Instantly the great ship lurched and trembled as, from its rear, came thunderous explosion on explosion. In a moment every one of its rear-tubes was firing, and the speed-dial's arrow was creeping steadily forward until in a few minutes more it registered the ship's top speed of ten thousand miles an hour. The long gleaming craft, stubby of nose and stern and fully five hundred feet in length, was like a giant projectile, as it tore through the void, belching fire behind it. From the squat pilot-house set atop it, Evans and Calden gazed ahead. The great gray disk of earth filled a quarter of the heavens before them, the outline of its continents and seas visible here and there through its shifting screen of clouds. Behind them the moon's silvery sphere was dwindling rapidly, as they had seen it dwindle now for hours. It was hours that the great Earth-Guard rocket with its half-hundred men had been hurtling toward earth after its weary week's vigil in space, before this call had come. And weary enough indeed was the vigil that the rocket ships of the Earth-Guard kept around the earth and its moon, and had kept up for more than fifty years. More than fifty years it had been since, back in 1954, the first crude rocket had thundered out from earth into the great void toward its shining satellite. Neither that first rocket nor the twenty-first had reached their goal, but the next one had. Thus had begun the commerce that now filled all the space-lanes between the earth and moon. In their first flame of exploration, men had headed out toward the nearer planets, too, but they had found them unapproachable because of the fierce guard maintained by their strange peoples. Every ship that had sought to explore another planet had been annihilated on reaching it and we had finally realized that our planetary neighbors were guarding fiercely their isolation. * * * * * There had remained to earth only its own moon. But that had become swiftly a lure to all adventurous earthlings. Upon the moon's other side were great mines in which men, dwelling in air-tight cities and toiling in hermetically-tight metal suits, worked the rare metals and minerals in which earth's satellite abounded. And upon the moon's earthward side were other great air-tight cities, glass-roofed and luxurious, to which went each year hundreds of thousands of the earth's wealthy--there to spend their vacations--enjoying the wondrous celestial views, the astounding strength and youth given them by the moon's lesser gravity, and the chance to view the earth from the outside.