would mean a gap in the chain that could not be covered adequately by interceptors from the adjacent stations.

His screens were never completely clear. And to complicate things, the Quadrantids, which start every New Year's Day and last four days, were giving him additional trouble. Each track had to be analyzed, and the presence of the meteor shower greatly increased the number of tracks he had to worry about. However, the worst was past. One more day and they would be over. The clutter on his screens would drop back to normal.

Even under the best of circumstances, his problem was bad. He was hemmed in on one side by physics, and on the other by arithmetic. The most probable direction for an attack was from over the Pole. His radar beam bent only slightly to follow the curve of the Earth. At great range, the lower edge of the beam was too far above the Earth's surface to detect anything of military 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 was about 40° Latitude. It certainly wasn't the satellite. Two more blips appeared on the screen, at velocities and altitudes similar to the first. Each swipe of the white line left more new tracks on the screen. And the screens for the adjacent stations were showing similar behavior. These couldn't be meteors.

The Launch Control Officer slapped his hand down on a red push-button set into the arm of his chair, and spoke into his mike. "Red Alert. Attack is in progress." Then switching to another channel, he spoke to his assistants: "Take your preassigned sectors. Launch one interceptor at each track identified as hostile." He hadn't enough interceptors to double up on an attack of this size, and a quick glance at the screens for the adjacent stations showed he could expect no help from them. They would have their hands full. In theory, one interceptor could handle a missile all by itself. But the theory had never been tried in combat. That lack was about to be supplied.

Harry Lightfoot heard the alarm over the intercom. He vaguely understood what would happen before his launch order came. As each track was identified as hostile, a computer would be assigned to it. It would compute the correct time of launch, select an interceptor, and order it off the ground at the correct time. During the climb to intercept, the computer would radio steering signals to the interceptor, to assure that the intercept took place in the most efficient fashion. He knew RI 276 had been selected when a green light on the instrument panel flashed on, and a clock dial started indicating the seconds until launch. Just as the clock reached zero, a relay closed behind the instrument panel. The solid-fuel booster ignited with a roar. He was squashed back into his couch under four gees' acceleration.

Gyroscopes and acceleration-measuring instruments determined the actual trajectory of the ship; the navigation computer compared the actual trajectory with the trajectory set in before take-off; when a deviation from the pre-set trajectory occurred, the autopilot steered the ship back to the proper trajectory. As the computer on the ground obtained better velocity and position information about the missile from the ground radar, it sent course corrections to the ship, which were accepted in the computer as changes to the pre-set trajectory. The navigation computer hummed and buzzed; lights flickered on and off on the instrument panel; relays clicked behind the panel. The ship steered itself toward the correct intercept point. All this automatic operation was required because no merely human pilot had reflexes fast enough to carry out an intercept at twenty-six thousand feet per second. And even had his reflexes been fast enough, he could not have done the precise piloting required while being pummeled by this acceleration.

As it was, Major Harry Lightfoot, fighter pilot, lay motionless in his acceleration couch. His face was distorted by the acceleration. His breathing was labored. Compressed-air bladders in the legs of his gee-suit alternately expanded and contracted, squeezing him like the obscene embrace of some giant snake, as the gee-suit tried to keep his blood from pooling in his legs. Without the gee-suit, he would have blacked out, and eventually his brain would have been permanently damaged from the lack of blood to carry oxygen to it.

A red light on the instrument panel blinked balefully at him as it measured out the oxygen he required. Other instruments on the panel informed him of the amount of cooling air flowing through his suit to keep his temperature within the tolerable range, and the amount of moisture the dehumidifier had to carry away from him so that his suit didn't become a steam-bath. He was surrounded by hundreds of pounds of equipment which added nothing to the performance of the ship; which couldn't be counted as payload; which cut down on the speed and altitude the ship might have reached without them. Their sole purpose was to keep this magnificent high-performance, self-steering machine from killing its load of fragile human flesh.

At one hundred twenty-eight seconds after launch, the acceleration suddenly dropped to zero. He breathed deeply again, and swallowed repeatedly to get the salty taste out of his throat. His stomach was uneasy, but he wasn't spacesick. Had he been prone to spacesickness, he would never have been accepted as a Rocket Interceptor pilot. Rocket Interceptor pilots had to be capable