No person alive has been more closely associated, for so long, with America's triumphs in space during the late '50s and early '60s than Ohio native John Glenn. Here, on the occasion of his 90th birthday (July 18), LIFE.com presents unpublished photos of the first American to orbit the earth; the decorated Marine Corps veteran (WWII and Korea); and the earnest, novice politician, taken by LIFE photographers during one of the most thrilling, inspiring, nerve-wracking eras in the nation's history: the Space Race....