The shuttle Atlantis is poised to launch Friday on the final flight of the 30-year American program, a journey that marks the end of an era of US dominance in human space exploration.
The liftoff is scheduled for July 8 at 11:26 am (1526 GMT) from Kennedy Space Center in Florida's Cape Canaveral, sending Atlantis on a 12-day mission to the International Space Station.
Four American astronauts will ride the shuttle on its last tour of duty to stock the orbiting research outpost with plenty of supplies -- 8,000 pounds (3,600 kilograms) worth in all -- and bring back a failed ammonia pump, NASA said.
Up to a million people are expected to turn out along Florida's Atlantic Coast to watch the 135th and final shuttle launch, an estimate between two and three times the typical crowd size, local officials said.
"The space shuttle has been very good to this country. It is an incredible ship that is difficult to let go," said retiring NASA astronaut Mark Kelly, who just finished commanding Endeavour's last flight in May.
Kelly and NASA administrator Charles Bolden spoke to reporters in Washington on Friday about their memories of the shuttle program, and what lies ahead as private industries hustle to build a next-generation space capsule.
"Some of my best friends died flying on the shuttle and I am not about to let human spaceflight go away on my watch," said Bolden, who grew tearful as he remembered the 14 people who died in the Challenger and Columbia disasters.
The Challenger exploded 73 seconds after launch in 1986, and Columbia broke apart on its way back to Earth in 2003. Technical failures and problems in NASA's workforce culture were blamed for the safety breaches.
"We are not ending human spaceflight. We are recommitting ourselves to it and taking necessary and difficult steps today to ensure America's preeminence in human space exploration for years to come," Bolden said.
The shuttle has toted heavy loads of equipment to the space lab and back many times over the years, building up what Bolden called "the pinnacle of our current achievement, a stepping stone to the rest of the solar system and the tip of what comes next."
Atlantis's visit will mark the 37th and final trip by a US shuttle to the orbiting space station.
Once the shuttle program ends, the world's astronauts will have to hitch a ride aboard Russia's three-seat space capsules at a cost of ê51 million each.
Local communities that depend on Kennedy Space Center for tourist dollars and employment are bracing for the disappearance of 8,000 jobs with the program's closure.
In the meantime, its successor remains uncertain. Several private companies are competing to build a more efficient space capsule that will once again be able to carry astronauts into space, though that will not be ready for several years at least.
NASA also has its sights set on exploring Mars and an asteroid in the coming decades.
Atlantis's four-member crew is smaller than usual because there is no other shuttle in operation to serve as a rescue ship should the need arise while they are in space.
All four astronauts have flown shuttle missions in the past. The commander is retired Navy captain Chris Ferguson, 49; the pilot is Doug Hurley, 44; and mission specialists are Sandra Magnus, 46, and Rex Walheim, 48.
The quartet wore dark-colored business suits as they addressed the media Thursday for a final time before entering quarantine ahead of the flight.
Asked by a reporter if they were dressed for mourning, Walheim said it was actually just a matter of limited wardrobe options.
"These are the color suits we interviewed with 16 years ago, so that's the main reason -- we don't have a lot of suits," he joked.
When Atlantis returns to Earth, the shuttle will remain at Kennedy Space Center to serve as a tourist attraction while the other orbiters take spots at museums elsewhere in the country.
"In some ways, I think the shuttle will continue to inspire people," said Walheim.
"People are going to have a chance to go right up to a space shuttle vehicle and see a real one and think, 'Wow, we used to be able to launch those things into orbit.'
"I hope it generates that same kind of enthusiasm -- that childlike enthusiasm -- so people will say, 'Wow, if that made it to orbit, let's do it again,'" he added.