Gone but not forgotten: An artist's impression of The Planetary Society's Cosmos 1 in orbit. The solar sail-equipped craft was lost during launch in 2005, but a replacement is already under development.
Credit: The Planetary Society
Under development: A four quadrant, 20-metre-across solar sail is fully deployed during testing in a space simulation chamber at NASA's Glenn Research Centre in Ohio, USA.
Credit: NASA
Five-kilometre-wide bright spot
Also in the 1970s, Louis Friedman – then at NASA's Jet Propulsion Laboratory – led a project to try the first solar sail flight. Halley's Comet was to make its closest approach to Earth in 1986, and NASA conceived the exciting idea of propelling a probe via solar sail to rendezvous with the comet. Eventually, the project was scrapped, but it demonstrated that the concept was feasible.
In 1993, the Russian Space Agency launched a 20-metre diameter, spinning mirror called Znamya 2, hoping to beam solar power back to the ground.
"Some call Znamya 2 a sail because it was made of a large, lightweight reflector and unfurled like a solar sail might be unfurled," says Les Johnson of the NASA Marshall Space Flight Centre in Huntsville, Alabama. Johnson is co-author of the soon-to-be-published book Solar Sails: A Novel Approach to Interplanetary Travel.
"In fact, if I were asked to demonstrate solar sail technology and was constrained to deploy it from a large spacecraft, I might design a 'sail' like Znamya," he says.
The foil reflector unfurled and when illuminated produced a five-kilometre-wide bright spot which crossed Europe from France to Russia at a speed of some eight km/h. Unable to control its own flight, however, the mirror burned up in the atmosphere over Canada. Russia's proto-sail program was abandoned in 1999 after a larger, follow-up mission failed to deploy properly.
Light pressure
Solar sails were an accessory on India's INSAT 2A and 3A communications satellites, circa 1992 and 2003. The satellites were powered by a four-panel solar array on one side. A solar sail was mounted on the north side of each satellite to offset the torque resulting from solar pressure on the array.
In 2004, the Japanese deployed solar sail materials sub-orbitally from a sounding rocket. Although it was not a demonstration of a free-flying solar sail that could be used for deep-space exploration, the deployment was nevertheless "a valuable milestone" remarks Friedman, who appreciates the challenges of deploying gossamer sheets from fast-moving spacecraft.
To date, no solar sail has been successfully deployed in space as a primary means of propulsion.
The Planetary Society hoped to demonstrate the technology with its Cosmos 1 mission in 2005 (see image). "Cosmos 1 was a fully developed solar sail spacecraft intended to fly only under the influence of solar pressure for control of the spacecraft's orbit," says Friedman.
"If all had gone as planned, the U.S.-based Planetary Society, working with Russia, would have been the first to fly a fully functional, though performance-limited, solar sail in space," agreed Johnson. "It would have been the first spin-stabilized, free-flying solar sail to fly in space."
Cosmos 1, however, was lost when the launch vehicle failed.
Meanwhile, NASA also continued to dabble in solar sailing. Between 2001 and 2005, the Agency developed two different 20-metre solar sails and tested them on the ground in vacuum conditions.
"These sail designs are robust enough for deployment in a one atmosphere, one gravity environment and are scalable to much larger solar sails – perhaps as much as 150 metres on one side," write Johnson and his co-author Giovanni Vulpetti in their upcoming book. They estimate that a NASA flight test is possible by 2010.
