Although the idea of a solar sail was first proposed some 100 years ago, to date none has been successfully used in space as a primary means of propulsion. The Japan Aerospace Exploration Agency (JAXA) is looking to change all that with its IKAROS project – not a misspelling of Icarus, rather an abbreviation of Interplanetary Kite-craft Accelerated by Radiation Of the Sun. Launched today aboard the H-IIA Launch Vehicle (H-IIA F17), IKAROS is a space yacht that gathers energy for propulsion from sunlight pressure (photons) by means of a square membrane measuring 20 meters (65.6 ft) diagonally.

The IKAROS won’t just employ a solar sail, but rather a “solar power sail” in that thin film solar cells on the membrane will be used to generate electricity. However, any electricity generated will not be used by the craft, as the goal of the mission is to verify acceleration by solar radiation and power generation of the thin film solar cells. If successful, JAXA then hopes to use the technologies to build an explorer that can travel to Jupiter by combining solar sail technology and a high-performance ion engine powered by electricity gathered by the solar cells.

JAXA calls IKAROS a spin-type explorer, as it will deploy its square membrane in space using centrifugal force. The sail, which measures 14 meters (46 ft) along each side, is made of polymide resin deposited with aluminum. It is only 7.5 micrometers thin, or about 1/10the the thickness of a human hair. Various devices, including the thin film solar cells, a liquid crystal device that controls attitude by changing light reflection characteristics, a temperature sensor, and a dust counter will be equipped on top of the sail.

Because just traveling randomly depending on where the Sun happens to be would obviously limit solar sail technology, the IKAROS team will adjust the direction of reflected sunlight so the craft will move along a targeted orbit. To change the direction of the sail IKAROS carries a gas-powered jet engine as well as a liquid crystal device on the sail that changes the sail’s light reflection characteristics. The latter will allow it to change direction using only solar power. The team will test both systems.

Previous attempts to test solar sail technology have been unsuccessful. A solar sail spacecraft, Cosmos 1, was launched in 2005, but the Volna rocket launch vehicle that was to take it into space failed and the spacecraft failed to reach orbit. A similar fate befell NASA’s NanoSail-D mission, which was lost in a launch failure aboard a Falcon 1 rocket in 2008.

It appears that IKAROS has already outdone these previous attempts with a successful launch aboard the H-IIA Launch Vehicle No. 17 (H-IIA F17) alongside the Venus Climate Orbiter AKATSUKI at 6.58am on May 21, Japan Standard Time. The H-IIA F17 flew smoothly, and, at 27 minutes and 29 seconds after liftoff, the AKATSUKI was separated from the H-IIA. IKAROS will travel towards Venus with the AKATSUKI for about a month, after which it will deploy its sail.

The JAXA team will first make sure the thin-film solar cell is generating electricity and, for about six months after that, the team will be demonstrating the solar sail's acceleration and orbit-control functions. AKATSUKI will decelerate to enter the Venus orbit, but IKAROS will pass by Venus and navigate around the Sun. Where it heads will depend on how well the solar sail's orbit control function performs.

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