Solar sail is a large, lightweight structure designed to use the pressure of sunlight to move a spacecraft through outer space. It may also be called a light sail or a photon sail. A solar sail consists of a framework covered in thin reflective material. Solar sails may make it possible for spacecraft to travel much farther, more cheaply, and for longer periods than is possible using rockets.
Solar sails are designed to move spacecraft as sails move sailboats. The pressure of the wind against a boat’s sails pushes the boat through water. The “wind” in space takes the form of photons (particles of light) given off by the sun. When a photon strikes a solar sail, the mirrored surface reflects the photon back into space. Some of the photon’s momentum (amount of motion) is transferred to the sail according to the third law of motion developed by the English scientist Isaac Newton. That law states that for each action there is an equal and opposite reaction. As the solar sail reflects the photons back into space, the photons exert a force on the sail.
A solar sail-powered spacecraft would travel in orbit around a heavenly body, such as the sun or a planet. The solar sail could be angled so that the photons would push the craft in different directions. By reflecting photons backward, the sail could increase the craft’s momentum, carrying it farther away from the object. By turning to reflect photons forward, the solar sail could reduce the craft’s momentum, moving it closer to the object.
The need to carry large amounts of heavy fuel limits how far and how fast rockets can travel. A spacecraft powered by solar sails would not need to carry fuel.
Characteristics of solar sails.
Solar sails must be large enough to catch many photons because the momentum carried by an individual photon is extremely small. Some sails could have as much area as 1 to 10 football fields. To obtain as much speed as possible, solar sails also must be lightweight. Some experimental sails have been made of material that is about one-third the thickness of typical plastic wrap. Solar sails must also be near-perfect reflectors. The more reflective the sails are, the more the photons’ momentum is converted into motion rather than being absorbed by the sails as heat.
Engineers have proposed sails in circular, square, and heliogyro shapes. A heliogyro is a pinwheel shape similar in design to a helicopter’s rotor.
The power produced by solar sails would not be sufficient to lift a spacecraft out of Earth’s gravitational pull and thick atmosphere. Instead, a solar sail-powered spacecraft could be launched into outer space on a conventional rocket. The sails and their frame would have to be packaged compactly to be launched on a rocket. The spacecraft would separate from the rocket once it reached a sufficient distance from Earth. In the future, solar sails could also be built by machines in space.
At first, a solar sail-powered craft would move at a slow pace compared with a rocket. However, the craft could travel for a longer period of time than a conventional rocket could because the sun would provide unlimited fuel.
A solar sail could carry a craft on comparatively short voyages, such as to Mercury or Mars. A solar sail-powered craft would also be suited for travel across vast distances beyond the solar system. Such a craft would slowly continue to increase speed, eventually moving faster than a conventional rocket. In time, however, the craft would travel too far from the sun to obtain sufficient sunlight. Scientists believe two methods could be used to give such a craft more speed. One method would be to send the craft close to the sun to gain as much momentum as possible before leaving the solar system. Another method would be to supply photons using banks of lasers in orbit around the sun or Earth.
Experimenting with solar sails.
On May 21, 2010, the Japanese Aerospace Exploration Agency successfully launched IKAROS (_I_nterplanetary _K_ite-craft _A_ccelerated by _R_adiation _o_f the _S_un), the first space probe propelled solely by a solar sail. IKAROS has a square sail measuring about 14 meters (46 feet) per side. Six months after launch, IKAROS performed a flyby of the planet Venus. Later that same year, NASA launched a satellite called the NanoSail-D. The satellite’s square sail was smaller than that of IKAROS, measuring about 3 meters (10 feet) per side. It spent more than 240 days in orbit around Earth.
On May 20, 2015, the craft LightSail-A (now called Lightsail 1) was launched into orbit. The LightSail project is funded by the Planetary Society, a non-profit space advocacy group headed by Bill Nye , and individual donors. Malfunctions plagued the mission, but the craft managed to deploy its solar sails. In June 2019, the Planetary Society launched the LightSail 2 aboard SpaceX’s Falcon Heavy rocket. It successfully deployed its solar sails and became the first steerable spacecraft to orbit Earth using sunlight as its power source.
