Background information Edit
From the Star Trek Encyclopedia (4th ed., vol. 1, p. 306), "Glenn flew the Mercury-Atlas 6 spacecraft Friendship 7, in 1962, during which he made three orbits. Glenn, who had previously served as a military officer, subsequently served his country as a United States senator before returning to space as an astronaut aboard the space shuttle orbiter Discovery in 1995 for mission STS-95."
In a note to the entry, the authors of the reference work had this to say, "John Glenn can be glimpsed in the main title sequence for Star Trek: Enterprise as one of the astronauts inside the space shuttle orbiter during the launch of STS-95."