Two of the four spacecraft in this series were intended to go to Venus but NASA redirected them to the Moon after the Soviet Luna 3 success.
The first spacecraft to reach Venus was Mariner 2 in 1962.
The U.S. and Soviet Union launched six unmanned lunar spacecraft in 1959. This was the last.
Two of the spacecraft in this series were pulled from planned Venus missions as a result of the Soviet Union's Luna 3 success.
This was America's 7th interplanetary mission.
The spacecraft carried a motor for lunar orbit insertion.