Electric solar sail moves closer to reality

It's a striking image made popular in sci-fi classics like the recent Star Wars films - a spacecraft hurtles through the galaxy propelled by gigantic reflective sails that use of solar radiation in place of on-board fuel . Space organizations around the world including NASA are pursuing this technology, but a rapidly evolving project from the Finnish Meteorological Institute has taken a radically different approach by using long metallic tethers and a solar-powered electron gun to create an "electric sail" that looks very different from the depictions of pressure sails with which we have become familiar.

Invented in 2006 at the Kumpula Space Centre and developed by Dr. Pekka Janhunen, the electric solar wind sail is based on the same principles that underly the lightweight reflective pressure sails such as those being tested by NASA: solar wind is harnessed to provide the thrust source allowing the spacecraft to fly without the need for fuel or propellant and providing a means of faster and cheaper space exploration. The electric sail however, looks more like an antenna than a conventional sail. A full-scale version would consist of up to 100 thin conducting wires as long as 20 km that are kept in a high positive potential by the spacecraft's on-board solar-powered electron gun. This electric field effectively turns the wires into 50 meter wide sails that can then make use of solar wind. It's estimated that a 20km long electric sail wire (which weighs only a few hundred grams and fits in a small reel) is equivalent to a one square kilometer solar wind sail when deployed in this way.

In just two years the project has moved swiftly towards implementation through work undertaken at the Finnish Meteorological Institute with component work is carried out at the University of Helsinki (where a method for constructing the ultr-thin metal wires using ultrasonic welding has been developed) and in Germany, Sweden, Russia and Italy.

"We haven't encountered major problems in any of the technical fields thus far. This has already enabled us to already start planning the first test mission", says Dr. Pekka Janhunen.

Apart from it's benefits to space exploration, the technology could assist in the development of solar powered satellites that orbit in the permanent sunshine of space and transmit electric power back to Earth by microwaves.

"The electric sail might cheapen all space activities and thereby for example help making large solar power satellites a viable option for clean electricity production," says Dr. Janhunen. "Continuous power would be a major benefit compared to e.g. ground-based solar power where storing the energy over night, cloudy weather and winter are tricky issues especially here in the far North".

In 2004 NASA's Solar Sail Propulsion Team successfully deployed two 10-meter solar sails made of reflective material 40 to 100 times thinner than a piece of writing paper in a laboratory vacuum environment and The Planetary Society is continuing in the push towards the realization of solar sailing after the first solar sail spacecraft ever constructed, Cosmos 1, failed to enter orbit when a launch was attempted in 2005.

Further reading: Finnish Meteorological Institute, Kumpula Space Centre