Back at the dawn of this century, many astronomers, and notably Dr. Percival Lowell, studied certain irregularities in the orbit and motion of Neptune, at that time believed to be the outermost planet. They decided that these eccentricities (or perturbations, as they are called) could only be caused by the presence of another, yet undiscovered planet beyond Neptune. Following this line of research, a young astronomer, Dr. Clyde Tombaugh, working at Lowell's own observatory, was able to announce on March 13, 1930, that he had finally found this ninth world, which he named Pluto. In the years that have followed, Pluto has proven to be a truly puzzling planet. Unlike its neighbors from Jupiter outward, it is not a giant world, light and gaseous in nature. Instead, it belongs physically to the small, dense inner planets of which Earth is one. The latest viewpoint on this planet, whose size and weight seem quite like those of Earth, is that it may not be a true child of the Sun, but an outsider captured as it roamed the trackless realms of galactic space. Its orbit is highly eccentric and rather lopsided, taking it as far away from the Sun as four and a half billion miles and as close to the Sun as two and three-quarter billion miles, thereby cutting inside the orbit of Neptune itself. In fact, during the period from 1969 to 2009 (covering most of the lifetimes of the younger readers of this book) Pluto will not be the ninth planet, but the eighth, for it will be at its closest in those years. Huge Neptune will thus regain temporarily the title of being the Sun's farthest outpost! This orbital eccentricity has lead some astronomers to speculate on the possibility that Pluto may once have been briefly held as a satellite of Neptune. And following that line of thought, the possibility also has been suggested that Neptune's larger moon, Triton, may once have been a companion of Pluto which failed to break away from Neptune's grip! I think that the first men to land on Pluto are going to make some very astonishing discoveries. But I am also sure that they will never go there in rockets. They will have to make the immense trip by some more powerful means--like the anti-gravitational drive. D.A.W. _The Secret of the Ninth Planet_ Chapter 1. _Special Delivery--by Guided Missile_ On the morning that the theft of the solar system's sunlight began, Burl Denning woke up in his sleeping bag in the Andes, feeling again the exhilaration of the keen, rarefied, mountain air. He glanced at the still sleeping forms of his father and the other members of the Denning expedition, and sat up, enjoying the first rays of the early morning. The llamas were already awake, moving restlessly back and forth on their padded feet, waiting for their tender to arise and unleash them. The mules were standing patiently as ever, staring quietly into the distant misty panorama of the mountains.