Which NASA spacecraft used a Venus flyby to reach Mercury in 1974?

Mariner 10 used a Venus flyby to reach Mercury in 1974.

Launched by NASA on November 3, 1973, Mariner 10 passed Venus on February 5, 1974. The planet’s gravity bent and slowed the spacecraft’s heliocentric trajectory, lowering its perihelion until it intersected Mercury’s orbit. This made Mariner 10 the first spacecraft to use a planetary gravity assist to reach another planet.

The maneuver was proposed using orbital calculations by Italian mathematician and engineer Giuseppe Colombo. After the Venus encounter, Mariner 10 flew past Mercury three times: on March 29 and September 21, 1974, and again on March 16, 1975. It became the first spacecraft to visit Mercury and returned the first close-up images of much of the planet’s cratered surface.

Mariner 10 also studied Venus. Its ultraviolet camera revealed detailed cloud patterns, while radio measurements helped determine atmospheric temperature, pressure, and density. A common mix-up is choosing Mariner 5, which visited Venus in 1967 but did not continue to Mercury. Mariner 10’s gravity-assist technique became a foundation for later planetary missions.