He knew there would be fifty seconds of free-fall before the rockets fired again. One solid-fuel stage had imparted to the ship a velocity which would carry it to the altitude of the missile it was to intercept. A second solid-fuel stage would match trajectories with the missile. Final corrections would be made with the liquid-fuel rockets in the third stage. The third stage would then become a glider which eventually would carry him back to Earth. Before the second stage was fired, however, the ship had to be oriented properly. The autopilot consulted its gyros, took some star sights, and asked the navigation computer some questions. The answers came back in seconds, an interval which was several hours shorter than a human pilot would have required. Using the answers, the autopilot started to swing the ship about, using small compressed-gas jets for the purpose. Finally, satisfied with the ship's orientation, the autopilot rested. It patiently awaited the moment, precisely calculated by the computer on the ground, when it would fire the second stage. Major Harry Lightfoot, fighter pilot, waited idly for the next move of his ship. He could only fume inwardly. This was no way for an Apache warrior to ride into battle. What would his grandfather think of a steed which directed itself into battle and which could kill its rider, not by accident, but in its normal operation? He should be actively hunting for that missile, instead of lying here, strapped into his couch so he wouldn't hurt himself, while the ship did all the work. As for the missile, it was far to the north and slightly above the ship. Without purpose of its own, but obedient to the laws of Mr. Newton and to the wishes of its makers, it came on inexorably. It was a sleek aluminum cylinder, glinting in the sunlight it had just recently entered. On one end was a rocket-motor, now silent but still warm with the memory of flaming gas that had poured forth from it only minutes ago. On the other end was a sleek aerodynamic shape, the product of thousands of hours of design work. It was designed to enter the atmosphere at meteoric speed, but without burning up. It was intended to survive the passage through the air and convey its contents intact to the ground. The contents might have been virulent bacteria or toxic gas, according to the intentions of its makers. Among its brothers elsewhere in the sky this morning, there were such noxious loads. This one, however, was carrying the complex mechanism of a hydrogen bomb. Its destination was an American city; its object to replace that city with an expanding cloud of star-hot gas. * * * * *