Project Gemini took place between the Mercury program, which was the United States' first human spaceflight program, and the Apollo program, which put the first human on the Moon. Mercury sent astronauts to space for longer periods of time than previously. This allowed NASA to learn more about space's effects on the human body—in preparation for sending humans to the Moon. NASA was also able to test and refine key technologies. The Gemini program flew 10 crews to space, and included feats such as the first U.S. spacewalk.