Mariner 10 was the first NASA mission to use a gravity-assist maneuver at Venus.
Launched in November 1973, Mariner 10 passed Venus in February 1974 and used the planet’s gravity to alter its trajectory toward Mercury. This technique allowed the spacecraft to reach Mercury without carrying enough fuel for a direct trip.
Mariner 10 then flew past Mercury three times and became the first spacecraft to visit that planet. It photographed roughly 45 percent of Mercury’s surface and found evidence of a weak magnetic field.
The mission’s Venus encounter was especially important because it demonstrated a powerful way to travel between planets. MESSENGER later used multiple planetary flybys before entering Mercury orbit, while Mariner 10 remained a flyby mission.