It is well known that if a solar sail can specularly reflect sunlight, it not only gains a factor of 2 in momentum, but it gains the ability to ``tack'', or generate thrust at an angle from the radial. Despite this advantage, one of the more promising solar sail candidates at the present is the black carbon-fiber sail, that can only absorb sunlight, and therefore cannot ``tack'' efficiently. It is promising because of its robust performance with respect to heat, tearing, deployment and mass-loading. It is instructive to examine microscopically how a carbon-fiber sail accomplishes this feat. It replaces a 2-D film with a 3-D mesh of carbon fibers which have on average, a smaller diameter than the thickness of the 2-D film. Yet it is remarkably robust, primarily because of its 3-D structure. Somehow an open mesh of mostly empty space can be stronger than a 2-D solid structure of the same weight.
In the same way, we view a plasma sail as an open mesh of mostly empty space, held together by long-range electrostatic and electromagnetic forces, rather than chemical bonds. In this sense, a plasma sail is just the extrapolation of a carbon sail to infinitesimal fibers. The components of a plasma are an even mixture of ions and electrons that form an electrically neutral fluid. The ions can be simple atoms that have lost a few electrons, or they can be massive particles with a net charge of many hundreds of electrons, or an admixture of both. The response of the plasma to a force depends on its magnetization as well as ion's mass. The properties of a plasma, then, are crucial in determining whether a plasma sail can withstand the pressure of sunlight and solar wind without ripping it to the shreds we see in comet tails.
The simplest plasma sail one can imagine is an atom that when ionized,
still absorbs or scatters sunlight. All ions that are not fully stripped,
that is, having at least one electron left, possess this property. Since
the solar wind is composed of mostly fully stripped H
and He
it remains invisible. But we could use for example, Ba, or Li ions that scatter some characteristic frequency of light, which are the
priciple means for coloring fireworks. The trouble with using an colored
ion for the plasma, is that it absorbs such a narrow part of the solar
spectrum, resulting in a small efficiency increase. Ideally, one would
like a ``black'' ion, one that absorbs all the sunlight. A viable
alternative is to use molecules or clusters that form charged dust
grains, or dusty plasmas. These plasmas occur naturally around comets or
within Jupiter's magnetosphere, where they can be observed with
telescopes. The field of dusty plasmas is brand new and many questions
have yet to be answered, but we can still draw some conclusions from
observations of comets.
Comets have two tails, a dust tail and a plasma tail. The dust tail somewhat follows the orbit of the comet, whereas the plasma tail is stretched radially in the direction of the solar wind. Indeed, these observations were the first indication of the existence of the solar wind. But this indicates that the dust does not stick to the plasma, it is not contained by the draped magnetic fields of the comet. If we are going to make a ``black'' magnetic balloon out of dusty plasma, we must find a way to get the dust to stick.
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These then form the two prongs of an experimental approach towards developing second generation plasma sail materials: an investigation of ``sticky'' dusty plasmas, and an investigation of ``black'' plasma materials. The first problem, of maintaining the integrity of a dusty plasma against an external force, is very similar to the problem of dusty plasma levitation against the pull of gravity, an experiment that has been addressed by Edward Thomas Jr at Auburn University. The second problem, of quantifying the scattering coefficients of a dusty plasma, has been addressed by Jim Spann and Mian Abbas of NASA/MSFC, albeit for selected dust types. In figure 2 we show the experimental setups of both groups. We propose to augment both experiments to measure candidate dust types for a plasma sail, measuring the radiative transfer functions for individual dust grains at MSFC, while examining their collective behavior at Auburn University.