Kelly had just turned 40 when Neil Armstrong took his first historic step on the Moon July 20, 1969. During the landing, ice had formed in a fuel line, clogging it. If the heat from the engine defrosted the clog, the fuel could detonate. Luckily for Kelly and the Grumman crew, the problem corrected itself and the crew was able to relax and realize what they had done. In an interview in 1998 Kelly stated, "It was the greatest thing in my career. And, in hindsight, it was even more significant than we thought at the time." However, the experience was not without strain and stress, manifested in a nervous tic he developed during the project.