Given this paragraph about the Mercury Seven. Who did not survive to reach retirement and what was the cause of their death?
The Mercury Seven were the group of seven astronauts selected to fly spacecraft for Project Mercury. They are also referred to as the Original Seven and Astronaut Group 1. Their names were publicly announced by NASA on April 9, 1959; these seven original American astronauts were Scott Carpenter, Gordon Cooper, John Glenn, Gus Grissom, Wally Schirra, Alan Shepard, and Deke Slayton. The Mercury Seven created a new profession in the United States, and established the image of the American astronaut for decades to come.

All of the Mercury Seven eventually flew in space. They piloted the six spaceflights of the Mercury program that had an astronaut on board from May 1961 to May 1963, and members of the group flew on all of the NASA human spaceflight programs of the 20th century – Mercury, Gemini, Apollo, and the Space Shuttle.

Shepard became the first American to enter space in 1961, and later walked on the Moon on Apollo 14 in 1971. Grissom flew the first crewed Gemini mission in 1965, but died in 1967 in the Apollo 1 fire; the others all survived past retirement from service. Schirra flew Apollo 7 in 1968, the first crewed Apollo mission, in Grissom's place. Slayton, grounded with an atrial fibrillation, ultimately flew on the Apollo–Soyuz Test Project in 1975. The first American in orbit in 1962, Glenn flew on the Space Shuttle Discovery in 1998 to become, at age 77, the oldest person to fly in space at the time. He was the last living member of the Mercury Seven when he died in 2016 at age 95.
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Only Gus Grissom did not survive to reach retirement. All others survived past retirement from service. The cause of his death is the Apollo 1 fire.
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