An Optical Lift for Solar Sails

While we wait for the NanoSail-D deployment, let’s talk about how to control a space-based solar sail. Japan’s IKAROS sail uses liquid crystal devices along the outer edge of the sail that actually allow the ground team to adjust the reflectivity of portions of the sail. Do that and you’ve created a situation where one part of the sail gets more of a boost from incoming photons than another, making it possible to maneuver the sail by light alone. It’s ingenious technology and, at least in early tests, seems to work.

Getting a sail to function seems easy. The tricky part is controlling what it does. We know that photons have no mass, but they do impart momentum, so that putting any large, reflective surface into space should result in forces acting upon it. James Clerk Maxwell demonstrated the basic principle as far back as 1873, although Johannes Kepler, observing the behavior of cometary tails, assumed they were affected by a ‘solar wind’ that could eventually be used to move a craft through space. The Echo-1 satellite was mightily manipulated by solar photons, a development startling to ground controllers. Space analyst James Oberg found a sports analogy to describe what happened, noting the need for stronger sail materials to handle the force:

> …flexible materials that were light enough, mechanically strong enough, and physically resistant enough to the disintegrating effects of solar radiation just weren’t available until recent decades. As far back as 1960, photon pressure played orbital soccer with the Echo 1 thin-film balloon in orbit, pushing its orbit around with astonishing force until the balloon’s skin shattered. The shards were then flung far and wide by sunlight.

We’ll see what IKAROS has to teach us about controlling a craft through liquid crystal technologies and ponder their applicability to future missions. Meanwhile, Grover Swartzlander (Rochester Institute of Technology) has found a considerably different way to adjust the attitude of a solar sail. Swartzlander’s recent paper discusses a way to create lift — think of a cambered airfoil — that is responsive not to air but to light shining through the sail material. A future sail, then, might use not just photons striking its surface for momentum, but arrays of ‘lightfoils’ that can change the sail’s attitude.

The idea here is that an asymmeterical deflection of light moving through a transparent semi-cylindrical rod in the shape of a wing — that’s the ‘lightfoil’ — can create a lifting force. Swartzlander’s experiments involved small rods with a cambered shape that were immersed in water. A laser beam was then switched on to strike the rods, which moved away from the light source as predicted, but also showed a ‘lift’ effect, a movement perpendicular to the light. Using symmetrical spheres instead of the rods caused the perpendicular movement to disappear.

Science News discusses the result in a recent story:

> Optical lift is different from the aerodynamic lift created by an airfoil. A plane flies because air flowing more slowly under its wing exerts more pressure than the faster-moving air flowing above. But in a lightfoil, the lift is created inside the object as the beam shines through. The shape of the transparent lightfoil causes light to be refracted differently depending on where it goes through, which causes a corresponding bending of the beam’s momentum that creates lift.

The shape of the lightfoil and the refractive index of the materials involved determine how much the photons change direction and ultimately control the direction of the lightfoil’s movement. The lift angle of the lightfoils turns out to be a hefty sixty degrees. Try taking an airplane off at that angle and you’ll be headed for a stall, but the paper on this work seems to show the lightfoil effect is stable. What lies ahead for Swartzlander is to test the effect with different materials, adjusting their shapes and refractive properties to find the optimal combination.

PUSHED ASIDE from Science News on Vimeo.

Are we talking about an effect that could be used for maneuvering a solar sail? In this New Scientist story, Swartzlander talks about using two perpendicular arrays of rods to control a future sail. Time will tell whether solar photons would be strong enough to generate sufficient force, and it may be that the ingenious IKAROS technology will have the upper hand. But we won’t know that until we find out how powerful an effect this is, which will call for a great deal more lab work and, if the effect proves out, eventual space testing.

The paper is Swartzlander et al., “Stable Optical Lift,” published online in Nature Photonics 5 December 2010 (abstract).


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