By Jonathan Amos
Science correspondent, BBC News
Japanese scientists are celebrating the successful deployment of their solar sail, Ikaros.
The 200-sq-m (2,100-sq-ft) membrane is attached to a small disc-shaped spacecraft that was put in orbit last month by an H-IIA rocket.
Ikaros will demonstrate the principle of using sunlight as a simple and efficient means of propulsion.
The technique has long been touted as a way of moving spacecraft around the Solar System using no chemical fuels.
The mission team will be watching to see if Ikaros produces a measurable acceleration, and how well its systems are able to steer the craft through space.
The Japan Aerospace Exploration Agency (Jaxa) said in a statement that its scientists and engineers had begun to deploy the solar sail on 3 June (JST).
On 10 June, Jaxa said, confirmation was received that the sail had expanded successfully. Some thin-film solar cells embedded in the membrane were even generating power, it added.
Space applications
The principle of solar sailing is a simple one. Photons, or particles of light, falling on a highly reflective, ultra-thin (in this case, just 7.5 microns) surface will exert a pressure.
The force is tiny but continuous, and over time should produce a considerable velocity.
Solar sails will never replace conventional propulsion systems like chemical thrusters, but they do have the potential to play a much greater role in certain types of space mission.
Louis Friedman, from the space advocacy group The Planetary Society, is a big supporter of the technology. The society's LightSail-1, a much smaller mission than Ikaros, could launch by the year's end. He told BBC News recently:
"The potential that we all seek is the ultra-lightweight, very fast spacecraft that doesn't use fuel.
"That's the future of interstellar travel; that's the long-term goal. The intermediate goals are to be able to use this technology to 'hover' in interplanetary space at particular points for monitoring, say, the Sun or monitoring the Earth's geomagnetic poles or magneto-tail; and then also to fly between the planets without using fuel."
Already some satellites in geostationary orbit above the Earth use flaps on the ends of their solar panels to catch the pressure of sunlight to maintain their correct attitude.
This leads to a considerable saving on the fuel that would otherwise have to be sent surging through the satellites' thrusters, and operators have found this strategy can extend the longevity of some missions by many months.
Venus 'piggy-back'
Deploying a large membrane in space is a challenging task, however.
The circular Ikaros was launched with the sail wrapped around it. The plan was to unbutton the four weighted corners of the membrane and allow them to fly outwards as the central module turned. This was expected to pull the sail taut. A camera mounted on the central hub of Ikaros confirmed the sail had indeed been drawn flat.
Japanese scientists must now hope they can control this huge spinning film. If instabilities develop in the sail, it could start to bend and fold, ruining the experiment.
Ikaros was a piggy-back payload to Japan's Venus orbiter, Akatsuki.
The pair were boosted in to space on 21 May (JST) from the Tanegashima Space Center.
Akatsuki will arrive at Venus in December. Key goals include finding definitive evidence for lightning and for active volcanoes.
SAILING TO VENUS -
HOW IKAROS UNFURLS ITS SOLAR SAIL
(1) For the deployment, the disc-shaped Ikaros spacecraft was first spun up
(2) The four weighted corners of the sail were then released and flew outwards
(3) Finally, the packed sail membrane was released
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