Also it is doubtful whether a mere ball of solid iron would smash up the works of an armored space-ship unless she met it at high speed on an absolute collision course. The more normal event would be for the mine to be picked up by the small but perceptible gravitational attraction of the space-ship and travel with it as a satellite. So the mines of space will probably have to be fairly large and contain heavy explosive charges, probably with proximity fuses. Still the advantages of mining are so great that they will undoubtedly play a large part in space-warfare. Asteroid or Moon bases can be defended by minefields, which would keep an enemy at a distance until he had painstakingly located the pesky objects and shot them up, during which time the ship would be an ideal target for guns on the ground. * * * * * And one striking feature about this space minefield must always be remembered, a feature that makes it different from anything encountered on the ocean. The fields will be constantly moving. Fixed in an orbit 25,000 miles above the Earth a space-mine would always remain over the same Earthly spot and there is a similar critical distance for the Moon, Mars, and the asteroids. But few of the mines will be at exactly that distance. Unless they are they will be tiny satellites, revolving around their primaries at greater or less speeds, constantly on the go. A space-ship won't have to hit them. They will hunt it up. There is also the fact that mining in war is not merely a defensive tactic. In World War I the German subs were pretty thoroughly mined in by the barrages across the North Sea and English Channel. In World War II what was left of the Japanese fleet was immobilized in its harbors by mines. Space-mines would serve a similar purpose of closing a planetoid or an area to all access. They could serve a tactical purpose as well by limiting avenues of approach. Not that they couldn't be eliminated--rocket-torpedo fire would be something quite different and more effective than the same kind of fire against a space-battleship capable of striking back. But this would take time and time is the one thing no one can afford to waste in a kind of war where everything will move at speeds exceeding those of the planets.