Status: Study 1961.
Mariner R would use be essentially a quick modification of a Ranger spacecraft, shot toward Venus rather than the moon. Instead the US government elected to wait a bit. Russian planetary probes failed repeatedly en route, and JPL's Mariner 2 became the first spacecraft to reach Venus in December 1962.
In its political desire to beat the Soviet Union to a planetary shot, the United States wanted to launch probes to the planets in 1962 if at all possible and chose Venus as the most likely target, since flights to Earth's closest neighbor would require less powerful rockets. On 28 August 1961, JPL proposed a 1962 Venus mission based on an Atlas-Agena launch vehicle, using hybrid spacecraft that combined features of JPL's lunar Ranger and Mariner A. This proposed spacecraft, called Mariner R, could carry about 11 kilograms of instruments. The 1962 project would not have a significant influence on the schedule for lunar Rangers, but a reallocation of launch vehicles would be required.