| An artist's depiction of the solar sail spacecraft LightSail-1.|
By Rick Sternbach and courtesy of The Planetary Society
Over the last decade the development of interplanetary spacecraft fitted with solar sails propelled by sunlight, much like sailboats dependant on the wind over a terrestrial sea, has evolved in fits and starts. The solar sail is not a new concept. In 1871 Scottish physicist James Clerk Maxwell postulated that light photons from the Sun exerted a force on exposed surfaces. Three decades later Maxwell´s theory was proved experimentally by the Russian physicist Peter Lebedev. Quantum theory suggests that photons act as small packets of energy with the character of matter. A photon striking a surface thus imparts a bit of momentum. In the 1920s Soviet rocket pioneer Friedrich Zander proposed that photons could propel a spacecraft. The notion of photons exerting a force should not be confused with the much weaker solar wind that gives comets their tails. Proof that solar radiation could be used as means of control and propulsion came in 1974 when the Mariner 10 spacecraft, on a mission to Mercury, was low on fuel. To stretch precious reserves NASA engineers successfully manipulated the pressure on the satellite's solar panels.
Two recent events have advanced the development of solar sails as a means of propulsion. First, on January 20 NASA deployed the NanoSail-D, a small experimental solar sail spacecraft, into low Earth orbit. NanoSail-D will eventually succumb to drag from the upper atmosphere, re-enter and burn up but not before providing important data on the capabilities of solar sails. The other event occurred in late January when the Japanese Aerospace Exploration Agency (JAXA) announced that it was extending the mission of its IKAROS spacecraft. IKAROS was launched in May 2010 and, partially propelled by solar sails, flew past Venus in December and is now on its way to the far side of the Sun. IKAROS has proved the practically of solar sails for interplanetary travel and gathered much relevant data.
The acceleration provided by light photons bouncing off the surface of a spacecraft's sails is very slight but given enough time very high speeds can be reached. The more reflective the surface of the sail the more momentum is imparted to it by the colliding photons. A lighter spacecraft has less inertia to overcome so considerable engineering ingenuity is required to cram extendable sails into the smallest and lightest package. A solar sail spacecraft, once blasted into a high earth orbit by a rocket would have no more need for conventional chemical fuels. The same sunlight also works on the spacecraft´s solar panels to produce electricity. The sails are made of ultra-thin aluminium covered mylar that can be folded compactly and stored for launch. Once in space the sails are deployed by either telescoping struts, or, as is the case of IKAROS, by spinning the spacecraft to create centrifugal forces that hold the sails open. The direction of the push, and thus, the spacecraft's trajectory can be controlled by changing the angle of the solar sails with respect to the Sun.
Later this year another experimental solar sail project is expected to go into space to gather more data. The Planetary Society, a US-based private non-profit organization, hopes to launch its LightSail-1 spacecraft into a high earth orbit. Solar sailing may prove to be more practical for interplanetary travel within the solar system rather than for deep space missions where the strength of the push rapidly diminishes as the craft moves away from the Sun. However, some scientists envision solar sail spacecraft powered by laser beams aimed from Earth. A sort of ongoing shove from the home planet, powering and directing the mission onwards.