By Irene Klotz
CAPE CANAVERAL, Fla (Reuters) - The United States space program was down and out when Alan Shepard climbed inside a one-man capsule on May 5, 1961, for a 15-minute ride.
Cold War rival Russia had flown cosmonaut Yuri Gagarin first -- less than a month before on a flight that was higher and longer -- and some in the nascent NASA thought the space race was already lost.
"When (Shepard) took off, he carried NASA on his shoulders," said Shepard biographer and long-time space journalist Jay Barbree. "If he failed, he knew it would be the failure of the space program."
At the wind-swept launch pad in Cape Canaveral, Florida, where Shepard blasted off, former astronauts, NASA guests and community leaders gathered Thursday to commemorate the 50th anniversary of the first flight of a U.S. astronaut into space.
The observation of Shepard's flight comes as NASA is finishing up a key chapter in its history. The agency is retiring the space shuttles this year after two more flights due to high operating costs and to free up funds to develop spaceships with longer range.
Shepard's 116-mile- -high jaunt beyond the atmosphere led to six U.S. moon landings and the space shuttle program.
"Project Mercury gave our country something new," said NASA administrator Charlie Bolden, a former space shuttle astronaut.
Shepard's fellow Mercury Seven astronaut Scott Carpenter also attended the anniversary ceremony, which featured a high-definition rebroadcast of Shepard's inaugural mission.
For 30 years, the shuttle program put hundreds of people, spacecraft and science experiments into orbit and built the International Space Station, a $100 billion, multinational orbital laboratory where astronauts -- mostly from the United States and Russia -- have been living continuously for more than a decade.
"We now move out on an exciting path forward where we will develop the capabilities to take humans to even more destinations in the solar system," Bolden said.
NASA hopes to hire commercial companies to fly astronauts to and from the station, breaking what will be a Russian monopoly on station crew ferry flights after shuttle retirement.
China, the only other nation that has flown people in space, is not a member of the station partnership.
Commercial suborbital flights, similar to what Shepard experienced, also are expected to begin as early as next year for tourists, scientists and business ventures.
In honor of Shepard's flight, the United States Postal Service on Wednesday unveiled two commemorative stamps. One depicts a space-suited Shepard with the Freedom Seven capsule and Redstone rocket in the background. A picture of NASA's Mercury-orbiting Messenger spacecraft is on the second stamp.
Both are "forever" stamps, which can be used at any time in the future for first-class letter postage.
Shepard, who went on to command America's third mission to the moon, died of leukemia in 1998 at the age of 74. Of the original seven astronauts, Carpenter and John Glenn, a former senator and the first American to orbit Earth, are still alive.
(Editing by Colleen Jenkins, Jerry Norton and Bill Trott)