Light Sails and Space Exploration

A solar sail
A solar sail set against the backdrop of the Milky Way Galaxy (artist's concept). The Planetary Society

Imagine a spacecraft that sails through space using light from the Sun as a propellant. Sounds like a story from the future, right? It turns out, however, that solar sail technology has flown, and the principles of using solar radiation to guide spacecraft are well-known to mission planners.  What's more, groups of scientists are foreseeing more solar sail exploration, including sending a fleet of tiny spacecraft to the star Alpha Centauri. If this happens, we could have probes in interstellar space after a trip of about 20 years!

The first solar sail was flown by the Japan Aerospace Exploration Agency in 2010; it was called IKAROS (short for Interplanetary Kite-craft Accelerated by Radiation of the Sun). The mission went to Venus, and was a successful test of the concept. The idea of using solar radiation pressure to help maintain attitude control of a spacecraft got a workout with the Mariner 10 mission to Merucry and Venus, and on the MESSENGER mission to Mercury. 

NASA leapt into the solar sail race by successfully launching NanoSail D2 for deployment in low Earth orbit. It worked for 240 days and allowed scientists to gather much-needed information about how to use this technology. NASA continues to research this useful technology.

After several years of trying, the Planetary Society launched its LightLight Sail spacecraft, which eventually unfurled a thin Mylar sheet to help propel it across space. It was a big step forward for proponents of this unique type of propulsion system. It sent valuable data and images before plunging back to Earth and burning up in the atmosphere on June 14, 2015. 

Why Solar Sails? 

As scientists on Earth prepare for more extensive and complex space missions to other planets, they always run into the same problem to solve: how to get explorers and equipment from Point A to Point B in space. Getting things to space requires booster rockets. But, you don't need those in space.

This is where light sails come in. Solar sail spacecraft can be used to move payloads from Earth orbit to other planets, such as missions to Mars. That could be very useful for missions where building materials and other equipment can be sent on fast trips and be waiting when the humans arrive to take up residence. The sail can then be sent back to Earth to ferry more materials. 

How Do Solar Sails Work?

Solar sails rely on a phenomenon called "radiation pressure" of light from the Sun. (This is not the same as radiation dangers to astronauts.)  Think of sunlight giving a "push" to the solar sail, which WANTS to feel this pressure. Given enough solar radiation, a solar sail-equipped spacecraft gets the benefit of a low-thrust (and relatively free) method of propulsion. 

If you placed a solar sail out in space at the same distance as the Earth is from the Sun (1 astronomical unit (AU)) the sunlight it receives produces about 1.4 kilowatts of power. Now, take  that 1.4 kw and divide it by the speed of light (186,252 miles per hour, or 300,000 meters per second) the constant force of sunlight on the spacecraft's solar sail could accelerate it up to speeds five times faster than a typical rocket could deliver. That's a considerable amount of power hidden inside sunlight! 

A solar sail needs to be very thin, much thinner even than a sheet of fine paper. It must also be aluminized for reflectivity, and must be able to survive under extreme conditions.

Materials such as Mylar are good solar sail material. Photons of light bounce off the sail and since the solar radiation pressure is constant, that gives the sail a constant source of the push it needs to move along. Solar sails pick up quite a bit of speed, and some scientists suggest that a solar sail could get up to a tenth of the speed of light, given the right conditions. And, when you get high speeds, then interstellar travel becomes a distinct possibility!