Sen—NASA’s New Horizons space probe is now just a year away from beginning its study of ex-planet Pluto, in the outer reaches of the Solar System. As the fastest probe ever fired into deep space, it has still taken many years to get there, having launched in January 2006 and racing to a speed of 55,500 km per hour.

It promises to be an exciting and revealing encounter, though one that is over in just a few months as New Horizons’ incredible speed sends it hurtling out into interstellar space. This space propulsion technique will not suit all remote voyages, and alternative technologies are being examined to reach the outer planets.

Among them is an interesting concept proposed by Finnish scientists which is a variation on the solar sail idea that is already being tested in space missions. The conventional sail is made of a thin fabric that catches photons in the solar wind much as a yacht’s sail is driven by the atmospheric winds.

The electric sail, or E-sail, being proposed by the Finnish team more resembles an umbrella’s pattern of spokes - wire tethers that when charged up will generate an electric field around the spacecraft and allow it to be pushed along by solar energy.

The concept of the E-sail was put together by engineer Pekka Janhunen, who is also leading a proposal to send a space probe to study the distant ice giant planet Uranus. And though it sounds like science fiction, a small Estonian satellite called ESTCube-1 is already testing the technique in orbit and ESA is also examining the concept.

Janhunen, of the Finnish Meteorological Institute, is Visiting Professor at Tartu University, Estonia, and believes probes powered like this could be used to visit several planets and moons in the outer Solar System.



He told Sen: “The E-sail can go on changing the vehicle’s course in the Solar System indefinitely without ever running out of fuel, and it can do so fast enough to be practically useful. It’s like an unlimited duration InterRail pass to the Solar System.”

An artist's image showing the ESTCube satellite in orbit. Credit: University of Tartu, ESTCube team

A full-sized sail, as shown in the main image, will consist of between 50 and 100 wires, each 20 km long but only a 25 or so microns in diameter, radiating away from the space probe. They will pack up into a tiny space for launch, but when unfurled they provide several square kilometres of wind-sail area. A solar-powered electron gun in the spacecraft will keep these tethers highly charged, so that they resemble a wall-like obstacle to the protons and alpha particles flowing in the solar wind.

Though the initial force of the wind pushing the spacecraft along will seem weak, the acceleration it provides means it could reach a speed of 30 km per second in a year, and the continued acceleration could send a small probe as far as Pluto in only five years.

For a larger probe, as envisioned for a mission to Uranus, nearly 3 billion km from Earth (1.8 billion km) the journey time is still estimated at about just six years at a speed of up to 72,000 km per hour. Changes in direction of the spacecraft can be applied by inclining the sail, or opening up gaps in it by killing the charge on certain tethers.

The main body of the space probe will still use some conventional rocket thrusters to allow for changes to the trajectory when it closes in on Uranus, which has an equatorial diameter of 50,700 km (31,500 miles). Once there, the team wants to send a probe into the planet’s atmosphere to tell us more about a world that still remains very much a mystery.

Janhunen’s team says an E-sail probe could study Uranus in a similar manner to how NASA’s Galileo measured Jupiter’s atmosphere in the 1990s and would help planetary scientists learn more about how the Solar System formed.

A video demonstrates how the Uranus probe will deploy its E-sail. Credit: Pekka Janhunen

Sen spoke to Chris Arridge, of University College London’s Mullard Space Science Laboratory in Surrey, who worked on plans for a mission called Uranus Pathfinder that ESA ultimately failed to back.

He said: “Travelling to the more distant reaches of our solar system will always take a relatively long time.

“This technology is at a very early stage of development and will require much more development for it to be used in a mission this far out in the Solar System. But if this can work we will be able to explore the Solar System much more cheaply and quickly.”