When we think of solar sails, I guess we mostly think of using radiation pressure of our closest star to achieve thrust, gracefully leaning on it with a large area sail of incredibly light and thin reflective materials. The idea, we now know, works and there's no doubt about it. We're now way past the early days of testing torque on a tiny carbon fiber sail by "light" alone in laboratory environment, something I still vividly remember watching nearly live (in those days that meant with months of delay, but it still felt as live if the science was cutting edge).

But the emissions from our Sun aren't exactly clean, and there's much more to it than what meets the eye, apparent to most of us only through photographs (and if we're lucky enough also seeing them in person) of polar auroras, a display of lights as the highly charged particles of solar winds hit the upper layers of our atmosphere and ionize the air molecules.

Such proton flux is however everyday's business as usual in the outer space, beyond protective shroud of Van Allen radiation belt, with not much in the way of decreasing kinetic energy of these highly charged, high velocity mass particles from solar emissions, so it's hardly radiation pressure alone that actually interacts with whatever we send there. Solar sails being no exception. But as they're meant to be extremely light and thin, I imagine most of such proton emissions would just push through them, with not much matter in their way to decrease their kinetic potential, and would slowly decay the sail's fabric. So here's my question:

With current materials used for solar sails, how fast would proton flux decay them and lower their performance, if used in regions outside of Van Allen belt? What is their life expectancy, and does this mean they wouldn't be suitable for long duration missions, say as means to regain some of the velocity of Aldrin Cyclers as it would be lost during taxiing?

From this question on How much would a solar sail's usefulness be reduced by a perforation? and its answer, we know that the expected performance is directly proportional to the area ratio of damage it would sustain. Obviously, this proton decay would also be relative to total proton flux hitting it, its density depending on the distance to the source of emissions. But as far as I understand it, it should be directly proportional to solar sail's expected performance (total insolation), and also a function of time, so there should be some way to infer that regardless of where in the outer space it would be used. Additionally, would this decay rate be any different above and below the solar equator, where I expect proton emissions from the Sun are less frequent?

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    $\begingroup$ ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/… $\endgroup$ Jan 15, 2014 at 10:06
  • $\begingroup$ @DeerHunter That's really cool, and I'll bookmark that for when I get a chance to read it, but at a first glance, it seems it's only about environment that's still well within the Van Allen belt (OK, less so over SAA, but that's a small fraction of the ISS's orbit). $\endgroup$
    – TildalWave
    Jan 15, 2014 at 10:09
  • $\begingroup$ You are right, that was the first iteration on aluminized kapton. Atomic oxygen, UV and thermal cycling seem to be dominant hazards on LEO missions. Would bet that DoE labs ran a few accelerator experiments with materials, could even put them on the 'Net via OSTI. $\endgroup$ Jan 15, 2014 at 10:13
  • $\begingroup$ DOI: 10.1016/j.radphyschem.2006.12.005. Ruiqi Li; Chundong Li; Shiyu He; Mingwei Di; Dezhuang Yang. Radiation effect of keV protons on optical properties of aluminized Kapton film. Radiation Physics and Chemistry, 2007 v.76 pp.1200-1204. $\endgroup$ Jan 15, 2014 at 10:18
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    $\begingroup$ There are a bunch of books and datasets on space weather; I'd start with Hastings & Garrett - Spacecraft-Environment Interactions, 1996, Cambridge Univ.Press. $\endgroup$ Jan 15, 2014 at 10:30

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Solar sails are generally envisioned from very lightweight reflective materials like aluminized mylar and this is what the Planetary Society vehicle used. This material is regularly used in multi-layer insulation (MLI) blankets which are on the outside of a great number of spacecraft. Structural degradation of these materials is slow but steady, even in relatively high radiation environments. Optical degradation occur quicker and can be quite rapid if you have contamination from the rest of your spacecraft. Because outgassing from the spacecraft gets less as time goes on, most of the optical degradation from contamination occurs in the first few weeks. Radiation darkening is steady, of course. To put time scales on it, it would likely take many years, maybe a decade, to see any significant perforation of the sail that would show up in its performance, say 25%. Some loss of reflectivity and thus performance will occur due to contamination, but it will take a very large flux of protons to affect the aluminum coating so the performance hit is likely to only be around 10% after a decade or two. Some interesting data on MLI performance can be found looking at material collected from Hubble servicing missions http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110015287.pdf


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