NASA's historic attempt to deploy a solar sail

Solar sails, also called photon sails or light sails are a proposed form of propulsion that uses large membrane mirrors to tap radiation pressure from a light source to propel a space craft. Although the thrust generated is small, it continues as long as the light source shines and the sail is deployed.

The possibility has excited NASA researchers for years, of sailing the solar system with sails propelled by sunlight instead of wind and rockets.

Though no one has, as yet, successfully deployed such a sail anywhere beyond Earth till now, Edward Montgomery of NASA's Marshall Space Flight Centre is optimistic, saying that "There's a first time for everything." Along with a team from Ames Research Centre, Montgomery's team hopes to create history and deploy a solar sail.

Named NanoSail-D, it will travel into space aboard a SpaceX Falcon 1 rocket, and is scheduled for launch from the Omelek Island in the Pacific Ocean during a window extending from 29 July to 6 August.

Some years ago, the Planetary Society attempted a mission similar to NanoSail-D, which was Cosmos I. However, the launch vehicle failure destroyed the undeployed spacecraft. Montgomery, who is also NanoSail-D's payload manager, says that the NanoSail-D will be the first fully deployed solar sail in space, and the first spacecraft to use solar pressure as a primary means of altitude control or orbital maneuvering.

The structure is made of aluminum and space-age plastic,and the whole spacecraft weighs less than ten pounds. Montgomery says it is carried around in a special suitcase, the size of an airplane carry-on luggage. Fully opened, the kite-shaped sail spreads out to around 100 square feet of light-catching surface.

Montgomery believes a successful deployment would be a huge step forward for the future of space exploration, since the limits of fuel are removed from the equation, "extending our reach as far as our dreams." Since there is no friction in space, and once a solar sail gets underway, it can sail virtually forever. Much after a rocket would have run out of fuel and started to coast, a solar sail propelled ship could still accelerate and achieve speeds that are much faster, while covering distances far greater than any rocket.

As of now, no rocket in existence can carry enough fuel to reach the outer solar system in as short a time. Like a marine sail, a solar sail could also bring the craft home by using the solar sail to tack the vessel, making it travel "against the wind," back to Earth.

Montgomery explains, "It's not so much about how far a sail will go compared to a rocket; the key is how fast. The Voyagers have escaped the solar system, and they were sent by rockets, but it's taken more than three decades to do it. A sail launched today would probably catch up with them in a single decade."

There is also an added bonus to this technology.

Montgomery says that the micro-satellites that are currently in orbit above a few hundred kilometres stay in orbit for decades after completing their mission, creating in the process an orbital debris collision risk for other spacecraft. The NanoSail-D will demonstrate the feasibility of using a drag sail to decrease the time satellites clutter the Earth's orbit. Although it looks like a kite, the sail can act like a parachute or a drag sail in the very thin upper atmosphere around Earth, slowing the spacecraft and making it lose altitude, so that it re-enters the Earth's atmosphere and burns up in a relatively short period of time. Montgomery says a drag sail is a much lighter alternative to carrying a propulsion system to de-orbit a satellite.

So what does the 'D' stand for in NanoSail-D? "We chose the 'D' in the name, not because it came after models A, B, and C, but because it can stand for demonstrate, deploy, drag, and/or de-orbit," says Montgomery. If all goes well, it could soon stand for "DID IT!"