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Long ago, men sailed the seas with ships of wood and sails of cloth. Some were traders, hoping to bring Indian spices and Chinese silk back to their homelands. Some were explorers, searching for new and exotic lands to discover. Other men were pirates, searching for buried treasure.
Eventually the corners of the map were filled in, leaving no more places to explore, nor buried treasure to dig up (at least on Earth). The sea is still used for transporting goods around the world, but few sailors use sails to power their vessels, instead relying on diesel engines.
Like seafaring ships, current spacecraft also rely on chemical energy from rocket fuel to travel through space, but that may soon change. The next advance in space travel is a sail. The sail for a spacecraft, however, is not powered by the wind. Instead, it is powered by the light of the sun.
The solar sail concept was proposed almost 400 years ago by Johannes Kepler, a scientist known for his three laws of planetary motion. Kepler observed that the tails of comets point away from the sun, as if blown away by some sort of solar breeze. He believed that a sail could be used to harness that breeze and travel through space, just as a sailing ship used the wind to travel the seas. While there is no such thing as a solar breeze, scientists have discovered that sunlight does exert enough force to move objects.
The solar sail is composed of a lightweight material that is both highly reflective and temperature-resistant. The reflective nature of the material is fundamental to the propulsion of the sail. Light particles, known as photons, have no mass, but they carry a small amount of energy and momentum which is transferred to the sail as they reflect off of it. This constant barrage of photons provides a small but continuous thrust of acceleration to the sail, allowing it to eventually surpass the velocity of a spacecraft powered by chemical rockets.
Currently, three organizations have had success with their solar sail projects: the National Aeronautics and Space Administration (NASA), the Planetary Society, and the Japan Aerospace Exploration Agency (JAXA).
NASA built a small satellite, known as Nanosail-D2, which was launched in December 2010. Due to technical difficulties, the Nanosail-D2 did not deploy on schedule but eventually took off on its own in January 2011, where it successfully deployed its 10-square-meter solar sail and traveled around the Earth for 240 days. NASA is building another solar sail, the Near Earth Asteroid Scout, or NEA-Scout, scheduled for launch in 2018. The NEA-Scout will have a sail measuring 85-square-meters in area.
The Planetary Society built a nanosatellite, LightSail-A, and launched it in May 2015 aboard an Atlas 5 rocket. However, they experienced several communication outages, which delayed the deployment of the sail until June, when the command for deployment finally reached the LightSail-A. It engaged its 32-square-meter sail and was nearly fully unfurled when communications broke down again. Because the mission objective of LightSail-A was to test the deployment mechanism of the sail, the mission was considered a success. The Planetary Society has plans for a second nanosatellite, LightSail-B, to be launched in 2016. This time, the objective will be to test ways that sunlight can be used to propel and steer the spacecraft.
JAXA introduced a spacecraft in May 2010 that officially became the first spacecraft to successfully use a solar sail. IKAROS (Interplanetary Kite-craft Accelerated by Radiation of the Sun) was launched aboard a H-IIA rocket and passed by Venus in December 2010, fulfilling its mission objective. As of May 2015, IKAROS is still flying, 110 million kilometers away from Earth.
There’s a whole new world to be explored in outer space, and solar sailing is becoming the preferred method of travel.