The first seven astronauts selected by NASA were called Mercury 7 because they were chosen for the Mercury program.

Here's why:

* Project Mercury: This was NASA's first human spaceflight program, aiming to launch a man into orbit and bring him safely back to Earth. The name "Mercury" was chosen for its association with the Roman god of travel and messenger, symbolizing the program's focus on space travel.

* Seven Astronauts: NASA selected seven highly qualified test pilots to be the first American astronauts. These men were:

* Alan Shepard

* Gus Grissom

* John Glenn

* Scott Carpenter

* Wally Schirra

* Gordon Cooper

* Deke Slayton

* "Mercury 7" as a Group Name: The term "Mercury 7" became a collective term for these seven men, recognizing their role as pioneers in the space race and their participation in the Mercury program.

So, "Mercury 7" was a combination of the program's name and the number of astronauts chosen. It became a iconic term in American space exploration history.