This trio of early short novels by Grand Master Robert Silverberg is straight-up pulp science fiction at its finest   As a young man, Robert Silverberg was a science fiction prodigy, turning out top-flight stories in the blink of an eye. Though written quickly, Silverberg's early prose already showed evidence of the literary and imaginative qualities that would make him a giant in the field. Here are three of his best early works.   In The Planet Killers, after Earth's supercomputer calculates that the inhabitants of the planet Lurion will destroy Earth in sixty-seven years, Roy Gardner is sent to stop them--by any means necessary. In The Plot Against Earth, Special Investigator Lloyd Catton's efforts to crack a ring of "hypnojewel" traffickers uncovers a galaxy-wide conspiracy. And in One of Our Asteroids Is Missing, when miner John Storm stakes claim to an asteroid with a king's ransom of rare minerals, he's set for life--or would be, if his discovery didn't also mark him for death.