Mercury astronaut Alan Shepard Jr., all business, became the first American in space on May 5, 1961. Photo Credit/NASA Photo

The  United States marks the 50th anniversary of the nation’s first human space flight on Thursday, the 15-minute voyage of Mercury astronaut Alan Shepard Jr.

The  May  5,  1961,  suborbital flight in the one-man Mercury 7 capsule designated Freedom 7 lifted off at 9:34 a.m., EST, and covered just 303 miles.

However, it sparked a national resolve to place the first human explorers on the moon.

Just 20 days later,  President John F. Kennedy stood before a joint session of Congress to set forth the goal of landing Americans on the moon and returning them safely to Earth before the end of the decade.

Shepard, a 37-year-old naval aviator, was one of seven test pilots chosen by NASA in 1959 to become the nation’s first astronauts. His suspenseful flight followed Soviet Cosmonaut Yuri Gagarin’s historic first human spaceflight on April 12, 1961. Gagarin’s Vostok spacecraft circled the Earth once before making its descent.

Gagarin’s flight ignited a space race between the two Cold War adversaries.

Alan Shepard, commander of Apollo 14, walks on the moon on Feb. 5, 1971. Photo Credit/NASA Photo

NASA’s response was dramatic as it marched through the closely sequenced series of Mercury, Gemini and Apollo missions leading to a triumphant first lunar landing by Apollo 11 commander Neil Armstrong and lunar module pilot Buzz Aldrin on July 20, 1969.

Shepard, who died on July 21, 1998, went on to become NASA’s chief astronaut.  Eventually, he overcame an inner ear problem that precluded spaceflight, to command the Apollo 14 mission that allowed him to land on the moon with Edgar Mitchell. The nine-day mission launched on Jan. 31,  1971.

A U. S. Postal Service stamp commemorating Shepard’s contributions was unveiled this week.