A solar sail is a propulsion technology for spacecrafts that uses electromagnetic radiation—primarily sunlight, with the possibility of using laser when it is far from sunlight—for actuation. The idea of using sails traces back to Kepler, and the concept of using solar power for space travel can be traced to Tsiolkovsky, the famous scientist, and his co-worker, Tsander. In fact, after the rocket, the solar sail is perhaps the most notable and promising method of propulsion in space.
The fundamental concept used in solar sailing is also not new, as Maxwell had shown us in the 1860’s that light exerts pressure. The light particles (photons) can therefore move the sail by transferring momentum to it, as long as the sails have a reflective surface for easy illumination. Since there is no shortage of photons from sunlight in space, a constant pressure is created on the sail that provides a constant acceleration to the spacecraft.
The force experienced by a conventional rocket is greater than a spacecraft propelled by solar sails. However, the constant acceleration produced on the solar-sail spacecraft increases its velocity over time. For this reason, it is imagined as the fabled tortoise in the space race.
Nonetheless, it must be noted that while the deployment of a solar sail should be possible in space, it is challenging to release a purely sunlight driven kite-like spacecraft directly from a low Earth orbit. A second rocket would typically be necessary to launch the sail.
Solar sails were considered by NASA in the 1970s. However, it was technologically too risky at the time. Later, NASA revisited solar sails with its partner, Able Engineering, and successfully demonstrated the deployment of solar sails in a vacuum environment during their testing period from April to May 2004. The same year, NASA and L’Garde, Inc., deployed their solar sail system successfully, and Japanese Aerospace Exploration Agency (JAXA) launched two large solar sails and deployed them through space.
The Planetary Society planned to launch its solar sail, Cosmos 1, in 2005. If the mission had been successful, it would have been the first of its kind to put a solar sail into orbit. Unfortunately, the launch vehicle did not reach its orbit due to a rocket failure.
JAXA launched IKAROS (Interplanetary Kite-craft Accelerated by Radiation Of the Sun) in 2010, the world’s first interplanetary spacecraft powered primarily by solar sails. IKAROS completed its mission six months after its launch in December 2010 when it passed by Venus; it was reported to be still alive in 2012.
A giant solar sail called the Sunjammer was due to be launched in January 2015 but it was cancelled last year. However, solar sails have been making the news again lately because the solar-sail spacecraft, LightSail-1 (formerly known as NanoSail-D), developed by the Planetary Society, will be tested in May 2015; a second LightSail spacecraft is expected to launch in 2016. These solar sail projects, in addition to the much-awaited James Webb Space Telescope of 2018, should make the next few years quite exciting.