LYING on a bench in Robert Winglee's lab is a small quartz tube laced with thin metal wires that looks suspiciously like a light bulb. It even casts a pale glow on Winglee's face when he turns it on. But switch it on in the darkness of space, and it will pump out a giant, glowing cloud that can pull a spacecraft to the distant reaches of the Solar System at fantastic speed.

Winglee is a physicist at the University of Washington in Seattle and this glowing cloud is a bizarre kind of space sail. There's a mighty wind blowing through the Solar System-a gale of hot ions and high-energy particles that are blasted outwards by the fiery heat of the Sun. This solar wind whistles past the Earth at more than 500 kilometres per second and Winglee is designing his sail to ride this storm.

Inflate the sail around a spacecraft and Winglee calculates that the wind will carry it along at over 100 kilometres per second-more than 6 times as fast as the swiftest of today's spacecraft. Adjust the sail's trim and you could travel almost anywhere in the Solar System-without heavy rocket propellants or a helpful gravitational tug from a nearby planet. You could tack upwind towards the Sun or bear away towards Pluto with the wind behind you. In about a year, you'd pass the blue, stormy clouds of Neptune and just four years later you'd reach the vast, mysterious regions near the edge of our Solar System.

Winglee isn't the first to dream of sailing to other planets. Almost 30 years ago, NASA engineers studied ways to build kilometre-sized sails using thin plastic sheets coated with aluminium. Supported by long struts, these sails would be pushed along by the pressure from billions of photons as they bounced off the sail's mirrored surface (New Scientist, 5 January 1991, p 31). Each photon exerts a minuscule force on the giant structure-like a ping-pong ball fired at an ocean liner-but in the friction-free vacuum of space, the photons add up to a steady pressure which slowly accelerates the sail from a standing start.

But the engineers faced one insurmountable problem: there was no reliable way to unfold these huge structures in space. The sails never got off the drawing board.

This is Winglee's edge. By harnessing the solar wind rather than the power of photons, Winglee doesn't need to unfurl slender beams and huge mirrors. Instead, his sailcloth is little more than an ethereal magnetic field: simply create a field around your spacecraft and the solar wind will do the rest. Since the ions and charged particles in the wind possess their own magnetic field, they are repelled by the sail and, like photons on a silvered sail, they exert a small but steady force that pushes sail and spacecraft before them.

It sounds simple. But to accelerate a magnetic sail beyond anything more than a crawl, the sail must be massive. On its own, a magnet produces a tiny field extending no more than a few centimetres in any direction. And although a powerful superconducting magnet can create a giant, kilometre-sized field, it would weigh at least ten tonnes-far too heavy to launch into space.

So Winglee has come up with an ingenious and remarkable idea to inflate a magnet's tiny field into a gigantic sail. He plans to pump a hot plasma of ionised gas into the field. Just as you can pump up a balloon by blowing air into it, the small field should inflate until it is kilometres across. The solar wind will have a huge surface area to blow against, says Winglee, and the sail isn't limited by mechanical structures.

This is where the quartz tube on the bench in Winglee's lab comes in: it is his prototype sail pump. Winglee built it using a $75 000 grant from the NASA Institute of Advanced Concepts (NIAC), a body that funds some of the more imaginative and revolutionary aerospace research. NIAC director Bob Cassanova is impressed. "We're looking for ideas that stretch the imagination," he says. "This is not just some flaky idea. They have done the computer modelling, and it looks very promising."

So far, Winglee hasn't had the chance to put his technique to the test: he is still perfecting the art of creating and trapping the plasma. To make a plasma, Winglee squirts a jet of inert gas-helium or argon, for instance-through a pipe into the centre of the quartz tube. Wrapped around the tube is a metal wire which, because of its helical shape, Winglee calls a "helicon". This wire is a heating element. With a high-frequency alternating current running through it, the helicon heats the gas atoms in the tube to almost 40 000 �C.

In this inferno, electrons are ripped off the atoms of gas, creating a plasma of hot ions and energetic electrons. Since the gas is pressurised, the plasma sprays out of the tube through a hole at either end and begins to diffuse away. But it doesn't get far. At each end of the tube is a solenoid, a tight coil of aluminium wire that creates a magnetic field when current passes through it.

The field generated isn't very strong, Winglee admits-it extends just 20 centimetres out from the tube. But this magnetic bubble traps and holds the plasma like a cage. The magnetic field lines curve in an apple shape around the quartz tube, with the tube lying along the apple's core (see Diagram, p 28). The ions and electrons in the plasma are far too excited to coalesce back into neutral atoms, says Winglee, so they spiral out along the field lines, moving from one end of the quartz tube to the other, where they are recycled.

Trapping the plasma is only half the story. Winglee must also get the magnetic "balloon" to inflate. For this, he will rely on the way that an electrical current passing through a conductor generates its own magnetic field. The plasma particles behave just like this current: as they spiral along the magnetic field lines, they create their own field which pushes outwards against the magnetic bubble.

Turn up the gas pressure, and Winglee calculates that the outward force on the magnetic bubble will increase until the plasma begins to stretch the field lines outwards and the bubble expands. Out in the vacuum of space, Winglee believes that his technique can create a luminescent plasma bubble almost 40 kilometres across.

Winglee has dubbed his sailing technique mini-magnetospheric plasma propulsion (M2P2). A spacecraft using M2P2 will be a remarkable sight. Depending on the gas in the plasma, the giant cloud will glow white, red or purple in the blackness of space, and as the solar wind blows against it, the huge sail will distort from an apple shape into something more like a giant tear drop (see Diagram). Pulled along by this bizarre cloud, the craft will accelerate to about 80 kilometres per second in three months, eventually reaching speeds of 100 kilometres per second or more.

Winglee's work has already attracted fans. Kevin Rudolph, a propulsion engineer at Lockheed Martin Astronautics in Denver, ran into Winglee at an advanced propulsion conference about six months ago and is convinced the M2P2 can work. And just a few weeks ago, the NIAC awarded Winglee another $500 000 to prove that pumping up the magnetic field with a glowing plasma is possible. "We have been stuck with chemical propulsion since the days of the V-2 rocket," says Ed Weiler, NASA's associate administrator for space science. "If humans are ever to reach the stars, we need a lot more of this innovative thinking."

Later this year, Winglee and his graduate student Tim Ziemba will test their sail pump at the University of Washington's Redmond Plasma Physics Laboratory. They'll clamp the quartz tube inside a small, drum-shaped vacuum chamber and use the helicon to expand a magnetic field until it's almost a metre across. No one knows for sure whether the field will actually inflate the way that Winglee predicts. But Winglee is optimistic: the theory ties up and all the technologies he needs to inflate the sail already exist.

If this test is successful, next year they'll move the M2P2 to a much larger chamber where they'll try to expand the sail until it reaches almost 10 metres across. But since there's no facility on Earth capable of containing a plasma cloud 40 kilometres wide, the ultimate proof of concept will have to wait until M2P2 is switched on in space.

Winglee is the first to admit that he doesn't have everything worked out yet-like how to steer a craft with his sail or how to stop the M2P2 from frying everything in sight. "You can change the shape of the magnetic bubble, possibly by tilting the device with respect to the solar wind, much like you shape the sails on a sailboat," he suggests. Pivot the sail, and the solar wind will push the spacecraft sideways.

One way to do this, suggests Winglee, would be to mount a small motor on the craft to tilt the quartz tube. "Once we get a flight demo, we can worry about it," he says. "That's our attitude at this stage."

Meanwhile, the tests will also help the physicists refine the way they generate and control the extraordinarily hot plasma. This is crucial. Mount the M2P2 on a spacecraft and the massive current flowing through the plasma cloud will short out every electronic component it touches. So the craft and its scientific instruments, including the solar panels which power the helicon and the solenoids, must be protected with insulation. This could be tricky. As the solar wind collides with the sail, the plasma will become hotter still. No one is sure exactly how hot, but possibly up to 100 000 �C. The way out, hopes Winglee, would be for the hollow quartz tube to sweep the plasma into a huge loop encircling the spacecraft. "That allows the plasma to circulate around without striking the spacecraft," he says.

Winglee also plans to measure his sail's acceleration by setting up a synthetic solar wind inside the vacuum test chamber. This will blow across the inflated sail and he'll measure the force it exerts. Although not expecting it to screech away like a Ferrari, he reckons his sail would be very efficient.

Rocket scientists measure an engine's efficiency by calculating its specific impulse-the ratio of thrust to fuel consumption. A high value means you can carry less fuel, go faster and carry more scientific instruments to your destination. Conventional rockets have a specific impulse of about 100 seconds. The electric ion thruster on Deep Space 1 has a specific impulse more than ten times as large (New Scientist, 24 October 1998, p 38). Winglee calculates that his sail will have a specific impulse of around 100 000 seconds.

But over time, the plasma will leak through its magnetic cage and disappear into space. If it's not topped up, the sail will sag like a leaky balloon. So a craft using M2P2 will have to pump fresh plasma into the sail to keep it inflated. The leakage will be slow-less than a quarter of a kilogram of gas per day, Winglee predicts-and for really long missions, the sail can be switched off when it isn't needed. So M2P2 will only need tens of kilograms of propellant, says Winglee, instead of the hundreds of kilograms required by the engines on conventional craft.

About 40 kilograms of gas should last for about a year. "Once you're out past the Moon, it's like being on the Pacific Ocean in a trade wind," says Rudolph. "You sail past Mars, Jupiter and beyond."

It wouldn't all be plain sailing. The solar wind eventually fizzles out when it merges with the interstellar gas at a region called the heliopause. And by about 600 million kilometres from Earth, sunlight becomes too weak to generate electricity from solar panels. Without power, the magnetic field would disappear, and the plasma cloud would disperse. The only way to keep accelerating would be to generate electricity with a small radioisotope thermoelectric generator such as that on NASA's Cassini probe.

The additional power from an RTG could even boost the strength of the magnetic field, enlarge the bubble, and crash the craft through the 100 kilometres per second mark. But even without a sail, there is no friction to slow a craft once it gets going: "By the time you get to the heliopause, you're going like a bat out of-wherever," says Rudolph. "You coast to where you want to go from there."

But ask Winglee if he has a particular destination in mind for an M2P2 mission, and he'll shake his head. He simply sees the technology opening the door to a whole new class of deep-space missions that would take slower craft a lifetime to complete.

How about a mission to take a good look at the Kuiper belt, for instance-a ring of rocky bodies near Pluto, or out beyond the edge of the Solar System to the Oort cloud, the birthplace of comets such as Hale-Bopp? Astronomers know very little about this cloud and the icy objects it contains. "There are millions of those suckers out there that we can't see," Winglee says. Out in the interstellar gas, you could even sample material thrown out by nearby stars.

Curiously, despite the growing respect for Winglee's ideas, don't expect propulsion engineers to give up on their conventional rockets and start blowing bubbles without a fight. "Sails have always had a bad press," says Colin McInnes, an aerospace engineer at the University of Glasgow, who is busily cataloguing missions that require sail propulsion. If Winglee is to get his sail into space, he'll probably need to find a scientist who can convince NASA that a particular deep space mission is simply too alluring to pass up.

"You find somebody with funding who cannot do their mission unless this sail works," Rudolph says. "You find that, and you've found your sugar daddy."

Ben Iannotta is a technology writer based in Florida

Further reading: Solar Sailing by Colin R. McInnes (Praxis Publishing, Chichester, 1999)

For more information on M2P2 see: http://www.niac.usra.edu/studies/9801/ 9801Final/WingleeFinal.pdf

New Scientist issue 4th September 99

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