Introduction

The concept of a sail is so ancient as to defy attribution, yet so modern that it is constantly re-engineered and redefined. For example, the parasail is a relatively new invention that remakes the simple sail into an airfoil, and thereby spawned a whole new sport. If we are permitted to broaden the scope of a sail to include any device that couples momentum from the atmosphere, we might be able to include sailplanes and autogyros. In exactly the same way, if the definition of solar sails were broadened to include any device that couples spacecraft momentum to sunlight, we would be able to include magnetic balloons and plasma sails, which may do for solar sailing what parasails did for parachutes.

The first recorded occasion when mankind could the ground and took to the air might be considered when the Montgolfier brothers sent a sheep up in a hot air balloon in the 1783. Balloons played an important role in warfare 100 years before aircraft, being used by both sides in the Civil War as forward observation posts. Even today, ambitious entrepreneurs are attempting to fly dirigibles equipped with cell phone antennas 10 miles above a metropolitan area and thereby cover 10,000 square miles with one ``tower". So we see that balloons fill two roles: they are often the first steps into conquering new territory, and they occupy some specialized niches much more effectively than the more versatile, higher technology aircraft.

These are the same two roles that magnetic ballooning fills with respect to solar sails. Solar sails are undoubtedly high technology. Materials that are incredibly thin and light have to be deployed over square kilometers of area and stretched to mirror smoothness. While great strides have been made in materials the long term survival of such fragile films, and the deployment of a large sail are still areas needing work. Knowledgable estimates suggest 5-10 years for the deployment of a 100 meter sail that will act as a testbed for future work. In light of the real technological hurdles facing solar sails, is there an easier way to harness the promise of infinite Isp solar propulsion? Yes, if one is willing to accept some limitations that come with the great simplification of ballooning.

Robert Winglee of the University of Washington has proposed and recieved funding from the NIAC to develop the concept of magnetic ballooning, which he prefers to call ``M2P2'' for mini-magnetospheric plasma propulsion (Winglee, JGR 2000). Winglee used a computer code for plasmas that he had developed over the past decade to show that a magnetic field can be inflated by a plasma to many times its original size. Then when this inflated magnetic field is placed in the supersonic solar wind, the wind transfers momentum to the magnetic field which accelerates at a constant rate until it approaches the speed of the solar wind, about 400 km/s. One convenient feature of his balloon is that it expands as it gets further from the sun, much like a weather balloon expands as it rises through the atmosphere, so that unlike a solar sail, a balloon experiences a constant force at all distances from the sun. This theoretical model has been tested with a scale model placed in a 30 ft vacuum chamber at MSFC, providing the first demonstration of the inflation of a balloon, and the deflection caused by hitting the balloon with a simulated solar wind. In figure 1 we show an artist's rendition of such a satellite employing a dusty plasma sail to rendezvous with Mars.

Figure 1: Left: Engraving of Montgolfier balloon; Right: An artists conception of a magnetic balloon with a plasma sail en route to Mars.

The principle of ballooning is simple, but involves some tradeoffs that require some quantitative discussion. The strength of sunlight at the Earth's orbit is about 1.4 kW/m$^2$. Einstein demonstrated that light is a particle and therefore has a momentum given by $p = E/c$. This means that the intensity of sunlight given above can be converted into a pressure (=force/area) by dividing by the speed of light, $c$. Working in SI units, that would be 4 $\mu$Pa of pressure generated by light on a black sail. But in addition to sunlight, the sun is evaporating, sending out heated hydrogen and helium in a supersonic radial flow known as the solar wind. The solar wind, like the Earth's wind, varies in speed from 300-1000 km/s, but on average is about 400 km/s. The density is quite low, however, between 1-10 atoms/cc, which has an average around 3/cc. So if we have a square meter of area intercepting this flow, the pressure exerted by the solar wind would be the momentum of all the particles in a volume 1m$^2$ x 400 km, or 1.2x10$^{12}$ proton x 1.67x10$^{-27}$ kg/proton x 400 km/s = 0.8 nPa. Since the solar wind is about 5% Helium, it adds about 25% to the mass density, which makes this a convenient round number of 1 nPa for the pressure exerted by the solar wind. The result of this calculation is that solar wind is about 4000 times less effective than sunlight in pushing a sail.

This calculation is why the solar sail calculations rarely add the forces due to the particles. A magnetic balloon, on the other hand, intercepts most of the solar wind pressure and very little of the sunlight pressure. So why would anyone prefer a magnetic balloon? Well, when calculating the acceleration of a spacecraft, it isn't the pressure that is needed, it is the total force divided by the total mass. If a solar sail is 1000 times heavier than a balloon of the same size, then even if it does experience more pressure, its acceleration is less. Worse than that, as the sail leaves the sun, its acceleration drops with decreasing light intensity whereas a balloon keeps a constant acceleration (theoretically at least out to the heliopause, which is beyond the orbit of Pluto).

Can we estimate the size/mass ratio of a balloon? This is the calculation Winglee has done. He argues that a simple magnetic field, say, from a permanent magnet, doesn't make much of a balloon. However, when plasma is injected into that magnetic field, much the way Jupiter fills its magnetic field with plasma, then the size of the balloon is increased 10 or 100 times. (A magnetic field filled with plasma was called a ``magnetosphere'' when it was discovered around the Earth 40 years ago, which accounts for Winglee's terminology). For example, if we could see Jupiter's magnetosphere from Earth, it would be larger than the full moon, being about 100 times larger than Jupiter itself. Therefore, Winglee argues, we can build a spacecraft that inflates a magnetic balloon much larger than the spacecraft itself. Winglee estimates that a 30 km wide balloon can be inflated with ``commercial off-the-shelf'' (COTS) components on a 1-meter sized spacecraft that weighs about 500 kg. In solar sail terminology, that is $<$0.001 gm/m$^2$, which is a very low mass-loading. Even with the 1/4000 times weaker solar wind pressure, the resultant acceleration is still equivalent to a 10 gm/m$^2$ solar sail/spacecraft.

Are balloons then, destined to be surpassed by solar sails when sails can finally be made with mass loading $< 10$ gm/m$^2$? Not necessarily, for if magnetic balloons can tap, if ever so slightly, into the sunlight pressure, they can increase their exfficiency up to 4000 times, in essence, becoming a solar sail. Imagine for a moment, that there exists a gas that is bright yellow when it is ionized. If the magnetic balloon were inflated with this yellow plasma, then it would not only deflect solar wind, it would deflect sunlight. Perhaps not all of the sunlight, but even 1% of the sunlight would be a factor of 40 more acceleration for the balloon. That is, a 1% opaque plasma filling a 500 kg balloon to a diameter of 30km would have an equivalent mass loading of 0.1 gm/m$^2$. Not only does this surpass the expectations of the AO, but such a plasma sail would be easily deployed, resistant to tearing, compactly stored, and radiation hard. This then, is the purpose of this proposal: to investigate the potential for magnetic balloon materials that are opaque to sunlight, which we refer to as ``plasma sails'' in response to A.2.B "Solar Sails: Long-term technology development".