1. The hab’s tape

Andy Weir’s The Martian has Mark Watney patching his habitat with whatever he has, including duct tape, because the difference between living and dying is sometimes the difference between a surface that keeps heat in and one that lets it leak. The desktop is not held together by duct tape, but the thermal finishes perform the same function: they are the thin boundary that separates the inside from the outside.

Entry 244 fixed the radiator configuration as body-mounted panels. This entry picks the surface finishes and insulation that make those panels work.

2. Radiator coatings

A radiator coating must do two things at once:

  • Emit infrared heat efficiently, so the radiator rejects waste heat.
  • Reflect solar energy, so the sun does not heat the radiator back up.

The two properties are expressed by the ratio α/ε, where α is solar absorptivity and ε is infrared emissivity. A good radiator finish has low α and high ε.

Common choices:

Finish α (solar absorptivity) ε (emissivity) α/ε
Black paint 0.85–0.95 0.85–0.90 ~1.0
White paint 0.20–0.30 0.85–0.90 ~0.25
Optical solar reflector (OSR) 0.05–0.10 0.75–0.80 ~0.1
Silvered Teflon 0.05–0.10 0.65–0.80 ~0.1
Polished aluminium 0.10–0.20 0.03–0.05 ~3.0

For the desktop’s radiators, optical solar reflector tiles or silvered Teflon are the right choice. They reflect most solar energy while radiating heat efficiently. OSR tiles are slightly heavier and more expensive than silvered Teflon, but they are more durable against atomic oxygen in LEO.

A practical radiator finish is OSR tiles over an aluminium radiator face, with edge sealing to prevent undercutting by atomic oxygen.

3. Solar array back-surface finish

The back sides of the solar arrays face deep space and can act as secondary radiators. They should also use a low-α/high-ε finish, though they do not need the same performance as dedicated radiator panels because the array substrate is warmer and the area is shared.

Silvered Teflon on the array back surface is a common choice. It keeps the arrays from overheating and contributes a small amount of heat rejection.

4. Multi-layer insulation

Surfaces that are not radiators should be wrapped in multi-layer insulation (MLI). MLI reduces radiative heat exchange by reflecting infrared radiation many times between layers.

A typical MLI blanket has:

  • An outer layer of aluminised Kapton or aluminised Mylar for durability and optical control.
  • Several inner layers of aluminised Mylar separated by mesh or scrim to keep the layers apart.
  • An inner layer that faces the spacecraft and stays clean.

The number of layers sets the effective thermal conductivity. A blanket with 10–20 layers can achieve an effective thermal conductivity of 10⁻⁴ to 10⁻³ W/m·K in vacuum, orders of magnitude better than any solid insulator.

Performance depends on:

  • Layer count. More layers reduce radiative heat transfer but add mass and bulk.
  • Edge sealing. Gaps at seams let heat bypass the layers.
  • Compression. Compressed MLI loses performance because layers touch and conduct heat.
  • Venting. Trapped air between layers must be allowed to escape during ascent, or the blanket inflates and tears.

For the desktop, 15–20 layers of aluminised Mylar with venting paths is a reasonable starting point for the main body blankets.

5. Where MLI goes

Not every surface needs the same treatment.

  • Radiator panels: bare finish, no MLI, to maximise heat rejection.
  • Electronics bay walls: MLI on the outside to isolate the bay from external temperature swings.
  • Batteries: MLI plus patch heaters to maintain a stable temperature band.
  • Propulsion tanks and lines: MLI to prevent propellant freezing or boiling.
  • Antennas and sensors: partial MLI or standalone thermal shields, because full wrapping blocks the view.
  • Deployment mechanisms: cutouts or blankets that do not bind moving parts.

The blanket pattern is part of the thermal design. It is drawn just like wiring: every cutout, seam, and fastener matters.

6. Second-surface mirrors and louvers

For surfaces that need to switch between rejecting heat and retaining it, second-surface mirrors and louvers exist.

A second-surface mirror is a piece of glass with a reflective coating on the back. It reflects solar energy while emitting heat, like OSR but in a different form. It is durable but heavier.

Louvers are mechanical shutters over a radiator. They open when the spacecraft is hot and close when it is cold, giving variable emissivity. They are useful for payloads with wide temperature swings, but they add mechanism and mass. For the desktop’s first version, fixed finishes and heaters are simpler.

7. Contamination control

Coatings and MLI are sensitive to contamination. Outgassing from adhesives, lubricants, and plastics can deposit on radiator surfaces and raise their absorptivity. A radiator that starts at α/ε = 0.1 can degrade to 0.3 or higher over a few years if contamination builds up.

The desktop should:

  • Use low-outgassing materials and adhesives.
  • Keep radiator surfaces oriented away from vents and thrusters.
  • Include margin in the thermal design for degradation.

This is one reason the radiator area estimate in entry 243 included a margin factor. Surfaces in space do not stay clean.

8. Mass estimate

For the desktop’s thermal finishes:

  • OSR tiles or silvered Teflon on roughly 1 m² of radiator area: 0.5–1.5 kg.
  • MLI blankets for the main body and non-radiator surfaces: 1.5–3 kg.
  • Adhesives, fasteners, edge tape, and venting: 0.5–1 kg.
  • Total coatings and insulation mass: roughly 3–5 kg.

This brings the thermal control attachment mass to roughly 8–13 kg including the radiator panels and heat pipes from entry 244.

What this changes

  • The desktop’s radiators use optical solar reflector or silvered Teflon finishes for low solar absorptivity and high emissivity.
  • Non-radiator surfaces are wrapped in 15–20 layer MLI blankets.
  • The solar array back surfaces contribute secondary heat rejection with silvered Teflon.
  • Contamination control and degradation margin are part of the thermal design.
  • The coatings and MLI add roughly 3–5 kg.
  • The next entry can close the thermal arc and pick the next attachment to define.