What was the first successful US space probe to fly by Venus?

The first successful US space probe to fly by Venus was Mariner 2. NASA's probe passed Venus on December 14, 1962, becoming the first spacecraft to successfully encounter another planet and return useful scientific data.

Mariner 2 was launched on August 27, 1962, after its predecessor, Mariner 1, was lost shortly after launch. The spacecraft survived several technical problems during its journey, including attitude-control difficulties and the failure of one solar panel. It still reached Venus after about 110 days of flight.

During the flyby, Mariner 2 measured Venus's extremely high atmospheric temperature, detected the planet's slow rotation indirectly, and made pioneering observations of the solar wind. Its results helped overturn earlier ideas that Venus might have a more Earth-like surface environment.

A common mix-up is Mariner 1: it was the earlier mission, but it failed before reaching space. The Soviet Venera 1 flew past Venus earlier, in 1961, but stopped transmitting before returning successful encounter data, so Mariner 2 is the correct answer for the first successful US probe.