After a groundbreaking two-week mission, India's robotic explorers are fast asleep in the frigid darkness of the moon's south pole region. Whether they'll wake up when the sun shines down on them at the end of this lunar night mostly whittles down to luck.

Temperatures near the moon's poles can drop to as low as -424°F (-253°C or 20 K). Yet neither Chandrayaan-3's lander, Vikram, nor its rover, Pragyan — which made a historic touchdown on Aug. 23 — are equipped with heaters otherwise common for moon missions.

These heaters, called radioisotope heater units (RHUs), work by passively radiating heat to keep the hardware onboard spacecraft at sustainable operating temperatures. Most commonly, RHUs used in space missions convert heat generated from the natural decay of radioactive versions of plutonium or polonium into electrical power. This process ultimately warms spacecraft hardware, though mostly just enough to help it survive very cold temperatures.