The space probe Mariner 10 became the first spacecraft to reach the planet
Mercury. It had been launched on November 3, 1973.The first encounter took
place at a range of 437 miles, passing on the shadow side of the planet. The
probe flew by Mercury again on September 21, 1974. Mariner 10 is still orbiting
the Sun, although its electronics have probably been damaged by the Sun’s
radiation.

Mercury, 6 hours before closest approach of Mariner 10

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