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Acknowledgment I would like to express my thanks for the invaluable help and advice given by Professor Marvin Minsky, Director of the Artificial Intelligence Unit of the Massachusetts Institute of Technology. A popular notion holds that science-fiction writers see today where science-fact will be going tomorrow; in reality, more often, the process tends to work the other way round. Jim Hogan Acton, Mass. February 1979 �0�2 �0�2 PROLOGUE The planetismal began as a region of above-average density that occurred by chance in a swirling cloud of dust and gas condensing out of the expanding vastness of space. Gently at first but at a rate that grew steadily faster as time went by, it continued to sweep up the smaller accretions in its vicinity until it had grown to a rough spheroid of compressed dust and rock measuring fifty feet across. Eventually the planetismal itself came under the pull of a larger body that had been growing in similar fashion, and began falling toward it. It impacted at a speed of over ten miles per second, releasing the energy equivalent of a one-hundred-kiloton bomb and blasting a crater more than half a mile in diameter. Shortly afterward, as measured on a cosmic timescale, a second planetismal fell close by and created another crater of similar dimensions;
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