Science: By Jove, It's Hydrogen

After a perilous 21-month journey across more than half a billion miles of space, a tiny, unmanned spacecraft named Pioneer 10 passed within 81,000 miles of the giant planet Jupiter last December. That first close-up peek at the sun's largest satellite lasted barely a day.

But before the little ship was boosted along by Jovian gravity on a flight path that will eventually carry it out of the solar system, it gathered more information about Jupiter than had all astronomers since Galileo first pointed his crude telescope at the planet more than three centuries...

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