13 May 2008—Next week, scientists will gather at the European Space Research and Technology Centre (ESTEC) in Noordwijk, the Netherlands, to discuss a radical idea in spacecraft propulsion. A Finnish invention known as an electric sail has been designed to propel a space probe through the solar system by repelling the protons contained in the solar wind. If successful, the electric sail would reduce the cost of launching deep-space probes that must carry heavy fuel tanks.
Invented by Pekka Janhunen, a research fellow at the Finnish Meteorological Institute, the electric sail produces thrust from the solar wind, comprising high-energy electrons and protons streaming from the sun. The electric sail is similar in some ways to the more conventional solar-sail idea. The solar sail requires a ultrathin aluminized Mylar sail 250 meters in diameter to catch the solar wind, which blows out from the sun at speeds between 400 and 800 kilometers per second. However, no one has ever deployed and tested the solar sail in space, and because of its extreme size, the sail presents a number of engineering problems. But, says Janhunen, the electric sail has many advantages over the solar sail—notably, not having to deploy a gigantic sheet of Mylar in space.
Instead, the electric sail uses 50 to 100 wire tethers, each 20 to 30 km long, that are unreeled in space like a fishing line and kept taut using centrifugal force, by spinning them on a central axis. Each tether is composed of four wires, 20 micrometers in diameter, that are interconnected by other smaller wires. The tethers are kept at a 20-kilovolt potential relative to solar protons. This voltage is achieved by an electron gun, which draws off the electrons from the tethers and fires them into space. The positively charged tethers repel the solar wind protons, thereby extracting momentum from them and producing thrust for the spacecraft.
For the past two years, scientists from the space research department at the Finnish Meteorological Institute, in Helsinki, have been conducting computer simulations, hoping to get their electric-sail concept accepted and funded by the European Space Agency (ESA). Janhunen and two other researchers, Giovanni Mengali and Alessandro A. Quarta, from the University of Pisa, in Italy, reported a performance analysis of the electric sail in the January�February 2008 issue of the Journal of Spacecraft and Rockets . According to their analysis, an electric sail would allow a spacecraft to travel just about anywhere in the solar system without having to carry large stores of liquid or solid rocket fuel. Their analysis shows that the electric sail could accelerate small 10- to 100-kilogram payloads to substantial speeds, even faster than is possible with conventional propulsion systems.
At first glance it might seem that the sail can only propel a craft in a straight line out from the sun. But Janhunen’s simulations show that by tilting the plane of the sail as much as 30 degrees the system allows for attitude course-correction adjustments, which are required to reach most objects in the solar system. ”This thrust vectoring allows one to spiral inward or outward in the solar system,” he says. ”It is not clear how this would be done with a solar sail.”
”Even with the baseline electric sail, one could ferry a multimetric-ton payload from a near-Earth asteroid to Earth orbit in a timescale of a few years,” said Janhunen.
For the electric sail to accelerate, the electron gun has to fire constantly. ”If one stops the gun, the tethers become neutralized by solar-wind electrons in a timescale of about half a minute,” says Janhunen. ”This, however, turns out not to be bad, since it provides us with an interesting electric throttling capability. For example, we can turn off and restart propulsion an unlimited number of times.” (He points out, though, that shutting down the power does not actually slow down the craft.) For Janhunen and his team, this is another selling point of the electric sail compared with the traditional solar sail, which cannot be turned off once it has been opened in space.
Ralph McNutt, project scientist for the NASA Messenger mission to Mercury, says that both the solar-sail and electric-sail concepts should be explored but adds that both have weaknesses. For one, they are both quite large and need to be deployed, even in a test version, rather far out in space. Because Earth’s magnetic field shields easily accessible low Earth orbits from the solar wind, sails would have to be launched into a high, and consequently expensive, orbit. ”By the time a launch vehicle is added, I think one is easily talking tens of millions of dollars,” says McNutt. ”Without an immediate need for development, [the money] will be the biggest stumbling block to even demonstrating that the technology will work.”
Janhunen’s cost estimates are lower—on the order of 5 million euros [US $7.7 million]—provided that his team could freely select where they purchase the parts. ”If we get this amount of funding, we estimate that we can fly a test mission three years from now,” he says.