A settlement of 20 people requires a minimum of 200 kilowatts of continuous power. Life support alone consumes roughly 5 kilowatts per person. Add ISRU water processing, habitat thermal management, communications, lighting, and equipment charging, and you are closer to 400 kilowatts for any meaningful operational capability. The question is whether solar or nuclear provides that most reliably.

At the Shackleton rim, the solar case is strong. Near-continuous illumination means capacity factors of 80 to 90 percent, compared to roughly 50 percent for equatorial sites with two-week nights. Modern multi-junction solar cells achieve 33 percent efficiency. At the lunar solar constant of 1,361 watts per square meter, a 400-kilowatt array requires approximately 900 square meters of panel area. That is a 30-by-30 meter footprint. Achievable.

Achievable in clean-room conditions. On the Moon, solar panels degrade. The CRaTER instrument measured a 1 to 2 percent annual degradation of silicon solar cells from radiation damage alone. Add lunar dust accumulation, which is electrostatically charged and adheres to everything, and you face an additional 1 to 3 percent performance loss per lunation without active cleaning. Within five years, your 400-kilowatt array produces 300 kilowatts. Within ten, it is below 250.

The NASA Kilopower project demonstrated a 10-kilowatt fission reactor using highly enriched uranium-235 and Stirling converters. The KRUSTY test in 2018 ran for 28 hours at full power. Scaling to 40 kilowatts per unit, ten units provide the 400-kilowatt baseline. Each unit masses approximately 1,500 kilograms. Total system mass of 15 tonnes, fitting within a single Starship HLS cargo allocation.

Nuclear has its own complications. Highly enriched uranium requires launch safety approvals that have historically taken years. The reactor must be positioned far enough from the habitat to keep radiation doses below limits, which means long power transmission runs and associated losses. And if a reactor fails, you cannot repair it. You are not sending astronauts to service an irradiated core on the Moon. Every failed unit is permanently lost capacity.

The pragmatic answer is both. Solar as the primary daytime source with battery storage for brief shadow periods. Nuclear as the baseload and backup, positioned 500 meters from the habitat with buried superconducting power lines. Redundancy through diversity. No single failure mode can collapse the power grid. The lunar settlement must be designed like a submarine: every critical system has a backup, and every backup has a backup.