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Some 40 years ago, a quarter-ton lump of circuits and sensors slipped Earth's surly bonds, sped past the moon and Mars, and hurtled toward Jupiter. The probe, Pioneer 10, and its sister ship, Pioneer 11, which followed a year later, were true trail blazers . They gave humanity its first close-up glimpses of worlds beyond the solar system's asteroid belt. They also left behind a mystery-one that has simultaneously baffled and inspired as trophysicists for years.