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Solar wind is composed of a mixture of materials travelling between 250 and 750 km per second.

Radiation pressure involves electromagnetic field travelling at light speed.

If considering a sail in circular heliocentric orbit at 1 AU, what proportion of the thrust the sail can generate, is due to solar wind, and what proportion is due to radiation pressure?

Is it correct to state that solar radiation pressure is the sum of solar wind and radiation pressure? Are radiation pressure and photonic pressure the same?

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  • $\begingroup$ Besides solar sails, which use mirrored surfaces to reflect light, there is the concept of magnetic sails, which use a current-carrying loop of wire to catch the solar wind. I say "concept" because, as far as I know, none of have been tested. A potential advantage is that, as it gets bigger, the mass of a loop of wire increases linearly with diameter, while that for a sail increases as diameter-squared. A disadvantage is the power supply required unless you have a superconductor that can operate in those conditions. $\endgroup$ – Greg Dec 28 '19 at 0:20
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According to wikipedia, solar wind pressure at 1AU is about 1-6 nN/m2. Solar light pressure on the other hand is apparently 4.54-9.08 μN/m2. It therefore seems unlikely that solar wind pressure will make up a substantial proportion of the sail's thrust.

The solar wind is generally a mix of electrons, protons and alpha particles with energies of (say) 1-10keV. Using the NIST PSTAR, ASTAR and ESTAR tools, you can see that the stopping powers for protons, alpha particles and electrons at those energies in kapton film are (approximately) 174.6-440.4, 171.5-529.5 and 108.2-20.59 MeV cm2/g. Given that kapton has a density of 1.42g/cm3 and that a common film thickness is apparently 2μm, that means that you might reasonably expect the sail membrane to be capable of stopping everything except the 10keV electrons. (I'm ignoring the nanometre layers of aluminium and chromium as the extra effort seems unwarranted, given the puny penetrating powers of solar wind particles at these speeds).

The photons hitting the sail are probably going to be incident on a nice, highly reflective surface designed to bounce them back off again. The particle radiation on the other hand is probably not going to be involved in nice elastic collisions, and is instead likely to scatter off in some odd direction, and/or ionise part of the sail material and heat it up rather than imparting much momentum. Alas, I can't tell you the likely average momentum flux of the solar wind on the sail, other than to say that it seems likely to be pretty negligible, probably less than half a percent of the contribution of light pressure.

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