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The first successful spacecraft in the NASA Mariner program, the Mariner 2, was the first robotic space probe able to conduct a successful encounter with Venus. The missions of Mariner 1 and 2 spacecraft are together sometimes known as the Mariner R missions. Original plans called for the probes to be launched on the Atlas-Centaur, but serious developmental problems with that vehicle forced a switch to the much smaller Agena B stage.