Solar sail technology is changing the way we think about deep-space missions and our ability to perform tricky spacecraft manoeuvres. Also called light or photon sails, this special form of spacecraft propulsion requires no fuel and operates using the power of the Sun.
To power a spacecraft, these sails use the pressure from radiation that is pushed against them from the Sun. This force is felt by any surface that is exposed to the Sun or even any type of electromagnetic radiation. The concept of radiation pressure was first imagined in 1619 when Johannes Kepler, a German astronomer and mathematician, noticed that the tails of comets in the night sky always pointed away from the Sun. Scotland’s James Clerk Maxwell adopted this observation in 1862, who proposed that electromagnetic radiation has momentum. This means that all radiation, like radiation from the Sun, can put a pressure on any surface that it touches. This theory was later proven through a variety of experiments in the early 1900's. Much like a sailboat that uses wind to push itself through the water, a solar sail uses the pressure from sunlight to move through space.
Typically, this force is too small for us to feel or use in everyday life. However, in the vacuum of space, this pressure cannot be ignored. Solar radiation exerts a force on all objects in our solar system, but it is the small objects that are the most heavily affected. This is because small bodies have a lower mass, so this pressure from radiation can be strong enough to change how these bodies move. The type of surface that the radiation is hitting influences the strength of the radiation pressure. For example, some surfaces absorb and reflect the incoming radiation better than others. Generally, solar sails reflect most of the radiation and the rest is absorbed.
This form of propulsion has allowed engineers to create new ways to change spacecraft trajectories and to imagine new possible deep-space mission concepts. These systems are low-thrust in nature, meaning that they do not provide strong, heavy forces to send the spacecraft that they are powering through space at high speeds. However, large sails can collect enough energy to propel a spacecraft for long distances. Since the earliest interplanetary spacecraft in the 1960's, this force has been accounted for in careful spacecraft designs. It has been used as a way to perform specific trajectory corrections, as done by NASA's Messenger probe when it was orbiting Mercury. These small and delicate manoeuvres allow for higher precision than typical fuel-powered thrusters because they give lower thrust and speed. Similarly, NASA's Mariner 10 spacecraft, launched in 1973, flew by the planets Mercury and Venus using solar pressure as a way to stabilise the spacecraft to save its valuable propellant.
NASA has developed more recent prototypes that only rely on this form of propulsion. Unfortunately the NanoSail-D (pictured below) was lost during a launch failure in 2008. This spacecraft was equipped with a 9-square-metre solar sail that folded for launch into a container comparable to the size of a loaf of bread. This was followed by the NanoSail-D2 in 2010, which was the agency’s first solar sail deployed in low Earth orbit and has helped us study new possible ways to use solar sails to bring back dead satellites and space debris.
A photo of the experimental solar sail, NanoSail-D (NASA/MSFC/D).
In 2010, the Japan Aerospace Exploration Agency launched the first spacecraft to succeed in solar sail flight. Named IKAROS, for “Interplanetary Kite-craft Accelerated by Radiation of the Sun”, the spacecraft hosts a solar sail measuring almost 200m2 (which is slightly smaller than a tennis court). The spacecraft completed its intended trajectory of a pass-by of Venus and has since had its mission extended to continue testing the sail’s capabilities. IKAROS is the first solar sail spacecraft that is fully propelled by sunlight (artist's depiction pictured below).
The non-profit organization The Planetary Society has developed the LightSail project to demonstrate this technology for small "Cubesat" satellites. The spacecraft, termed LightSail 2 measures 10cmx10cmx30cm, and operates with a kite-shaped sail that deploys to cover an area of 32m2 (comparable to the size of an average living room). The LightSail 2 is intended for launch on Space X's new Falcon Heavy rocket in late 2018 for operation in low-Earth orbit. A similar, smaller demonstration spacecraft, LightSail 1, was launched in 2015, which successfully deployed its solar sail.
When looking forward, successful testing and demonstrations of solar sail technologies will encourage future low-cost missions with longer lifetimes.