First page Back Continue Last page Summary Text
Notes:
Whenever one talks about solar sails, invariably someone asks why solar wind doesn’t also push on solar sails. This is much like asking why we drive on the parkway but park on the driveway. The answer is, not much. But it is helpful to calculate the number, since this will be important later. The solar wind, which is primarily hydrogen and helium ions boiling off the surface of the sun (also known as plasma), has a variable density of about 3 particles per cubic centimeter, and a variable speed peaked around 400 km/s. By multiplying by the mass of a hydrogen ion (and increasing by the 25% helium enrichment) we get a solar wind pressure of 1 nanoPascal. That’s nearly 10,000 times less than the solar photon pressure, and a lot less dependable. Given this disparity in pressure, most solar sail designs ignore the solar wind.
However, for those physicists such as I who specialize in solar wind and space plasmas, this oversight is most unfortunate. While considering Jupiter’s effect on the solar wind, Dr. Robert Winglee a space plasma physicist at the University of Washington, realized that this low solar wind pressure might still be significant if a large enough obstruction were made. Jupiter has the largest obstruction (called a magnetosphere since it is generated by a magnetic field) in the solar system, primarily because one of its moons, Io, has sulfur volcanoes, and inflates the magnetosphere by filling it with plasma. This plasma is still too tenuous to see, but if it were visible, the magnetosphere of Jupiter would appear larger than the full moon! If one could build a mini-magnetosphere, Winglee reasoned, and inflate it with plasma, it might be possible to sail on the wind from the sun.