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LightSail Mission FAQ

How does a solar sail work?

When the light from the Sun reflects off the surface of the solar sail, the energy and momentum of light particles known as "photons" are 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, unlike rockets using chemical engines that fire until the spacecraft reaches a cruising speed, when they are turned off to conserve fuel. A solar sail can accelerate constantly and 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.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 and create a force less than one percent of that from light pressure, so if we want to go really fast, we’ll travel with light.

What is the advantage of using a solar sail?

The great advantage of a solar sail is that it requires no fuel. A solar sail spacecraft can, in time, move the spacecraft even faster than a chemical rocket.  For a round trip to another planet and back, solar sails have great advantage since it doesn’t need to carry fuel for the return leg.

How thin are the reflective sails, and what material are they made of?

The sails are made of aluminized, reinforced Mylar™ 4.5 microns (.18 mil) thick, about 1⁄4 the thickness of a trash bag. The sail must be as light as possible to maximize the acceleration.

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 sunlight. The acceleration from sunlight is very small and depends on the size and weight of the sail and spacecraft.  For our LightSail-1, the acceleration from the solar force will be approximately 0.06 mm per second per second.

The real advantage of solar sailing is that, unlike a chemical rocket that applies a lot of thrust for a very short time, sunlight hitting the sail applies thrust continuously.  In 100 days, a sail-propelled craft could reach 14,000 kilometers per hour.  In just three years, a solar sail could reach over 150,000 miles per hour. At that speed, you could reach Pluto in less than five years.

What can a solar sail be used for?

Solar sails can be used to boost or decrease the orbits of spacecraft, hold a spacecraft in position to monitor the Sun for solar storms, provide stable Earth observation platforms, travel between the planets within our solar system, and someday take us to worlds around other stars.  However, once you get much beyond the orbit of Jupiter, energy from sunlight is too weak to keep you accelerating. Far away from the Sun, the highly focused beams of lasers can be directed at the sails to boost them onto interstellar trajectories.

How much does your spacecraft weigh and how is it configured?

LightSail-1 is an ultra-light spacecraft with a mass of less than 5 kilograms.  With a sail area of approximately 32 square meters, this spacecraft will actually have a lower mass-to-area ratio and hence a higher acceleration from sunlight than our earlier, larger Cosmos 1 spacecraft.  LightSail-1 takes advantage of nanosat technology and will be built with three Cubesat spacecraft, each measuring 10 cm by 10 cm by 10 cm.  One Cubesat will form the central electronics and control module, and two additional Cubesats will house the solar sail module.  The sail itself will consist of four triangular blades that together form a diamond shape resembling a kite.

What is the mission destination for LightSail-1?

Our goal is to fly in Earth orbit to demonstrate control and increase our velocity under sunlight pressure.

Will LightSail-1 be visible from Earth?

Details of LightSail-1’s visibility will depend on details of its orbit, which are not known at this time.  However, most possible orbits will enable viewing of LightSail-1 at some point.  In those cases, LightSail-1’s large reflective sail will act like a mirror.  As it moves, its mirror-like (specular) reflection will move across the surface of the Earth.  For locations where there is a mirror-like reflection, LightSail-1 will appear – for a brief period like a minute – to be much brighter than the brightest star or planet.

What is the LightSail program?

The LightSail program will launch three separate spacecraft over the course of several years, beginning with LightSail-1. Lightsail-2 will attempt a longer duration flight to higher Earth orbits, demonstrating that solar sails can increase their orbital energy and taking the next major step toward using solar sails for missions in and beyond Earth orbit.  The longer flight times will require that we further advance solar sail technology.  LightSail-3 will fly to the Sun-Earth Libration Point, L1, where solar sail spacecraft could be permanently placed as solar weather stations, monitoring the geomagnetic storms from the Sun that potentially endanger electrical grids on Earth as well as satellites in Earth orbit.

Who oversees the project?

Dr. Louis Friedman, Executive Director of The Planetary Society, is the Program Manager of LightSail. He was the program leader of the NASA Solar Sail program in the 1970s and is author of the book, Starsailing: Solar Sails and Interstellar Travel. The Project Manager of LightSail-1 is Jim Cantrell, CEO of Strategic Space Development, Inc, who is an experienced aerospace engineer and a veteran of The Planetary Society’s Cosmos 1 team

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