They left the building and rode to the rocket field. Night had fallen and the spaceport lay stark and cold in the beams of large floodlights. Three spaceships stood on the field, their bluish sides gleaming in the beams of the floodlights. To the south, a transcontinental rocket rose into the night like a spark from a chimney. The air was bitter with the temperature at eighteen below. "Take a look," the police officer handed Roger a pair of binoculars. Roger placed the instrument to his eyes, and the side of the center rocket leaped toward him. He saw a man in the red overalls of a fuel technician climb the gantry alongside the center rocket and push something into a valve on the side of the booster stage, near its juncture with the main part of the ship. "Do you see that mechanic on the center rocket?" Roger asked. "Let's see," the officer replied and looked toward that rocket. "Yes, I see him now. A mechanic shouldn't be pushing anything into that valve. That particular valve is used to jettison fuel in an emergency. A blast of compressed air will usually clear anything out of it. If that doesn't work, the valve has to be taken apart to be cleaned. I'd like to know just what he shoved into that valve." The officer spoke briefly into his pocket radio. Four policemen moved toward the entrances that led into the deep pit where the rocket stood. The technician closed the valve and climbed down the ladder. As soon as his feet touched the concrete floor of the pit, he was seized by the waiting policemen. A pistol shot cracked, and the prisoner sagged to the floor with a hole in his chest. Instant confusion reigned in the pit, and in that confusion the assassin somehow escaped. When the officer and Roger arrived, they found the policemen talking with a fuel technician. The technician left the group and climbed the ladder to the valve. He opened it and inserted a spring operated probe. "The valve's clean," he shouted down. "I'll take off some of the nitric acid." He did so, collecting the liquid in a small sample bottle which he carried on his belt. Climbing down the ladder, he handed the bottle to the officer in charge, who handed it to Roger. Roger unscrewed the cap and cautiously sniffed the contents. "I can't be sure, but if it's what I think it is, you'd better not have the tanks drained until morning. Give it a chance to dissolve. Otherwise you'll have some left in the tanks. It doesn't react very rapidly at low temperatures." "Just what do you think it is?" the officer asked. "Well," Roger replied, "it's probably some organic compound that would react with the nitric acid to form an explosive nitrate. Of course, it could be an ammonium compound that would react to form ammonium nitrate. That would do the job just as well." * * * * *