First Solar Sail - getting closer

by Jeremy Hsu, Space.com on 16 Aug 2009 Never yet has a vehicle successfully flown in space using a solar sail as its primary means of propulsion. But it hasn't been for a lack of trying.

Solar sails have long promised to harness sunlight for space exploration, just as early sailing ships harnessed the wind.

Now the Planetary Society hopes to refit a NASA sail design and make a third U.S. attempt to fly the first solar sail in space.

The California-based Planetary Society attempted to fly its Cosmos-1 solar sail in 2005, but it foundered because of a Russian rocket malfunction. Then NASA's NanoSail-D was lost in the third failed flight of SpaceX's Falcon 1 rocket in 2008.

Japan managed to deploy solar sail materials from a sounding rocket in 2004, but did not attempt to demonstrate controlled flight. The Planetary Society's new venture could combine technologies and lessons from both earlier spacecraft.

'In Cosmos-1 we had a more conventional spacecraft, but the technology has moved ahead now,' said Louis Friedman, executive director of the Planetary Society in Pasadena, Calif. 'You can actually get a higher performance sail with a lower mass spacecraft.'

NASA's spare sail:

NanoSail-D may provide the foundation design for that future, smaller spacecraft. The sail was meant to test both solar wind pressure and Earth's atmospheric drag, but lacked actual maneuvering capabilities for controlled solar sailing. By contrast, Cosmos-1 had a radio system, imaging system and a micro-accelerometer to enable control over the spacecraft.

NASA has backup NanoSail-D hardware in storage on Earth and a Planetary Society working group could make a decision by the end of summer on how best to integrate that design with its goals. Any future design may also come significantly cheaper than the roughly $4 million price tag on Cosmos-1.

'If we can reduce it by half, we'd be pretty happy,' Friedman told SPACE.com. However, he added that the size reduction would not come with performance reduction, and expects acceleration 'at least as good and maybe better' compared to Cosmos-1.

That equal or better speed boost becomes possible because acceleration is proportional to area divided by mass, and so a smaller spacecraft mass can make do with a smaller sail. Another mini-sail suggestion for the Planetary Society comes from Russian researchers at the Space Research Institute in Moscow, who also collaborated on Cosmos-1.

Small sails, big possibilities

A smaller-sized solar sail also opens up new launch possibilities, ranging from the Russian Soyuz rockets to private launch firms in the United States and elsewhere. The sail could likely hitch a ride as a secondary payload, similar to the smaller cubesat missions which have allowed scientists to launch numerous experiments into space.

Ideally, a companion spacecraft could also launch with the solar sail and watch while the sail unfurls, Planetary Society officials said. That would also provide researchers with valuable data on how the thin, gossamer-like sail structure behaves in microgravity.

Figuring out how solar sails operate in space could eventually lead to spacecraft which maneuver as confidently as sailing ships on Earth, and even allow for sailing or 'tacking' into the face of the sun's oncoming solar wind.

The new endeavor may still face some friendly competition from a Finnish solar sail concept under development. That solar sail uses a slightly different concept of electrical charge repulsion, which takes advantage of charged solar particles.

Planetary Society officials have said they plan to announce the results of their feasibility study once this summer's analysis of NASA's spare NanoSail-D and Russia's mini-sail concept is complete.

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