An Apollo mission to the Moon was expected to take at least a week, so one of the objectives of Project Gemini was to test the ability of the crew and spacecraft components to operate in space for that length of time. When Shepard was grounded in October 1963, Grissom and Borman became the prime crew of Gemini 3. Grissom invited Borman to his house to talk to him about the mission, and after a long discussion, decided that he could not work with Borman. According to Gene Cernan, "the egos of Grissom and Borman were too big to fit into a single spacecraft". Slayton therefore replaced Borman with John Young.