What Happens to a Moon Rover During 14 Days of Lunar Night: The Chandrayaan-3 Story

The Moon takes about 27.3 days to complete one rotation on its axis, the same time it takes to orbit the Earth. That synchrony is why we always see the same face of the Moon from the ground. It also means that any single point on the lunar surface spends roughly 14 Earth days in sunlight, then 14 Earth days in complete darkness. Not a few hours. Not an overnight. Fourteen days of unbroken night, followed by fourteen days of unbroken day.

During the lunar day, surface temperatures near the equator climb to around 127°C. When the sun sets, those same patches of ground drop to -173°C. The swing is 300 degrees Celsius across a fortnight. No atmosphere buffers it. No cloud cover slows it. The heat simply radiates away into space the moment the sun disappears.

This is the environment every rover must be built to survive, or not.

Electronics have operating ranges. Most commercial components stop functioning reliably below -40°C. Spacecraft-grade hardware is rated lower, but even purpose-built systems face serious risk in the -150°C to -173°C range that the lunar night routinely delivers at the poles and mid-latitudes.

The first problem is battery chemistry. Lithium-ion cells, the same basic technology in every smartphone, lose capacity sharply in the cold and can suffer permanent internal damage if discharged below a threshold temperature. A rover that cannot keep its batteries warm enough will find them dead or degraded when sunrise finally arrives.

The second problem is mechanical. Metal contracts in extreme cold. Joints, wheels, and actuators built to tight tolerances can seize. Lubricants used in gears and bearings can solidify or change viscosity in ways that make movement impossible. NASA's Opportunity rover, which operated on Mars from 2004 until 2018, survived over 5,000 Martian days partly because engineers obsessed over thermal management for every moving part. Mars nights are cold, but they are short compared to a lunar fortnight.

The third problem is the electronics themselves. Temperature cycling, repeated expansion and contraction of circuit boards and solder joints, causes micro-fractures over time. A rover that survives one lunar night may carry invisible damage that compounds across the next.

There are three broad strategies, and most missions combine them.

The first is passive insulation. Multi-layer insulation blankets, similar in principle to a thermos flask, wrap critical components and slow the rate of heat loss. These add mass, which is expensive at launch, so engineers make careful trade-offs about what gets wrapped and what doesn't.

The second is active heating. Electrical resistance heaters, powered by the rover's battery, keep sensitive components above their minimum operating temperature. The problem: heaters draw power, and power comes from solar panels. In lunar night, the solar panels produce nothing. The battery must supply heat for 14 days from whatever charge it held at sunset.

The third is radioisotope heating. NASA's Curiosity rover on Mars carries Radioisotope Thermoelectric Generators, or RTGs, which produce both electricity and heat from the natural decay of plutonium-238. RTGs work in darkness, in dust storms, at the poles, anywhere. China's Yutu-2 rover, which landed on the Moon's far side in January 2019 aboard the Chang'e 4 mission, uses a radioisotope heat source to survive lunar nights. As of the time of writing, Yutu-2 has survived multiple lunar nights and continues to operate, making it the longest-running lunar rover in history.

ISRO chose a different path for Pragyan.

Chandrayaan-3 landed on the Moon's south polar region on 23 August 2023, making India the first country to land a spacecraft near the lunar south pole. The Vikram lander deployed the Pragyan rover, which spent approximately 10 days traversing the surface, collecting spectral data, and confirming the presence of sulphur, aluminium, iron, calcium, and other elements in the regolith. ISRO also detected temperature gradients in the lunar soil that were sharper than pre-mission models had predicted.

As the first lunar night approached in early September 2023, ISRO placed Pragyan in sleep mode. The rover's solar panels were oriented to receive maximum light at the next sunrise. Its battery was fully charged. ISRO engineers publicly stated they would attempt to re-establish contact when the sun rose again over the landing site, roughly two weeks later.

Contact was never re-established. ISRO confirmed in late September 2023 that Pragyan had not woken up. The official assessment: the rover's battery or electronics did not survive the temperature drop. Pragyan carried no RTG. It had no active heating system capable of sustaining it through 14 days of -120°C to -180°C cold at the south polar region, where temperatures are even more extreme than at the equator. The mission was always designed with the understanding that lunar night survival was uncertain. The science was front-loaded into those first 10 days for exactly this reason.

Pragyan is still on the Moon. It sits where it stopped, a small machine on a grey plain, in permanent shadow for part of every month.

The lunar night is hostile to rovers, but it is scientifically valuable in ways the day is not. The south polar craters that never receive sunlight, permanently shadowed regions, are where water ice is most likely to be preserved. The Chandrayaan-1 mission's Moon Impact Probe and NASA's LCROSS mission both contributed to confirming the presence of water ice in these regions. The cold is what kept that ice stable for billions of years.

Understanding the thermal behaviour of the lunar surface at night also matters for any future crewed mission. Astronauts on a 14-day surface stay would need habitats and suits engineered for the same temperature swings that defeated Pragyan. The data Pragyan collected on soil temperature gradients in those 10 active days feeds directly into those engineering calculations.

Future missions are being designed differently. NASA's VIPER rover, planned for the lunar south pole, will use a combination of solar power management and mission planning to avoid the worst of the night. ESA and JAXA are studying radioisotope options. ISRO has not publicly confirmed the thermal architecture for any follow-on rover mission, but the Pragyan experience is now a hard data point in every conversation about what lunar night actually costs.

The rover that cannot survive the dark still tells you something precise about the dark. Pragyan's silence is its own measurement, a 14-day gap in transmission that describes, by its absence, exactly how extreme the lunar night is and what it will take to outlast it.