This coincidence meant that a space vehicle could get a speed boost from the gravitational pull of each giant planet it passed, as if being tugged along by an invisible cord that snapped at the last second, flinging the probe on its way. Using a favorite precision tool of 20th-century engineers-a pencil-he charted the orbital paths of those giant planets and discovered something intriguing: in the late 1970s and early 1980s, all four would be strung like pearls on a celestial necklace in a long arc with Earth. Flandro, who was working part-time at NASA's Jet Propulsion Laboratory in Pasadena, Calif., had been tasked with finding the most efficient way to send a space probe to Jupiter or perhaps even out to Saturn, Uranus or Neptune. It was 1965, and the era of space exploration was barely underway-the Soviet Union had launched Sputnik 1, the first artificial satellite, only eight years earlier. The first person to call attention to it was an aeronautics doctoral student at the California Institute of Technology named Gary Flandro. For a while the rare planetary set piece unfolded largely unnoticed. Some 60 years ago they were slowly wheeling into an array that had last occurred during the presidency of Thomas Jefferson in the early years of the 19th century. In this case, the stars were actually planets-the four largest in the solar system. If the stars hadn't aligned, two of the most remarkable spacecraft ever launched never would have gotten off the ground.
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