Mariner 10 flies by Mercury, 1974

Mariner 10, launched November 3, 1973, became the first spacecraft to encounter Mercury, the closest planet to the Sun. It made ultraviolet observations of Comet Kohoutek in January 1974 and passed Venus February 5. It made its first flyby of Mercury March 29, 1974.  The second was September 21 and the third (and last) March 16, 1975.

It was decommissioned about a week later.