In satellite thermal design: when a communication satellite enters Earth's shadow (eclipse) with no direct solar heating, its external surface temperature can plunge to approximately what value (express your choice in kelvin, K)? Provide the best typical textbook approximation used for GEO/LEO eclipse discussions.

Introduction / Context:
When a spacecraft passes into Earth’s shadow, the Sun's radiant heating suddenly disappears. The thermal balance shifts to internal dissipation and deep-space radiation. This question checks fundamental awareness of eclipse temperatures for communication satellites and the typical order-of-magnitude value used in basic satellite engineering courses and competitive exams.

Given Data / Assumptions:

• No direct solar input during eclipse.
• External surfaces radiate mainly to deep space (~3 K background).
• Only internal equipment dissipation and Earth infrared/albedo (much reduced in full shadow) contribute to heating.
• Looking for a commonly cited approximate temperature for an unheated external surface in eclipse.

Concept / Approach:

Spacecraft surface temperature is governed by radiative balance: absorbed power equals radiated power. Without solar input, temperatures rapidly drop until a new equilibrium is reached. Classic first-order figures used in many texts put “cold case” external temperatures around 120 K to 130 K for unheated surfaces, which is a helpful design anchor for thermal control systems (MLI, louvers, heaters).

Step-by-Step Solution:

Verification / Alternative check:

Thermal analyses vary by coating emissivity/absorptivity and geometry. However, exam-centric values often cite ~120 K as a representative eclipse external temperature, validating selection of 123 K over higher values like 170–273 K.

Why Other Options Are Wrong:

• 273 K: near room temperature; unrealistic without solar input and with deep-space radiation prevailing.
• 243 K and 170 K: still warmer than the classic baseline “cold case”; 170 K may occur with significant internal dissipation or specific surfaces, but 123 K is the standard lower benchmark taught.
• —: not a valid engineering choice.

Common Pitfalls:

Confusing average internal avionics temperature (actively controlled) with uncontrolled outer surface skin temperatures in eclipse; also forgetting that coatings and MLI can push actual values up or down around the textbook baseline.

Final Answer:

123 K