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What is a Solar Sail?

A solar sail, simply put, is a spacecraft propelled by sunlight. Whereas a conventional rocket is propelled by the thrust produced by its internal engine burn, a solar sail is pushed forward simply by light from the Sun. This is possible because light is made up of packets of energy known as “photons,” that act like atomic particles, but with more energy. When a beam of light is pointed at a bright mirror-like surface, its photons reflect right back, just like a ball bouncing off a wall. In the process the photons transmit their momentum to the surface twice – once by the initial impact, and again by reflecting back from it. Ever so slightly, propelled by a steady stream of reflecting photons, the bright surface is pushed forward.

A solar sail is made up of just such a reflective surface, or several surfaces, depending on the sail’s design. When the bright sails face the Sun directly, they are subjected to a steady barrage of photons that reflect off the shiny surfaces and impel the spacecraft forward, away from the Sun. By changing the angle of the sail relative the Sun it is possible to affect the direction in which the sail is propelled – just as a sailboat changes the angle of its sails to affect its course. It is even possible to direct the spacecraft towards the Sun, rather than away from it, by using the photon’s pressure on the sails to slow down the spacecraft’s speed and bring its orbit closer to the Sun.

In order for sunlight to provide sufficient pressure to propel a spacecraft forward, a solar sail must capture as much Sunlight as possible. This means that the surface of the sail must be big – very big. Cosmos 1 is a small solar sail intended only for a short mission. Nevertheless, once it spreads its sails even this small spacecraft will be 10 stories tall, as high as the rocket that will launch it. Its eight triangular blades are 15 meters (49 feet) in length, and have a total surface area of 600 square meters (6500 square feet). This is about one and a half times the size of a basketball court.

For a true exploration mission the requirements are far greater: when a NASA team in the 1970s, headed by Louis Friedman, suggested using a solar sail spacecraft for a rendezvous with Halley’s comet, they proposed a sail with a surface area of 600,000 square meters (6.5 million square feet). This is equivalent to a square of 800 meters (half-mile) by 800 meter – the size of 10 square blocks in New York City!

Even with such a gigantic surface, a solar sail spacecraft will accelerate very slowly when compared to a conventional rocket. Under optimal conditions, a solar sail on an interplanetary mission would gain only 1 millimeter per second in speed every second it is pushed along by Solar radiation. The Mars Exploration Rovers, by comparison, accelerated by as much as 59 meters (192 feet) per second every second during their launch by conventional Delta II rockets. This acceleration is 59,000 times greater than that of a solar sail!

But the incomparable advantage of a solar sail is that it accelerates CONSTANTLY. A rocket only burns for a few minutes, before releasing its payload and letting it cruise at a constant speed the rest of the way. A solar sail, in contrast, keeps on accelerating, and can ultimately reach speeds much greater than those of a rocket-launched craft. At an acceleration rate of 1 millimeter per second per second (20 times greater than the expected acceleration for Cosmos 1), a solar sail would increase its speed by approximately 310 kilometers per hour (195 mph) after one day, moving 7500 kilometers (4700 miles) in the process. After 12 days it will have increased its speed 3700 kilometers per hour (2300 mph).

While these speeds and distances are already substantial for interplanetary travel, they are insignificant when compared to the requirements of a journey to the stars. Given time, however, with small but constant acceleration, a solar sail spacecraft can reach any desired speed. If the acceleration diminishes due to an increasing distance from the Sun, some scientists have proposed pointing powerful laser beams at the spacecraft to propel it forward. Although such a strategy is not practicable with current technology and resources, solar sailing is nevertheless the only known technology that could someday be used for interstellar travel.

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