A particle of light transfers up to nearly twice its momentum to an object it bounces off of. After 100 days, a solar sail could reach 14,000 kilometers per hour; after three years it could be zipping along at 240,000 kilometers per hour. At that rate it could get to Pluto in less than five years, rather than the nine years the plutonium-powered New Horizons spacecraft, now on its way, is taking.
The actual acceleration and performance depends upon how the solar sail is constructed. If the sail can be thin and light enough the acceleration and performance can be a lot better.
One of the hottest things in satellite technology today is the CubeSat, a box just 10 centimeters on a side that weighs about 1 kilogram. Such boxes can be mixed and matched in “nanosatellite” combinations of up to three cubes yet still be launched using a shared deployment system. CubeSats are thus relatively cheap and easy to work with, so researchers have used them to carry a variety of science experiments. A small solar sail, thinner than a trash bag and weighing just grams, turns out to be nearly the perfect payload to fly on a CubeSat.
Planetary Society and Other Solar Sails
Planetary Society LightSail’s design calls for the main CubeSat bus to unfold four rectangles covered with solar panels, then unfurl blades of Mylar film to form a kite 5.6 meters on a side. It will have cameras to photograph itself, accelerometers to measure solar pressure and a motor to help keep it pointed on course. As it goes around the Earth, the sail will have to turn 90 degrees twice every 90 minutes.
And this time, just in case, the society is building two copies: Twin LightSails are in the final stages of construction at Stellar Exploration in San Luis Obispo, Calif.
Finding a ride is next. To get above 825 kilometers in altitude, where solar radiation pressure begins to dominate over atmospheric drag, LightSail needs a launch vehicle that goes higher than most CubeSat launches. The project is now waiting for that lift, Friedman says.
Never one to give up dreaming, Friedman envisions two other LightSails to come. LightSail-2 would aim to do a longer flight in a higher Earth orbit, and LightSail-3 would fly to the gravitationally stable L1 Lagrangian point between the Earth and sun.
In England, a consortium from the University of Surrey and its industry partner Astrium is building two prototypes for yet another CubeSat-based solar sail 5 meters on a side, called CubeSail. Engineers have constructed one sail that relies on booms of metal tape that unroll like party poppers, and a second that uses rigid carbon fiber booms that unfold directly. The team will test both in the laboratory and by December decide which design to fly, says project leader Vaios Lappas of the University of Surrey. He expects CubeSail to launch in early 2012.
Lappas’ team is also working on a larger European Union–funded project, called DEORBIT SAIL, for launch in 2014, and an inflatable sail for launch that year or the next.
Yet another approach to solar sails is taking shape in a clean room in an Illinois laboratory. Researchers there have designed a sail that would unfurl from bobbins into a giant space ribbon 250 meters long, says Victoria Coverstone, an aerospace engineer at the University of Illinois at Urbana-Champaign. This project, also dubbed Cube Sail, is basically ready to fly, she says, if the team can find money for a launch and to upgrade the Mylar film that makes up the sail. The Illinois group next aims to test a spinning deployment of sail blades, on the way to an ambitiously large spinning sail whose rotating blades could measure up to 5 or even 10 kilometers long.
Meanwhile, the German space agency DLR and the European Space Agency are planning their own series of solar sails dubbed Gossamer. The first of these would launch a 25-square-meter sail into Earth orbit in 2014, followed by bigger ones over the next several years.
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