Solar sails have been touted as one of the most promising means of propelling spacecraft to the farthest reaches of our solar system. This August engineers at the Institute of Space and Astronautical Science in Japan successfully deployed two 30-foot-wide solar sails, each less than 1/3,000 of an inch thick, that had been carried into space aboard a solid-fuel rocket. At an altitude of 75 miles, one sail unfolded like a clover leaf. Then, at 100 miles, the other unfolded like a Japanese fan.
Solar sails gain thrust from the energy of solar photons reflecting off their mirrorlike surface. The force exerted by those photons is minuscule but persistent. Because the supply of sunlight is essentially limitless, a solar sail could keep accelerating long after a conventional rocket would run out of fuel.
The Japanese team constructed its two test sails out of polyamide, a plastic film strong enough to withstand temperature changes, radiation, and micrometeorites. Although neither sail remained aloft long enough to pick up any significant speed, simply deploying them in space was a crucial achievement. Project manager Junichiro Kawaguchi says that one of the key challenges is ensuring that the fragile sails unfurl without folds or wrinkles. “In our rocket experiment, the sails were deployed passively using natural spin motion,” he says. A truly autonomous solar sail will need an onboard mechanism to unfurl itself without the help of a spinning rocket. Kawaguchi and his teammates hope to test that device aboard a balloon flight in May.
Meanwhile, the Planetary Society in the United States and private Russian space organizations are developing Cosmos 1, the first fully functional solar sail. They currently plan to launch it in March, but this maiden flight has been canceled many times. The sail consists of eight 45-foot-long aluminized Mylar sheets. Initially orbiting 500 miles above Earth, Cosmos 1 will slowly increase its altitude by riding the pressure of solar photons.
Huge solar sails may someday propel ships big enough to carry passengers, Kawaguchi says: “Round trips between Earth’s vicinity and other planets are not a dream, but realistically possible.”