A new kind of spacecraft that sails on sunlight has just been successfully deployed above Earth. The LightSail satellite yesterday overcame a series of glitches to unfurl its solar sails, a propulsion system that’s entirely different to traditional rockets, thrusters or even solar panels.
The prototype craft, built by the US Planetary Society and funded by public donations, is testing the sail deployment mechanism. Another mission due to be launched next year will trial the full propulsion system. And it’s not the first spacecraft to fly using solar sailing. That honour went to the Japanese craft Ikaros in 2010. But the growing interest in the technology creates some exciting possibilities for the future of space travel.
Solar sailing is the space equivalent of boat sailing. A boat is pushed along by the air particles that strike its sail when the wind blows. The idea behind solar sailing is the same except, instead of wind, particles of light emitted by the Sun drive the craft forward.
Light is made up of moving particles known as photons that can be reflected by a mirror. When this happens, the mirror experiences a pressure across its surface called solar radiation pressure. This pressure is very small, so you couldn’t feel it just by holding up a normal mirror in sunlight while on Earth. But in the vaccuum of space it’s enough to move a very light mirrored sail.
To understand why solar sailing is important, we need to go back to the emptiness of space. Propulsion is the ability to accelerate a vehicle. Vehicles on Earth accelerate by interacting with something around them. For example, a car accelerates by producing a force on the tarmac.
In space, however, there is no medium to interact with. This is where rocket propulsion comes in. By ejecting propellant in one direction, the rocket is pushed in the opposite direction. This is because of the scientific law (Newton’s Third Law of Motion) that says every time an object exerts a force on another object it experiences an equal and opposite force.
The catch is the propellant on a spacecraft is limited, and once it is all used up, no further no acceleration is possible. Because of this, some distant targets in space are impossible to reach with pure rocketry. Solar sailing, instead, goes elegantly around this this problem by not requiring any propellant mass at all. It is like having a rocket that can thrust for an indefinite amount of time, for free.
