How do Solar Sails Work

A cosmic kite blown by photons is our greatest hope for interstellar travel.

When the Space Shuttle fuels up for a short commute to the International Space Station, 95 per cent of its weight is in the gas tank. The sheer weight of rocket fuel is one of the greatest obstacles to interstellar space travel. That’s why space futurists are so excited about solar sails, a ‘fuel-free’ craft powered by beams of sunlight.

Sunlight travels in packets of energy called photons. When a photon reflects off a mirrored surface, it imparts two minuscule taps of energy: once during the initial impact and once as it’s reflected. For decades, scientists theorized that if you could make a reflective surface big enough and light enough, it could be nudged through space by a constant barrage of photons.

In 2010, that theory will be tested when the Planetary Society, co-founded by the late Carl Sagan, will launch a 350-square-foot solar sail made of aluminized Mylar (1/5,000 of an inch thick) into space.

Solar sails don’t have dramatic blast-offs, but rely on a more patient form of power: constant acceleration.

A massive solar sail of 600,000 square metres would accelerate at an underwhelming one millimeter per second. After a day, however, the sail would be moving at a rate of 310kps (195mph). After 12 days, it would reach 3,700kps (2,300mph).

Imagine its velocity after six months – enough, scientists hope, to sail out of our solar system into the great beyond.