle, unrolling it in four different quadrants, says Alhorn. The satellite will remain in low earth orbit for between five days and two weeks, during which researchers will track and analyze the satellite.

The deployment mechanism is the most interesting part of the spacecraft, says Friedman. His craft will use an inflatable deployment system to expand the sail to a diameter of 30 meters. Unlike with NanoSail-D, his plan is to control the satellite with the sail.