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The Atlas V booster set to launch from Cape Canaveral on Wednesday will carry some wild stuff to space.
Nestled in the nosecone will be the X-37B, an uncrewed reusable orbital vehicle that flies classified missions for the Department of Defense. This time, the X-37B will be sporting a Hall thruster, a propulsion system that uses electrically-accelerated noble gas instead of burning rocket fuel. Or, to put it another way: It's a secret spaceplane with an ion drive!
And tucked in behind that, the Atlas will have a package of CubeSats---small, flexible-mission satellites designed for customization with experiments and other space-bound payloads. The packages this time (PDF warning) include a plasma propulsion system, another ion drive, some communications equipment … and an idea from science fiction's golden age that could reinvent space travel: the solar sail.
Fuel is heavy, finite, and expensive. So why not use energy from the stars? Every sun, including the one Earth orbits, kicks off a steady stream of charged subatomic particles called the solar wind---not to mention a whole mess of photons from up and down the electromagnetic spectrum. Throw up a piece of reflective material and that energy will---theoretically---push it into the universe.
The CubeSat in question, called LightSail A, comes from a Planetary Society project, funded by the crowd. If successful, solar sails could lead to more missions beyond the solar system, and put more endurance behind orbital projects closer to home.
LightSail's computer brain will come alive 15 to 20 seconds after the CubeSat separates from the rocket into orbit. But the sails won't deploy until 28 days later, to give ground control enough time to identify radio signals from the craft. Once they get a lock, they'll be able to guide peoples' eyes to where the satellite will be when the sails unfurl.
A timer will start the process of burning through super thin wires that release four spring-hinged solar panels from the satellite's sides. "This exposes the compartments where the solar sail material is stashed," says Rex Ridenoure, CEO of Ecliptic Enterprises Corporation and one of the lead engineers. Then, a drive motor kicks into gear, and the four booms unfold. "At the end of each one of those booms is a clasp that hooks onto a triangle-shaped section of the solar sail material." Fully deployed, the square sail will be 8 meters from corner to corner. As the sails deploy, cameras will snap a picture every other second.
The sail is mylar---a super-tough polyester film---powder-coated in aluminum. "Contrary to what you might read on NPR and other places, this does not work by using the 'solar wind,' which is ions and electrons," says Ridenoure. No, this baby uses pure photons---sunlight. The particles are massless, but energy rich. When they reflect off the sail, they also give it some energy.
Scientists have understood this effect since at least 1899, when a Russian scientist named Pyotr Lebedev measured the pressure light exerts on a solid object. Fast forward to 1976, and Carl Sagan is on the Tonight Show showing showing Johnny Carson drawings of a photon-sailing satellite.
The LightSail project isn't the first to attempt solar sailing. The Russians successfully deployed a sail from Mir in 1991, and the Japanese Space Agency did it again in 2004. However, none has successfully demonstrated propulsion.
The Planetary Society got moving after the LightSail proposal won a ride aboard an Atlas V launch. This made the project financially feasible, but it also limited the scope of the first mission. "Unfortunately solar sails need to be way out of the atmosphere to work," says Ridenoure. The solar sail, once deployed, will cause perturbations in the orbit, and the satellite will tumble to its fiery, upper atmospheric end. So this first mission is proof-of-concept, to make sure the sail can deploy at all. (Here's a great story about it.)
The second mission, slated for next summer, will deploy its sails further out, and attempt to demonstrate propulsion and maneuverability. LightSail B will be almost identical to its predecessor, except for some software updates and the addition of a wheel that ground control will be able to spin to change the satellite's orientation to the sun. "Change the spin of that wheel, the craft goes the other way, through conservation of momentum," says Ridenoure. Because the sail is around the satellite's center of mass, orienting it at different angles can let the satellite move obliquely to the angle of the inbound photons. In other words, it'll tack to change its altitude.
At its raddest, the LightSail has space enthusiasts like Ridenoure dreaming of escape from the solar system. "The absolute best mission is to use conventional propulsion to get as close to sun as possible," he says, because you get solar energy increases exponentially as you approach. "Then deploy your sail. The sun puts this huge force out and it just screams you out of solar system. By time you get to Mars you’re just screaming, and you can dump the sail." And NASA has a mission on the books planning to use a solar-sailed satellite to get a glimpse at the permanently darkened craters on the Moon’s poles.
Rocket launches are always impressive---this Atlas has a payload that makes it one to keep watching once it gets to space, too.
