It was further stipulated that candidates should be between 25 and 40 years old, no taller than , and hold a college degree in a STEM subject. The college degree requirement excluded the USAF's X-1 pilot, then-Lt Col (later Brig Gen) Chuck Yeager, the first person to exceed the speed of sound. He later became a critic of the project, ridiculing the civilian space program, labeling astronauts as "spam in a can." John Glenn did not have a college degree either, but used influential friends to make the selection committee accept him. USAF Capt. (later Col.) Joseph Kittinger, a USAF fighter pilot and stratosphere balloonist, met all the requirements but preferred to stay in his contemporary project. Other potential candidates declined because they did not believe that human spaceflight had a future beyond Project Mercury. From the original 508, 110 candidates were selected for an interview, and from the interviews, 32 were selected for further physical and mental testing. Their health, vision, and hearing were examined, together with their tolerance to noise, vibrations, g-forces, personal isolation, and heat. In a special chamber, they were tested to see if they could perform their tasks under confusing conditions. The candidates had to answer more than 500 questions about themselves and describe what they saw in different images. Navy Lt (later Capt) Jim Lovell, who was later an astronaut in the Gemini and Apollo programs, did not pass the physical tests. After these tests it was intended to narrow the group down to six astronauts, but in the end it was decided to keep seven.