Sunshine Sails

ON A REMOTE ISLAND in the South Pacific, a black box the size of a loaf of bread houses what could be the future of space flight. The precious cargo is the National Aeronautics & Space Administration's NanoSail-D, a small satellite equipped with a solar sail that's scheduled to launch into orbit aboard SpaceX's Falcon 1 rocket sometime between July 29 and Aug. 6. If all goes according to plan, three days after the launch, NanoSail-D will unfurl its shiny wings into a 10-m² square and become the first orbiting demonstration of solar sail technology.

Just about everything we've sent into space has been propelled by a rocket. But a rocket can carry an object only so far before it runs out of fuel. Solar sails get a continuous push from the sun's photons.

Although a solar sail would initially travel much slower than a rocket, the constant press of photons would eventually give it a greater speed. For example, it should take only a decade for an appropriately sized solar sail to catch up with the Voyager probes, which have spent the past 30 years making their way to the edge of the solar system.

NanoSail-D won't be going that far. In fact, the satellite will use its sails simply to maneuver in orbit for a few days. Still, the scientists and engineers behind the mission say it's an important next step in proving that solar-sail-powered space flight is possible.

Previous solar sail missions have had limited success. In 2004, a team from the Japanese Institute of Space & Astronautical Science deployed two prototype solar sails in space, but neither was used for propulsion. The following year, the Planetary Society's Cosmos 1 spacecraft lost the chance to try out its solar sail technology when its launching rocket failed to reach orbit.

To sail a ship through the heavens, it takes a special kind of material. ManTech SRS Technologies, based in Huntsville, Ala., made NanoSail-D's gossamer sails. Greg Laue, who manages spacecraft products for the engineering firm, tells C&EN that a solar sail material needs to be highly reflective, lightweight, extremely thin, and durable enough to stand up to the sun's heat and ultraviolet radiation. It has to be flexible so that it will fold neatly inside the package that carries it into space, yet the sail material must also be strong so that it doesn't tear as it unfurls.

Engineers have tried aluminum-coated Mylar, but Laue says Mylar's polyethylene terephthalate polyester doesn't hold up well in the intense radiation of space.

For NanoSail-D, Laue and coworkers at ManTech SRS turned to a fluorinated polyimide called CP-1. CP-1 was originally developed as a replacement for Mylar coatings on large spacecraft, according to Anne K. St. Clair, a retired chemist who invented the polymer as part of a team at NASA's Langley Research Center, in Hampton, Va.

St. Clair decided to investigate variants of Kapton, an amber-colored polyimide that tolerates high temperatures. She needed a colorless polymer, so St. Clair did some digging through the literature and found that Kapton's color arises from electron transfer between the polymer's chains. St. Clair began adding bulky or repellent substituents to the polymer in order to keep the strands away from each other. She found that fluorine substituents worked best, creating a colorless, soluble polymer that's resistant to high temperatures and UV radiation.

"We knew CP-1 could handle the rigors of a space environment," Laue says. To turn it into a solar sail, the ManTech SRS engineers developed a continuous-roll production process for making the polymer extremely thin, and they figured out a way to vapor-coat it with aluminum so that it would be reflective.

A SWATCH of the sail material was sent to C&EN by Laue. It looks like the inside of a potato chip bag, and at just 2.5 µm thick, it's about four times thinner than common plastic wrap. "The slightest whiff of air will send the material flying," Laue notes. That gossamer quality is critical to making a successful solar sail. It's basic physics. "Out in space the pressure from sunlight pressing on the surface is very small," Laue says, so the less mass a structure has, the greater its acceleration will be.

"The aluminized CP-1 is the best option we have today" for solar sails, says David Murphy, chief of research at ATK Space Systems, in Goleta, Calif. Murphy, a designer of solar sail systems, notes that while existing materials will be useful for solar sails as large as 100 m², new materials that can be made even thinner will probably be needed in the future. "The really exciting missions will be possible if we have thinner sail materials and systems that can scale from 100 m² to 500 m²," he says.

For now, though, solar sail enthusiasts are just hoping that NanoSail-D's launch and deployment goes smoothly so that the satellite has a chance to spread its wings and fly.