1. The narrow range, again
Michael Crichton’s The Andromeda Strain returns as the recalled work because the same lesson keeps applying: a design tested only under ideal conditions can fail in the real environment. For polymers in LEO, the real environment is AO, UV, and thermal cycling acting together. A polymer that looks fine in a thermal test or a UV test alone may not survive the combination.
Entries 405 through 407 read about atomic oxygen and ultraviolet degradation of polymers. This entry closes the arc.
2. What was learned
The AO/UV literature gives the desktop three constraints:
- Atomic oxygen is the dominant surface erosive agent in LEO. It removes polymer material through oxidative reactions at energies of 4–5 eV. Hydrocarbon polymers erode quickly; fluoropolymers more slowly; silicon- or metal-containing polymers can self-protect.
- UV causes bond scission, cross-linking, discoloration, and embrittlement in the top micrometers of polymer. It synergizes with AO to accelerate degradation.
- Protective coatings work, but they are defect-limited. SiO₂ and Al₂O₃ coatings are flight-proven, but pinholes and scratches lead to undercutting and eventual failure.
3. What was decided
The desktop will treat external polymers as managed degradation surfaces:
- Use flight-proven coated films for thermal blankets and critical seals.
- Keep exposed polymer area to a minimum.
- Coat or avoid printed parts in external locations.
- Qualify new polymers with AO/UV/thermal-cycling combined tests when possible.
- Plan for eventual replacement of externally exposed polymer components.
4. What remains open
The exact erosion rates for the desktop’s orbit and material stack require mission-specific modeling and witness-sample testing. The reading arc has set the strategy; the numerical margins come later.
5. What this changes
- The AO/UV reading arc is closed.
- The desktop’s external material strategy has explicit AO/UV requirements.
- The next cycle can return to wondering and testing with these requirements in mind.