No flight of fancy

Finnish scientists have proposed a concept of space propulsion that could reduce the time taken to reach interstellar space.

R. RAMACHANDRAN recently in Helsinki

THE four most distant man-made spacecraft launched so far - the National Aeronautics and Space Administration's (NASA) Pioneer 10 and 11 probes launched in 1972-73 and the Voyager 1 and 2 missions launched in 1977 - took about 11 to 12 years to fly by the farthest planets in the solar system. NASA's latest mission to Pluto, New Horizons, which was launched in January 2006, is expected to fly by Pluto in July 2015 at a speed of about 14 kilometres a second. But if the new concept of space propulsion without the use of propellants - the electric solar wind sail or electric sail - proposed by two Finnish scientists can be made to work, this distance could be covered by a medium-sized satellite moving at speeds of 50-100 km/s, in less than five years, and the interstellar space beyond the heliosphere can be reached in less than 15 years. The idea, in fact, holds out the potential of being realised in the near term with available technology. It is not in the realms of science fiction or futuristic propulsion technology.

The physics behind the electric sail concept is the solar wind momentum that can be made to give thrust to the satellite. The solar wind is a very tenuous but high-speed (300-800 km/s) stream of charged particles, or plasma, consisting mainly of protons and electrons, blowing radially outward from the sun. The average dynamic pressure (force per unit area) due to the solar wind at the earth's distance from the sun - 1 Astronomical Unit (AU) in physics parlance - is about 2 nanoPascal (nPa). This corresponds to a mere 0.2 grams weight a square kilometre (g/km2). This is extremely weak compared with even solar radiation pressure. The latter, on which the more well-known and older, but yet unrealised, concept of solar sail - as against this new electric sail - is based, is about 5,000 times stronger.

To generate sufficient thrust to push a satellite at high speeds with such a weak force would obviously require a sail with a very large area, much larger than what can conceivably be provided by a physical solid surface. But the novel concept evolved over the last three years by Pekka Janhunen and Arto Sandroos of the Finnish Meteorological Institute and published in the journal Annales Geophysicae on March 29 makes use of the fact that the effective width that a positively charged wire presents to the solar wind plasma, owing to the surrounding electric field, is a few tens of metres. A thick obstacle, about 50 m across, thus repels the positively charged protons in the solar wind and transfers the solar wind momentum to the wire efficiently. If a satellite is so configured that it has a deployed sail in the form of a number of (say, 50-100) very long (say, 20 km) and thin (about 20 micrometre, which is thinner than the human hair) conducting wires (or tethers), the momentum transferred can be maximised (see diagrams, which show only eight wires for simplicity).

If such a complex sail can be deployed in outer space, the effective area of each such wire becomes one square kilometre, which is significant, and the force on a single wire due to a 2 nPa pressure from the solar wind becomes 2 milliNewton (mN). (One Newton is the force required to give one kilogram mass an acceleration of one metre per second per second, or m/s2.) And if there are 100 such wires, the total force will be 100 times this, which is 0.2 N and this is no longer small. That is, for a typical satellite probe weighing 200 kg, this corresponds to an acceleration of 1 mm/s2. This may seem small but the speed picks up fairly quickly, and after acting for about one year, this acceleration would result in a speed of over 30-40 km/s and the satellite would have reached 6 AU, which is already beyond Jupiter. The problem, of course, is how to deploy several very long thin wires stretching out radially like spokes of a wheel into space and how to maintain a high positive potential on them. Janhunen and Sandroos have interesting ideas on deployment, which they believe are simple and workable, though yet to be demonstrated even in the laboratory scale.

It all began with a study on magnetic propulsion that Janhunen had undertaken for the European Space Agency (ESA). This concept dates back to 1988 when D.G. Andrews and R. Zubrin of the United States suggested creating an artificial magnetosphere around the spacecraft by using the high magnetic field of a large loop of superconducting wire, which would deflect the charged particles in the solar wind plasma and thus impart a force to the satellite. Magnetosphere is the region around an astronomical object where the phenomena are controlled by its magnetic field, like the magnetic field surrounding the earth, for instance. The ESA, according to Janhunen, got interested in magnetic propulsion when a particular realisation of this concept, called Mini-Magnetospheric Plasma Propulsion or M2P2, was proposed by Robert M. Winglee of Washington University in 1999. In M2P2, plasma injected from an on-board device is used to "inflate" the magnetic field to a larger size. This is aimed at forming a large magnetic sail even with a relatively compact magnet, where current in the plasma forms the field instead of a loop of cable. But Janhunen found that both the basic idea as well as M2P2 had basic problems.

"For magnetic propulsion to work, we need a way of making superconducting cable in space," points out Janhunen. "But any type of active cooling is probably too heavy because the cable has to be very long and thin. Therefore, we need a superconducting material that can be cooled in outer space by passive cooling. Unfortunately, with the present state of technology, we can cool materials in outer space only to about - 120o C, and that is still much too warm even for the high-temperature superconductors," he adds.

"M2P2, on the other hand, has a fundamental physical reason why it cannot work. Though this is well known to the people working in the field, it's a bit sad it is not published anywhere. The flaw with M2P2 is that the author of the paper completely ignores the momentum that is carried away by the escaping plasma. The injected plasma cannot remain in the vicinity of the spacecraft and it eventually escapes with a rather high velocity. And this force cannot come from anywhere else but from the solar wind. When we analysed this, we found that a large fraction of the solar wind force will not be used in accelerating the spacecraft but in accelerating the plasma. It is the same physical phenomena as in a comet," explains Janhunen.

"But we continued to think along those lines. The solar wind exists there. If the magnetic field doesn't work, there should be some other way to use the solar wind momentum. What about the electric field? It actually turned out that no one had actually looked at this simple idea before. In 2004, we published the basic idea and talked of a mesh or a grid that is kept at a high positive potential to stop the solar wind and produce a force. However, at that time we had no idea of how to technically implement this because the grid has to be tens of kilometres across," he says.

That is where it remained for nearly a year and a half when Androos, a graduate student who had done his master's in Space Physics from Helsinki University, came to the institute and learnt of the problem in a casual conversation. He did simulations on his own by adapting the publicly available XOOPIC software from Berkeley to the problem of interaction of solar wind with a single wire. The simulation showed promising results. "This got me interested to think about it further," says Janhunen. "Okay, we don't know how to deploy this mesh technically. Could we think of some simpler configuration that can be deployed with current technology and demonstrate that this physical principle at least works? And then we came up with this idea of using just these radial wires which are independent of each other."

The principle behind the deployment of the wires is to spin the spacecraft with the help of on-board fuel and propulsion arms (shown in the schematic diagram) that have little rockets at their tips and use the centrifugal force to deploy the wires and keep them stretched. The spacecraft has reels on its perimeter with the thin wires, which begin to unwind simultaneously with the rotation of the spacecraft. The deployment is a slow process, according to Janhunen, because the speed at which it occurs is about 1 cm/s and, therefore, in a full-scale mission, the deployment could last for two weeks. Once the spacecraft acquires a sustained rotation and the wires are deployed, the fuel tank and the arms are jettisoned. The spacecraft is ready to fly in this configuration.

The deployment has to be done in solar wind, which exists at a height greater than about 10 times the earth's radius, about 64,000 km. So long as there is no charge on the wires, there is no effect of the solar wind. The positive potential is achieved by shooting electrons perpendicularly outward and away from the solar wind using an on-board electron gun (blue stream in the diagram).

The baseline potential that is required for achieving high speed is about 20 kV and this can be achieved with about 50 milliamperes of electron current in the gun. The requisite power can be derived from solar panels of about 500 watts (shown in violet-blue in the diagram), which would also power the on-board instruments and motors for the reels.

By losing the negative charge on the electrons, the whole spacecraft and the wires, which were electrically neutral originally, will now acquire a net posi tive charge. When the wires are charged, they are no longer thin and the effective width is 50 m or so because of the extent of the electric field around the wires (shown in red in the diagram). The distance to which the electric field penetrates the plasma depends on the potential on the wires and the solar plasma. Consequently, because of the increased effective area, the solar wind begins to push the sail of wires on the spacecraft, as shown schematically in the diagram.

In practice, the wire cannot be a single filament because of the possibility of breaks caused by micrometeor impacts. It is more like a tether consisting of more than one subwires that are bonded to each other at regular intervals. The structure has to ensure that even two or three breaks in a wire does not break the whole structure, points out Janhunen. The minimum perpendicular width between the subwires, according to him, is determined by requiring that there is a 95 per cent probability that a complete break does not occur during the lifetime of a typical mission and this is about 2 cm.

The authors do not yet have a technique to manufacture a tether made of 20-micron wires but are looking at copper-silver and aluminium-silicon alloys as possible materials. "But how to reel is the main problem," says Janhunen, showing a small model of the reel. "Of course, here it works; whether it works in space, we don't know yet. But we have some ideas. Right now we are studying how to bond these 20-micron wires, which is not easy." One of the ideas of unwinding the reel that Janhunen has is to use two flexible wheels (may not be rubber) and two motors besides the main wire carrying reel, which draw out the wire much like the mechanism in a cassette player.

The thrust is always radial because the solar wind is extremely fast (about 400 km/s), and the high speed is an important factor, points out Janhunen. The wires would bend a little but not significantly to cause any problem. The rotation speed near the tip of the wires being much smaller, about 100 m/s, the rotation does not slow down the wind in practice. The solar wind speed remains roughly constant throughout the solar system, that is, up to 100 AU or so where the solar wind stops completely. This is known as the Heliopause where there is a plasma shock wall and the interstellar plasma begins. Solar wind is variable because of solar flares and coronal ejections - which is why we have space weather - because of which the wires may begin to oscillate. This is not a problem, says Janhunen, but something that needs to be taken into account.

The one-dimensional and two-dimensional simulation results (see picture) show clearly the pushing action of the solar wind with a near ion-free cavity near the wire. Trajectory calculations show that the spacecraft speed saturates at about 50 km/s farther out where solar wind density goes down. But this speed is quite high; the satellite reaches 6 AU (beyond Jupiter) in about a year, 14 AU (Uranus) in two years and 30 AU (Neptune/Pluto) in about four years. Though the workability of the concept has not been demonstrated in a laboratory, Janhunen proposes to do it in the plasma chamber of the ESA soon.

"I think everything else works except this capability to reliably reel. This has not been validated. Everything else this is already known technology," he says. According to him, the performance can even be enhanced by making the wire potential higher. "It depends on the relative properties of the wires and the electron subsystem that are used. It depends on technology and the consequent design options: either make the voltage higher or make the wires longer," he says. "In principle, the wires could be as long as 100 km, beyond which we begin to have a problem because the conductivity of the wire is no longer sufficient to carry the current and also the tensile strength of the wire is no longer enough to keep it stretched. The centrifugal force required will become too high. But that limit is rather high and the solar wind can be still regarded as perpendicular. However, if the wire is very long, the deployment will take a long time besides more fuel. But there are also other ideas as to how to initiate the spin," Janhunen adds.

But what are the current limitations of the basic concept? "The spacecraft speed is crucially dependent on the weight of the spacecraft and the weight of the wires. This implies that this works best for experimental probe missions. We cannot use it for large-scale missions. The other most important limitation is that the thrust direction is always radially outward. We can change the direction by 20-30o by turning the spin plane but we cannot turn it too much because the velocity would drop. More importantly, there is some concern that the system might go mechanically unstable if we turn it edge-on, for example. We turn the spin plane by having a tunable resistor between each tether and the spacecraft, which enables each tether to have slightly different potential than the spacecraft. The trajectory of the spacecraft can also be changed, both in magnitude and direction, by changing the electron gun current and the orientation respectively," says Janhunen.

The Janhunen-Sandroos electric sail concept seems to have given the ESA an idea. The Agency, according to Janhunen, is interested in seeing this as an enabling technology for an Interstellar Heliopause Probe (IHP) mission in about 15 years' time frame. "The ESA would like to have a spacecraft that flies through the boundary into the interstellar space. The scientific objective would be to measure the interstellar magnetic field and plasma undisturbed by the sun," says Janhunen. A technical feasibility study would first be undertaken over the next two years, basically demonstrating the reeling concept in the laboratory and building a prototype. If that is successful, the ESA would decide on a demonstration mission that will test and demonstrate all concepts for an IHP mission, including control of the spin plane, not necessarily with as long wires as would be needed for a full-scale mission, says Janhunen. This could be within the next 10 years and the IHP mission in 15 years.

"This also has other important applications. The most important one that I consider is fetching material from asteroids. If the system weighs only 100 kg and produces a thrust of 0.1-0.2 N, we could fetch about 5 tonnes of material from some near asteroid. It would take about five years to fetch it," Janhunen envisages. The material could be water, which could be used to make rocket fuel in the form of liquid oxygen and liquid hydrogen by electrolysis in high altitudes without requiring active liquefaction. The electric sail principle can also be used for interstellar probes into distant planetary systems, for instance. Explaining how the concept would work for such missions, Janhunen says, "Interstellar plasma is not stationary but is moving very slowly at about 20-30 km/s, which means it is roughly stationary with respect to the satellite. So the plasma cannot be used for accelerating the satellite but for breaking small satellite probes which have been `shot' from the earth at ultra-high speeds using laser or microwave sails."