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Stafford also piloted Gemini 9A and Apollo 10. Gemini 9A was a joint US-Soviet space mission in 1966. Stafford and the Soviet cosmonaut Aleksei Leonov performed the first US-Soviet spacewalk, which was the first spacewalk in which two people were outside the spacecraft at the same time. Apollo 10 was a dress rehearsal for the first lunar landing, and Stafford became the first American to fly around the Moon.