In a few days, the Planetary Society’s LightSail 2 will be launched on a SpaceX Falcon Heavy rocket to test a unique approach to orbital travel around Earth. By using a solar sail, and no other means of propulsion, the loaf-size craft will move around our planet, even changing its orbit.
Solar sails work by harnessing the momentum of light particles called photons. To be efficient, the material of the sail needs to be super-light – a development that has only become possible in recent decades. Despite this, solar sails are not a new idea. In fact, Kepler made of note of it in 1608.
The mission, which is crowdfunded, will take LightSail 2 to an orbit of about 720 kilometers (45 miles) high. The craft will be stored in a suitcase-sized spacecraft called Prox-1 for a week to give time for the other spacecraft carried by the rocket to spread out.
Afterward, Prox-1 will open its front door and a large spring will propel LightSail 2 into deep space. The craft will circle our planet for a few days so that the team can assess its functions and begin operations to eventually deploy the solar sail. This will be roughly two weeks after the launch.
Then, the fun part begins. For about a month, LightSail 2 will be on a raising orbit mission. Thanks to sunlight, the craft will slowly but surely increase its semi-major axis by several hundred meters every day. The goal of the mission is to demonstrate that solar sails are a valid method to propel small, low-cost satellites. The craft is not designed to maintain a circular orbit, so when one side of the orbit grows, the other shrinks. Eventually, the craft will be slowed down by the (very tenuous) atmosphere and fall back to Earth, burning during re-entry.
As the name indicates, LightSail 2 has a predecessor. They share similarities, but the new mission will go to a higher orbit than LightSail 1 and the new design has upgrades that will allow better maneuverability of the sail as well as improved tracking of the craft.
The solar sail is made of mylar, a special polyester film. It is 32 square meters (344 square feet) but is only 4.5 microns thick, thinner than your average spider web strand. It is designed with the danger of micrometeorites and small space debris in mind, so there are seams every few inches to make sure a rip won’t spread across the whole sail.
Several missions with solar sails are being planned, including NASA's NEOScout and Breakthrough Starshot. The Japanese mission IKAROS was the first one to show solar sail interplanetary spacecraft was possible when it reached Venus in 2010.