significance. On a minimum altitude trajectory, an ICBM aimed for North America would not be visible until it reached 83° North Latitude on the other side of the Pole. One of his interceptors took three hundred eighty-five seconds to match trajectories with such a missile, and the match occurred only two degrees of latitude south of the station. The invading missile traveled one degree of latitude in fourteen seconds. Thus he had to launch the interceptor when the missile was twenty-seven degrees from intercept. This turned out to be 85° North Latitude on the other side of the Pole. This left him at most thirty seconds to decide whether or not to intercept a track crossing the Pole. And if several tracks were present, he had to split that time among them. If too many tracks appeared, he would have to turn over portions of the sky to his assistants, and let them make the decisions about launching. This would happen only if he felt an attack was in progress, however.

Low-altitude satellites presented him with a serious problem, since there is not a whole lot of difference between the orbit of such a satellite and the trajectory of an ICBM. Fortunately most satellite orbits were catalogued and available for comparison with incoming tracks. However, once in a while an unannounced satellite was launched, and these could cause trouble. Only the previous week, at a station down the line, an interceptor had been launched at an unannounced satellite. Had the pilot not realized what he was chasing and held his fire, the international complications could have been serious. It was hard to imagine World War III being started by an erroneous interceptor launching, but the State Department would be hard put to soothe the feelings of some intensely nationalistic country whose expensive new satellite had been shot down. Such mistakes were bound to occur, but the Launch Control Officer preferred that they be made when someone else, not he, was on watch. For this reason he attempted to anticipate all known satellites, so they would be recognized as soon as they appeared.

According to the notes he had made before coming on watch, one of the UN's weather satellites was due over shortly. A blip appeared on the screen just beyond the 83° latitude line, across the Pole. He checked the time with the satellite ephemeris. If this were the satellite, it was ninety seconds early. That was too much error in the predicted orbit of a well-known satellite. Symbols sprang into existence beside the track. It was not quite high enough for the satellite, and the velocity was too low. As the white line swept across the screen again, more symbols appeared beside the track. Probable impact point 

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