If all goes according to plan, Friday morning's launch of shuttle Atlantis on a 12-day mission to the International Space Station will mark the end of an era in the U.S. manned spaceflight program.
But veteran former astronauts say the space program is in "disarray" and fear the end of the shuttles could mean a permanent decline in U.S. space leadership as well.
Even one senior NASA official voiced pointed criticism recently about what he described as "poor policy" and the lack of any coherent leadership from Washington.
The White House and NASA's leaders have insisted, however, that America still has a bright future in space.
NASA is just retooling, officials have said, while adding that the U.S. space agency now plans to use some of the shuttle's budget to develop spaceships that can travel beyond the space station's 220-mile-high orbit, where the shuttles cannot go.
"When I hear people say or listen to media reports that the final shuttle flight marks the end of U.S. human space flight, I have to say ... these folks must be living on another planet," NASA administrator Charlie Bolden said last week at a National Press Club luncheon.
Scraping the shuttle also enables NASA to maintain the space station through at least 2020 -- five years beyond original budget projections, officials say.
But what is most troubling to space enthusiasts is the gap between the shuttle's end and the start of a new program, with the roll-out of a new generation of spacecraft.
"We're all victims of poor policy out of Washington D.C., both at the NASA level and the executive branch of the government and it affects all of us," NASA's launch director Mike Leinbach told his team after a final shuttle training run last week.
"I'm embarrassed that we don't have better guidance. Throughout the history of the manned spaceflight program we've always had another program to transition into," he said.
MIRED IN DEBATES
NASA had been planning to return to the moon under a program called Constellation, but that was quashed due to funding shortfalls. The Obama administration instead called for a flexible approach to exploration that includes visits to an asteroid and eventually a human mission to Mars.
Congress is mired in debates about what type of rocket to build and how much shuttle legacy hardware should be included.
NASA points to the space station, a $100 billion project of 16 nations that was assembled in orbit over the past 11 years, largely by space shuttle crews, as a major achievement.
But with construction complete, NASA wants to turn over station crew ferry flights to private companies, even though none are expected to be ready to fly until around 2015.
In the meantime, the United States will pay Russia to fly its astronauts, at a cost of more than $50 million a seat.
Critics say launch-ready spaceships are a critical component of human space flight.
Without that, the fear is that Russia, increasingly, or China and even Europe may step in to fill the void.
"We're basically decimating the NASA human spaceflight program," said seven-time shuttle flier Jerry Ross. "The only thing we're going to have left in town is the station and it's a totally different animal from the shuttle."
That sentiment is echoed by several Apollo-era luminaries, including the normally reticent Neil Armstrong, the Apollo 11 commander who 42 years ago was the first person to set foot on the moon.
Armstrong and colleagues Gene Cernan, commander of the final U.S. moon mission in 1972 and Jim Lovell, commander of the nearly fatal Apollo 13 flight, publicly decried the state of the U.S. space program in a widely distributed column.
"NASA's human spaceflight program is in substantial disarray with no clear-cut mission in the offing," the astronauts wrote recently.
"After a half-century of remarkable progress, a coherent plan for maintaining America's leadership in space exploration is no longer apparent."
(Editing by Tom Brown and Philip Barbara)