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Posted on May 6, 2010 by  & 

Photovoltaic solar sails in space

Solar sails are not energy harvesting in the common definition because they do not produce electricity. However, this is a story of a solar sail that is coated with photovoltaics so it has a second function of generating electricity for small electronic devices on board - energy harvesting indeed. The Japanese Space Agency succeeded with an origami space orbiter and zero-gravity sushi experiments. Now comes the first "space yacht". This month in a trial run that will captivate space researchers and science-fiction writers alike, a Mitsubishi H-IIA rocket goes into orbit from the island of Tanegashima and releases its small satellite into the void.
Having spent a few weeks first settling into a slow rotation Ikaros will then unfurl a microscopically fine 20m sail that some believe to be the future of interplanetary travel.
Over the following six months - and if the theory of "solar yacht" propulsion works - Ikaros will begin its silent journey to Venus, driven by the tiny but relentless force of solar particles buffeting the sail and nothing else. With every second Ikaros should gather a tiny amount of speed.
The craft derives its name from Icarus, the character from Greek mythology whose ill-planned flight took him too close to the Sun and ended in disaster when his wax wings melted. Keen to avoid this association, the Japan Aerospace Exploration Agency (Jaxa) notes that Ikaros stands for Interplanetary Kite-craft Accelerated by Radiation of the Sun. A larger version of the vessel could eventually travel at tens of thousands of miles per hour without any fuel.
In his final novel, The Last Theorem, the late Arthur C Clarke imagined solar yacht races with astronauts competing to reach the Moon and back by photon power. All this may be sailing, but it is not energy harvesting in the sense of harnessing ambient energy to create electricity for small devices. However, with the Japanese invention, there is an energy harvesting twist.
The sail, which cost about $15 million to create, is about the thickness of a Cellophane sandwich wrapper (32.5micrometers) and covered with a second experimental material - so called "thin film" solar panels, which also have potential applications on Earth. Boeing Spectrolab makes most of the photovoltaics used in space because its GaAs:Ge multilayers have the greatest efficiency per unit of weight.
The panels coat the sail so that Ikaros has a source of electrical power. It can then use it to ionise gas and fire it from small jets - a method of propulsion already used in conventional satellites. Japan is not the only country pursuing space sail technology. Russia is close to producing a version of the space yacht and much of the material science behind the sails has been developed in the United States.
Even if the prospect of sending sail powered craft through the galaxy remains distant, the technology could improve conventional satellites. Without the need for fuel and cumbersome propulsion mechanisms, sails would allow satellites to be built smaller and lighter, requiring less energy to launch them into space.
The maiden Ikaros mission will last six months but the Japanese agency has further ambitions for the technology if it proves successful. It hopes to send a device with larger sails towards Jupiter early in the next decade.
Top image of Ikaros by Japan Aerospace Exploration Agency

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Posted on: May 6, 2010

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