Science: Canceling the Tour

It was a mission that would have fired the public imagination and severely tested NASA's engineering ingenuity: an eleven-year flight to the very edge of the solar system. On one "Grand Tour," the spaceship would have swooped by Jupiter and with a whiplike assist from that planet's powerful gravitational field, flown past the ringed Saturn and finally Pluto, the outermost planet. In another version, the spacecraft would have used a similar "gravity assist" from Jupiter to swing by Uranus and Neptune instead of Pluto. Scheduled for the late 1970s, the Grand Tours would literally have been once-in-a-lifetime opportunities. The outer planets...

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