Mariner 10's Venus flyby was fine-tuned a bit to put the spacecraft on a path to Mercury. The change, which then included the Venus encounter and the initial Mercury flyby, gave Mariner 10 an orbital period twice as long as Mercury's. Thus, Mariner 10 would encounter Mercury every time it went twice around the sun. The result was that Mariner 10 looked at Mercury three times before its fuel was exhausted, rather than just once. The first encounter was March 29, 1974; the second was September 21, 1974; and the final encounter was on March 16, 1975.