Carl Sagan’s solar sail spacecraft is finally getting a real-world test

By Ryan Whitwam

The Planetary Society is preparing to test a spacecraft proposed by legendary astronomer (and founder of the Planetary Society) Carl Sagan. The vessel is called LightSail, and as you might expect, it uses a light sail for propulsion. The fascinating design has been successfully tested on Earth, but now it’s going to be launched to the upper atmosphere to test the deployment of its huge mylar sails in flight.

Solar sail technology relies upon a well-understood fact of spaceflight. While light doesn’t have mass, it does have momentum, which can be transferred to a vessel. It’s something space agencies have had to correct for since the early days of spaceflight. Of course, the effects on a small spacecraft are almost nil, but that’s why LightSail has big sheets of mylar. Basically, as photons make contact with the solar sail material, some of it is absorbed, while the rest is reflected. This exerts a small amount of pressure on the sail — enough to push a craft along.

This isn’t the first time a solar sail craft has been used, as both Japan and the US have deployed small solar sails in the past. LightSail, on the other hand, will have 32 square meters of mylar sail material. In addition to the sail, the craft needs to carry a comm system, batteries, solar panels, a computer, and other flight hardware. It all fits in a tiny 3U cubesat body that’s about the same size as a loaf of bread.
Solar sails fall into the same category as ion engine technology in that they are very low thrust, but highly efficient. In the case of solar sails, you don’t need to bring any fuel at all, so it’s really infinitely efficient if you don’t mind the long acceleration times. You get just a few newtons of force from even a large solar sail, which is why some of the proposed designs for interplanetary sails have surface areas many times bigger — in the hundreds or thousands of square meters. Scientists are still working on how you’d deploy something that size.
These bigger solar sail designs could reach respectable speeds with enough time to accelerate. It is estimated a large solar sail might reach a significant fraction of the speed of light. Some proposed designs that could be built with current technology might hit 20,000 meters per second during a transfer to one of the outer planets.
The Planetary Society’s Linux-powered LightSail probe won’t be large enough to get to other planets in the solar system, but it’s a step in that direction. The upcoming May 20th test will place it just high enough above Earth that it can deploy the sails and perform a system check before falling back down. If all goes as planned and the sails deploy correctly, The Planetary Society will start preparing for a real mission in April 2016 that would see the craft deployed in space where it can put that sail to real use.