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What are Solar Sails?
Last Updated on:  06/25/2015 06:31 AM

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How does a solar sail work? 
A solar sail is a spacecraft with a large, lightweight mirror attached to it that moves by being pushed by light reflecting off of the mirror instead of rockets.

When the light from the Sun reflects off the surface of the solar sail, the energy and momentum of light particles known as "photons" is transferred to the sail. This gives the sail a "push" that accelerates it through space. Although the acceleration is very slight, it is also continuous, enabling the sail to reach very high speeds in a relatively short time. The direction of the push is controlled by the angle of the sail with respect to the Sun, adding to or subtracting from the orbital velocity.

How does light push a solar sail?
Photons, which are "particles" of light, bounce off the reflective material of the sail. (Newton's Third Law of Motion states that for every action there is an equal and opposite reaction.) The reaction here causes a change in momentum, pushing the sail and accelerating the spacecraft. A photon reflecting off the mirror-like surface of a solar sail gives the sail a double kick -- a push equal to twice the photon's momentum (one push from the sail stopping the photon and one from it reflecting the photon and accelerating it away).

Does a solar sail fly on the solar wind?
No! The solar wind is made up of ionized particles ejected by the Sun. These particles move much slower than light. A solar sail does not stop or reflect them, although they also may impart some of their momentum to the solar sail. However, the force from the solar wind is less than one percent of that from light pressure.

How fast does a solar sail go?
The speed of an interplanetary solar sail spacecraft will depend on how long it has been propelled by the pressure of sunlight. The acceleration from sunlight is very small -- approximately five ten-thousandths of a meter per second per second, depending on the size and weight of the sail and the spacecraft. Over one day, that is a velocity increase of 45 meters per second or about 100 miles per hour.

But the real advantage of solar sailing is that, unlike a chemical rocket that applies a lot of thrust but only for an instant, sunlight hitting the sail applies thrust continuously. In 100 days a sail could reach 16,000 kilometers per hour (10,000 miles per hour); in one year it could reach 58,000 kilometers per hour (36,000 miles per hour). In just three years, a solar sail could reach a speed of over 160,000 kilometers per hour (100,000 miles per hour). At that speed you could reach Pluto in less than five years. In comparison, the New Horizons misson to Pluto, using chemical propulsion and a gravity-assist from Jupiter, is planned to take nine years to reach its target.

Still, 160,000 kilometers per hour (100,000 miles per hour) is still only 0.00015 the speed of light. It would take about 1,000 years for a solar sail to reach one-tenth the speed of light, even with light shining on it continuously. This emphasizes just how hard interstellar flight is. It will take advanced sails much thinner than today’s technology, plus a laser power source in space that can operate over interstellar distances to reach one-tenth the speed of light in less than 100 years. Some researchers of beamed-power sailing think that use of high-temperature materials may make such speeds possible in a few decades.

What can a solar sail be used for?
Solar sails can be used to boost or decrease the orbits of spacecraft, travel between the planets within our solar system, and someday may take us to worlds around other stars.
However, once you get much beyond the orbit of Jupiter, energy from sunlight is too weak. When far from the Sun, lasers can be directed at the sails. Lasers stay in a tight beam so that most of their energy can be imparted to the sail and not diffused into space. Very large lasers in Earth orbit or in the inner solar system could be used to help us travel to other stars. In the future, people may travel to distant stars using laser powered solar sails.

What is the advantage of using a solar sail?
The great advantage of a solar sail is that it requires no fuel. Today, we use chemical rockets to give our spacecraft a quick boost into Earth orbit, and then out of Earth orbit. The spacecraft then coasts most of the way to its destination, with some small blasts from thrusters to adjust its trajectory. This requires a lot of fuel. Solar sails give a very low thrust, but they can work continuously, pushing spacecraft faster and faster. A solar sail can, in time, move the spacecraft even faster than a chemical rocket. For a round trip solar sails have great advantage since no fuel is needed for the return.

Can a solar sail only provide thrust away from the Sun?
No, thrust can be generated inward or outward with respect to the sun. By turning the sail at different angles, we can add or subtract velocity to the spacecraft. When we add velocity, the sail flies away from the Sun. When we subtract velocity, its orbit spirals inward.

Why hasn't anyone flown a solar sail before?
No one had been able to organize a simple flight using the very low cost launch vehicles in Russia. The Planetary Society has unique international team building capability, a willingness to take risk, and accepts the very limited objective of a first solar sail flight.

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