In low Earth orbit, residual atomic oxygen (O) slams into a spacecraft's surfaces at ~7.8 km/s orbital speed. How hard a given surface gets hit depends on its orientation to the ram (flight) direction — a face pointed straight into the flow takes the full flux, while a face angled away sees far less, following the same cosine law that governs any flux through a tilted surface:
Flux(θ) = Flux₀(h) · max(0, cos θ) θ = incidence angle from ram
Fluence F(t) = ∫ Flux(θ) dt [atoms / cm²]
Depth d = Ey · F [cm], Ey = erosion yield [cm³ / atom]
Flux₀(h) ≈ F₀ · exp[-(h - 200 km) / H], H ≈ 60 km scale height
The cross-section on the right tracks four representative surfaces around a cylindrical spacecraft body: the ram-facing nose (θ=0°, full flux), a side panel (θ=55°), a shallow grazing panel (θ=80°, barely touched), and the wake-facing tail (θ=180°, aerodynamically shadowed — effectively zero AO exposure). Erosion yield Ey is a material property: reactive polymers like Kapton (~3.0×10⁻²⁴ cm³/atom) recede fast; PTFE resists via a fluorocarbon surface; bare aluminum's native Al₂O₃ oxide is already fully oxidized and barely erodes. Toggling the protective overcoat applies the same ~100× Ey suppression a real SiO₂ or silicone coating gives — the mechanism NASA adopted after uncoated Kapton multi-layer insulation on Shuttle and ISS showed measurable AO thinning within months.
- Altitude slider — sets the ram-facing AO flux via the exponential density falloff above.
- Material dropdown — swaps the erosion yield Ey for all four tracked surfaces at once.
- Protective overcoat — layers a ~100× erosion-yield suppression on top of whichever material is selected.
- Mission clock speed — mission-years of exposure played per real second.
- The live chart below the cross-section plots each surface's remaining thickness against mission time — note how the wake-facing surface barely moves while the ram-facing nose can breach.