Explain the thermal control challenges and strategies for spacecraft operating in orbit.

Challenges: extreme environments (sun-facing surfaces >150°C, shadowed <-150°C), no convection in vacuum (radiation only), orbital variations (eclipse, solar angle changes), long mission life with degradation, and tight mass/power budgets. Passive strategies: Multi-layer insulation (MLI) with 20+ aluminized layers reducing heat loss by 99%; Surface coatings controlling α/ε ratio (white paint for radiators: low α, high ε; gold for insulation: low both); Heat pipes redistributing power; Thermal mass for eclipse protection. Active strategies: Heaters with thermostatic control preventing cold-soak; Louvers modulating radiator area; Fluid loops for high-power payloads; Cryocoolers for IR sensors requiring <100K. Thermal model: node-network analysis with orbital heating terms (Q_solar, Q_albedo, Q_IR) varying with β-angle. Thermal balance test in vacuum chamber validates flight hardware. Design margins: +10°C qualification, +5°C acceptance from operating limits.