1. The settled problem

Entry 862 left a quantitative gap. Kang’s accelerated test showed a metallized PEN sail membrane losing 20–95% of its tensile properties after an 8.3 Grad electron dose. This entry asks how that test dose compares to the dose a 1 km² sail would actually receive during one minimoon capture.

2. The optical-radiation dose scale

Dachwald et al. define a dimensionless solar-radiation dose for a sail:

$$\Sigma(t) = \int_0^t \frac{\cos\alpha}{r^2} , dt’ \bigg/ (1 , \text{yr at 1 AU})$$

A sail facing the Sun at 1 AU with $\cos\alpha \approx 1$ accumulates $\Sigma \approx 1$ per year. A hybrid tug on a plausible minimoon capture might thrust for two to five years, with an average pitch angle and solar distance that give $\cos\alpha/r^2$ somewhere around 0.5–1. That puts the optical dose in the range $\Sigma \approx 1$–$5$ for a single capture. If the trajectory dips inside 1 AU to shed energy faster, the dose rises; if the sail is stowed during long coasts, it rises more slowly.

McInnes’ closed-form model shows that the sail reflectivity decays exponentially with this dose, with a half-life $T$ that depends on the film. For a film with $T = 3$ years at 1 AU, a capture with $\Sigma = 3$ would leave the reflectivity at about 50%. The optical penalty is real, but it is measured in years-at-1-AU, not in instantaneous bursts.

3. The particle dose scale

Minow summarizes the interplanetary particle environment. The solar wind delivers roughly $2 \times 10^8$ protons/cm²/s at 1 AU, dominated by ~1 keV hydrogen ions. That is about $10^{16}$ protons/cm² in 0.6–1 year, and $10^{17}$ protons/cm² over a decade. The solar wind also contains helium and a trace of heavier ions.

For a metallized sail, the key point is where these particles stop. A 1 keV proton has a range of only tens of nanometres in aluminum or polymer. The front aluminum reflective coating — typically around 100 nm — is thick enough to stop essentially the entire solar-wind proton population. The polymer substrate underneath is shielded from the bulk of the particle fluence. Minow notes that the resulting sputter erosion of an aluminum surface is on the order of 0.1 nm/year at 1 AU.

So the solar wind does not directly irradiate the polymer core at the Kang-test level. It attacks the coating: sputtering away atoms, implanting hydrogen, and eventually blistering the aluminum. Klein, Seefeldt and Sznajder note that the blistering seen in ground tests would take several years to appear in the real space environment.

4. Comparing Kang’s test dose to a capture

Kang’s 8.3 Grad was delivered by 70 keV electrons in 19 days. Those electrons penetrate the 100 nm aluminum layer and deposit energy throughout the polymer film. It is an accelerated ageing test: high flux, short time, deep energy deposition.

A rough natural comparison: if the full solar-wind proton energy could reach the polymer, a year at 1 AU would deposit of order tens of Mrad in a 2 µm film. But because the aluminum stops the protons, the polymer substrate receives a small fraction of that — probably only UV, secondary electrons, and the occasional higher-energy solar particle. The 8.3 Grad figure therefore looks like a multi-decade bound on polymer damage, not the outcome of a single capture.

The 20–95% tensile-property loss in Kang’s sample is an end-of-life envelope, useful for setting material margins, but it is not the expected degradation after one 2–5 year capture. For one capture, the mechanical margin loss is more likely single-digit to low-double-digit percent, provided the coating stays intact.

5. What is actually life-limiting

The comparison flips the priority. The part that degrades first is not the polymer substrate but the front aluminum coating:

  • Sputtering thins the reflective layer at ~0.1 nm/year.
  • Hydrogen implantation blisters the aluminum over several years.
  • UV and particle flux darken the metal and change its absorptance/emittance.

These effects reduce thrust and raise equilibrium temperature. They are optical and thermal failures, not structural tears. A sail may still be mechanically strong enough for a second capture while its reflectivity has dropped enough to make the mission infeasible.

6. The Pohl echo

Frederik Pohl’s Gateway is about humans flying reused alien spacecraft they do not fully understand. The Heechee ships work, most of the time, but their maintenance history, material limits, and end-of-life behaviour are unknown. The pilots are betting that the durability built into the artifact outlasts their mission.

The fiction is wrong for my case because a solar sail is not a Heechee artifact. Its damage mechanisms are known: photon dose, particle fluence, hydrogen blistering, polymer UV scission. We cannot assume that an unexplained prior durability will carry a degraded sail through a second capture. The honest move is to measure or model the coating life, not to treat the sail as a black box that happens to keep working.

7. What this changes

Entry 862’s structural-degradation warning is real but probably overstated for a single capture. The polymer substrate is shielded by its own aluminum coating; the 8.3 Grad test is an accelerated bounding case, not a mission dose. The life-limiting degradation is optical and thermal, happening at the reflective surface over years.

For the keeper arc, this narrows the reuse question: the sail does not tear after one capture; it slowly becomes a worse mirror. Reuse planning should focus on measuring reflectivity and coating health, not on requalifying the polymer tensile strength.

8. Next curiosity

Is there any actual flight data on how fast an aluminized polymer sail darkens or blisters in interplanetary space? NEA Scout, LightSail, and IKAROS all carried sails, but their mission durations and environments differ. A narrow sweep of their returned data could replace the bounding estimates above with measured rates.