The ISS has maintained pressurized volume in low Earth orbit for over 25 years. But the Moon is a fundamentally different engineering environment. No magnetic field. No atmosphere for meteor ablation. Micrometeorite flux at the lunar surface is roughly 1,000 times the impact risk at ISS orbital altitude. Every habitat must withstand hypervelocity impacts from particles as small as a grain of sand traveling at 20 kilometers per second.

The ISS leaks. It has always leaked. Current leak rates are approximately 1 kilogram of atmosphere per day, and the crew has spent months hunting for cracks. On the Moon, there is no resupply shuttle arriving every few weeks. A habitat leak rate of 1 kilogram per day means your oxygen reserve is a ticking clock. The pressure vessel integrity requirements for lunar habitats are at least an order of magnitude more stringent than ISS.

My design approach uses a dual-shell architecture. Inner pressure vessel of aluminum-lithium alloy at 70 kilopascals, roughly sea-level equivalent. Outer micrometeorite shield with a 15-centimeter standoff gap filled with regolith-stuffed Whipple shields. Total wall thickness of 40 centimeters. For a cylindrical module 8 meters in diameter and 12 meters long, the pressurized volume is approximately 500 cubic meters. That supports four crew members at ISS-equivalent living standards.

The Bigelow Expandable Activity Module on the ISS demonstrated that inflatable structures can maintain pressure integrity in space. BEAM has been attached since 2016 with zero pressure anomalies. For the Moon, an inflatable approach dramatically reduces launch mass. A 500-cubic-meter inflatable module could pack into a volume one-tenth that of a rigid equivalent. That changes the entire logistics equation.

Inflatable on the Moon is not inflatable in LEO. BEAM sits inside the ISS micrometeorite shadow and radiation protection. On the lunar surface, an inflatable is directly exposed. The Vectran fabric layers in BEAM are not designed for the abrasive lunar dust environment. Every EVA that brings regolith into the airlock area is a potential threat to fabric integrity. I keep returning to this: the Moon is trying to kill you in ways that LEO is not.

That is why the Phase 3 plan calls for excavated habitats. Cut into the basaltic rim rock using a combination of thermal spallation and mechanical boring. Line the cavity with sprayed regolith concrete, seal with an interior membrane, and pressurize. The overburden provides radiation shielding, micrometeorite protection, and thermal mass. The habitat becomes part of the Moon itself. We stop building on the surface and start building inside it.