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Electromagnetic radiation exerts pressure on physical matter. The more the radiation is reflected from the surface it strikes, the greater the pressure–so sunlight presses harder on a mirror than on, say, an ostrich with the same surface area. I’m not going to dwell on the physics underlying this fact (although I will provide a link), but rather on the remarkable ramifications. Contingent on the amount of radiation, this force is rather weak. However, taken in aggregate, across a large surface, light (or any form of EM radiation) can move an object. Hence…solar sails!
Much in the manner that wind pushes a sailboat through water, light can push an object through space. Although using such a sail for space travel was demonstrated to be feasible in the laboratory, the great national space programs—NASA, Russia, ESA, and China—have never successfully tested a solar sail in interplanetary space (despite several failed attempts). However, this year on May 21st the Japan Aerospace Exploration Agency (JAXA) successfully launched the IKAROS solar sail. IKAROS stands for “Interplanetary Kite-craft Accelerated by Radiation Of the Sun”–an acronym which somehow is both unwieldy and an allusion to a badly botched aerospace venture (JAXA can be forgiven for the awkward name however thanks to the success of the mission). IKAROS is a square sail with a diagonal diameter of 20 meters. It is made of polymer 7.5-micrometres thick. A solar array is embedded in the sail to supply the craft’s power needs. To provide attitude control, the sail also contains LCD panels with adjustable reflectivity. Various sensors, dust counters and controls are located on different parts of the craft.
IKAROS deployed its sails when it was approximately 4.8 million miles from Earth (smoothly deploying a delicate lattice of sails in the grim void of outer space has been a major obstacle to this sort of mission in the past). The spacecraft is currently somewhere between Earth and Venus. When it reaches the cloud planet, it will embark on a three year trip around the sun.
To follow up its success JAXA is planning to launce a 50 meter solar sail to the asteroid belt and Jupiter sometime late in the decade. Other space agencies have taken note and are now playing catch-up with the Japanese. NASA has plans for several solar sail missions in the coming years (provided poor national leadership does not botch the plans or scrub the funding). Since rocket fuel is heavy (and therefore a major sorce of missin costs), solar sailing technology has interested space agencies and space exploration enthusiasts for some time. The Planetary Society, an international group dedicated to space exploration, has long advocated solar sails as a revolutionary step forward in space travel. In fact, the Planetary Society chartered a submarine launched Russian rocket to deploy its own solar sail into space but the mission sadly failed when the rocket malfunctioned. Fortunately, the society has regained its old maniacal chutzpah and is launching a new solar sail mission (additionally, and even more importantly, it continues to lobby national governments for additional space funding)
In the near future, solar sails might be used for interplanetary missions or for de-orbiting old satellites and space debris (this latter task is growing in importance as humankind fills up near earth orbit with junk). Hybrid drives which utilize solar sails and solar powered ion drives in tandem are also on the drawing board. In the farther future, who knows? So far this is the only possible option for interstellar travel which utilizes technology humankind currently possesses (well, aside from ridiculous nuclear fission designs). It has been proposed that giant space lasers could be used in tandem with the sun to accelerate probes to nearby stars. Unfortunately such lofty prospects are still science fiction at present.