1. The winter planet rule
Ursula K. Le Guin’s The Left Hand of Darkness takes place on a world locked in ice, where warmth is scarce and every shelter is a negotiation between inside and outside. The desktop’s problem is the reverse: it generates warmth and must build shelters for the cold outside, but the principle is the same. Heat always goes somewhere, and the engineer’s job is to decide where.
Entry 242 defined the thermal control attachment’s purpose. This entry estimates the heat load and the radiator area required to reject it.
2. The heat stack
Almost every watt of electrical power consumed by the desktop becomes heat. The main sources are:
| Subsystem | Heat to reject |
|---|---|
| Platform bus electronics | 20–30 W |
| Compute attachment | 35–70 W |
| Storage attachment | 8–20 W |
| Communications attachment | 12–30 W |
| Sensor attachment | 8–16 W |
| Thermal control heaters and pumps | 15–30 W |
| Robotics attachment | 5–10 W average, higher intermittent |
Total average heat load: roughly 100–200 W. A practical planning number is about 180 W, close to the average electrical demand from entry 238.
Peak heat load, when imaging, processing, and downlinking coincide, can reach 300–500 W. The radiator must handle the average continuously and the peaks for limited durations.
3. Radiator physics
A radiator rejects heat by radiation to deep space, which is effectively at 3 K. The power rejected per unit area is given by the Stefan-Boltzmann law:
q = εσT⁴
where ε is emissivity, σ is the Stefan-Boltzmann constant, and T is the radiator temperature in kelvin.
A black radiator at 280 K with emissivity 0.85 rejects about 330 W/m². At 300 K, the same radiator rejects about 390 W/m². At 260 K, it rejects about 230 W/m².
The choice of radiator temperature is a trade. Higher temperature means smaller area but requires the heat sources to run hotter. Lower temperature is easier on electronics but demands more radiator area.
4. Radiator area estimate
For 180 W average at 280 K:
- Required radiator area: roughly 0.55 m².
Adding margin for view factor losses, degradation, and non-ideal pointing, the practical area is about 0.7–1.0 m².
This is smaller than the 3 m² solar array. However, the radiator must be oriented away from the sun and Earth, which makes its placement more constrained than the solar arrays. It also competes with antennas, sensors, and payload apertures for the limited outward-facing surface.
5. Heat transport
The heat is generated inside the desktop’s body, not on the radiator surface. It must be moved from the electronics, batteries, and payloads to the radiator.
For the desktop’s heat flux levels, heat pipes are the simplest and most reliable transport. A heat pipe is a sealed tube with a wick and working fluid. Heat evaporates the fluid at the hot end; vapor flows to the cold end and condenses; the wick returns the liquid by capillary action. There are no moving parts.
Aluminium-ammonia heat pipes are common for LEO spacecraft. They operate well across the temperature range the desktop needs and have decades of flight heritage. Heat pipes can be embedded in radiator panels or in mounting plates beneath electronics.
6. Coatings and insulation
The radiator surface should have high emissivity and low solar absorptivity. A common choice is black paint or a treated optical solar reflector over aluminium. The combination gives high emissivity for heat rejection and low absorptivity for solar heating.
Surfaces that should not radiate, or that should stay warm, are covered with multi-layer insulation. MLI reduces radiative heat exchange with the environment. It is lightweight and effective, though its performance depends heavily on layer count and edge sealing.
7. Heaters
Even with good insulation, some parts of the desktop will get too cold during eclipse. Batteries, in particular, lose capacity below 0 °C. Mechanisms with lubricated bearings may also need warming.
The thermal control attachment includes patch heaters for these cold-sensitive items. The total heater power is part of the electrical budget. From entry 238, the thermal control subsystem is allocated 20–40 W; much of that is for heaters during eclipse.
What this changes
- The desktop must reject roughly 180 W of heat on average, with peaks to 300–500 W.
- The radiator area required is roughly 0.7–1.0 m² at 280 K.
- Heat pipes move heat from sources to radiators without moving parts.
- Radiator coatings are high-emissivity, low-absorptivity; MLI keeps other surfaces warm.
- Heaters protect cold-sensitive components during eclipse.
- The next entry can decide whether the desktop uses deployable radiators or body-mounted panels.