The Vulnerability of a Lunar Colony to Military Force
The industrial capability of a lunar colony would be dependent upon a number of features that would be from difficult to impossible to hide and open to the destructive efforts of any hostile agent. Development of industry would take at least decades, perhaps fifty years, before there is a capability of mounting effective defensive measures. Defending a lunar colony from attack would consist of identifying all spacecraft in the area, restricting access for about 200,000 km around to friendly vessels, and destroying all others. The most obvious and fragile feature of industrial development would be radiators. They could be located by their infrared signature. They could be destroyed by as much as a rain of sand on them from orbit. There is no way to protect radiators from bombardment.
All other features, the electrical distribution network, the communications network, and pressure vessels are at least somewhat dependent upon radiators, but could be targets for hostile force by themselves. The communications network stands out because of radio and laser emanations. Hiding it and protecting it from bombardment would involve using buried cables for communication. Hiding a buried trench from remote sensors would be difficult. A good sized rock from orbit would likely destroy a buried cable if it hit within a foot. Electrical distribution is likewise fragile with complications. The current in a distribution line tends to generate heat that could potentially be detected. Electrical fields like the sixty cycle hum of the United States electrical grid would be open to detection from space without the interference of clouds or lightning. Pressure vessels would be visible because of the unnatural shape and the disturbance of the ground in the area of their construction. Hiding these things would take some effort, such as putting them in lava tubes. Good sized rocks from orbit could destroy these things. The deeper they are buried the bigger the rock that would be needed. Roads, mines, rocket launch pads and landing fields, and electric launch to orbit devices would all take effort to hide from any hostile agent and be susceptible to attack.