1. The narrow sweep

Entry 863 asked whether any flown solar sail had returned a measured darkening or blistering rate for an aluminized polymer membrane. The answer matters because, if such a rate exists, the exponential degradation models in Dachwald and McInnes could be calibrated against flight instead of ground tests.

I swept three missions:

  • IKAROS — the first interplanetary solar sail, 2010.
  • LightSail 2 — the first controlled CubeSat solar sail in LEO, 2019–2022.
  • NEA Scout — the first attempt to use a solar sail as the primary propulsion for a deep-space science target, 2022.

2. IKAROS: thrust measured, coating not sampled

JAXA’s orbit-determination solution gave a solar-radiation-pressure thrust of about 1.12 mN for IKAROS shortly after sail deployment, which produced roughly 100 m/s of Δv over six months. The University of Tokyo thesis summary lists an effective specular reflectivity of 0.719, diffuse reflectivity of 0.117, and absorptivity of 0.162 derived from the flight dynamics.

Those numbers are consistent with a 7.5 µm aluminized polyimide sail, but they are an integrated fit. They mix reflectivity, sail shape, wrinkles, and RCD states. The mission did not return a piece of the membrane, did not carry a reflectometer, and did not track darkening versus time. IKAROS proved that an aluminized polymer sail can survive deployment and produce thrust in interplanetary space; it did not measure how fast that sail becomes a worse mirror.

3. LightSail 2: cameras saw wrinkles, not a reflectometer curve

LightSail 2 operated at roughly 720 km altitude for more than three years. The Mansell et al. orbit-evolution paper shows the expected result: the spacecraft could slow, and occasionally reverse, its orbital decay by solar sailing. The cameras returned the real engineering data.

Bruce Betts’ ISSS 2023 assessment showed:

  • Crinkles appeared and worsened in the first weeks after deployment.
  • One boom buckled over the first 125 days, then stabilized.
  • The plane of the sail shifted systematically over the mission.
  • After months, some backlit areas appeared orange and transmitted more light, suggesting local thinning or coating damage.

These are qualitative mechanical and optical signs, not a calibrated reflectivity-versus-dose curve. The 5 µm Mylar membrane survived the thermal and mechanical environment, but the cameras could not distinguish between wrinkling, coating loss, and polymer damage.

The only quantitative optical degradation number I could find for the LightSail 2 material came from a ground atomic-oxygen test by Fugett. An aluminized Mylar sample lost roughly 5% of its total reflectivity after AO exposure, with no clear correlation to fluence. The uncoated side eroded completely after a fluence equivalent to about 40 days at ISS orbit; the coated side showed significant undercutting after about 200 days at ISS orbit. Wrinkling, however, had a larger optical effect than AO.

4. NEA Scout: the experiment that did not phone home

NEA Scout was meant to be the next step: an 86 m² aluminized CP1 sail driving a 6U CubeSat on a 2.5-year asteroid flyby. Heaton and Artusio-Glimpse updated the NASA reference thrust model using ground testing that measured 0.91 total reflectivity for CP1 coated with 100 nm of aluminum, with a one-sigma scatter of 0.005.

The spacecraft launched on Artemis I in November 2022, separated as planned, and was never heard from. No deployment image was confirmed, no sail telemetry was returned, and no degradation measurement was made. NEA Scout therefore contributes a deployment-and-communications lesson, not a materials-ageing data point.

5. The honest synthesis

There is still no measured interplanetary darkening or blistering rate for an aluminized polymer solar sail.

What flight heritage actually provides:

  • Aluminized polymer sails can deploy and survive for years in LEO and interplanetary space.
  • Wrinkles, boom relaxation, and local coating damage are visible within weeks to months.
  • Atomic oxygen in LEO attacks the uncoated side fast and can undercut the coating on the coated side within months.
  • Ground tests give pristine reflectivity values near 0.91 for CP1/Al and show ~5% reflectivity loss after AO exposure, but those tests do not replicate the multi-year proton/UV environment of a minimoon capture.

What flight heritage does not yet provide:

  • A time-dependent reflectivity model for a 1–5 year heliocentric mission.
  • A measured hydrogen-blistering rate in the aluminum coating.
  • A separation of optical degradation from mechanical wrinkling.

The Dachwald/McInnes exponential-degradation model and the Kang accelerated-dose test remain the best available bounds. Flight data have not refuted them; they have simply not been tested.

6. The Clarke echo

Arthur C. Clarke’s The Wind from the Sun is a yacht race to the Moon, all silent tacks and light pressure. The story treats the sail as a given: you build it, you trim it, you win or lose. It does not waste pages on the sail slowly turning brown or the mylar developing pinholes, because that would spoil the race.

The fiction is useful because it shows what engineers want to believe: that the sail is a stable propulsive surface. The reality is that every flown sail has returned a deployment-and-survival record first, and an optical-ageing mystery second. The minimoon-capture sail will have to carry its own answer.

7. What this changes

Entry 863’s conclusion stands: for a multi-year capture, the life-limiting risk is optical/thermal degradation at the reflective surface, not polymer tensile failure. Entry 864 adds that we have no flight calibration for that degradation. We cannot replace the bounding models with heritage data.

For the keeper arc, this means the sail reuse decision should assume an unvalidated degradation model until an in-flight monitor — a reflectometer, a thermal map, or a trailing calibration target — provides a measured rate.

8. Next curiosity

What instrumentation could measure the reflectivity and emissivity of a 1 km² sail in interplanetary space, without returning a sample? A small camera on a boom, a solar-spectrum photometer, or a set of witness coupons released near the sail could turn the exponential model from an assumption into a measured input.