Mariner 10 skimmed within 3,600 miles of Venus, capturing the first close-up images of its thick cloud cover and testing a novel gravity-assist maneuver. The flyby redirected the probe toward Mercury, which it would reach weeks later, becoming the first craft to visit that innermost planet.
Launched in November 1973, Mariner 10 carried out an unprecedented feat of orbital mechanics: using Venus's gravitational pull to slingshot itself toward Mercury without expending additional fuel, a technique that would become standard for later deep-space missions to the outer planets.
As it swept past Venus, its cameras revealed swirling ultraviolet patterns in the planet's dense atmosphere, confirming ferocious high-altitude winds far faster than anything on Earth. Scientists had long known Venus's surface was scorching, but the flyby data sharpened understanding of its runaway greenhouse effect.
The mission's chief scientist, Bruce Murray, later called the Venus-Mercury trajectory one of the cleverest pieces of navigation in the space program's early history. Mariner 10 went on to make three passes of Mercury, mapping nearly half its surface and discovering a surprising magnetic field, before running out of attitude-control gas in 1975.