The Gemini astronauts were pilots who flew in Project Gemini, NASA's second human spaceflight program, between projects Mercury and Apollo. Carrying two astronauts at a time, a senior Command Pilot and a junior Pilot, the Gemini spacecraft was used for ten crewed missions. Sixteen astronauts flew on these missions, with four flying twice. All Gemini astronauts–excluding the Mercury Seven astronauts already included–were inducted into the U.S. Astronaut Hall of Fame in 1993. Astronaut participation in Project Gemini was also a strong predictor for future achievement during the Apollo Program: