Atlantis is poised to blaze a path into history Friday when it propels toward the heavens on the last-ever mission of the 30-year-old American space shuttle program.

The crew of four US astronauts boarded the shuttle as NASA kept a close eye on the gloomy weather forecasts, which have predicted rain, clouds and a 70 percent likelihood of a delay in the launch planned for 11:26 am (1526 GMT).

Atlantis's 12-day mission to the International Space Station marks the end of an era in human spaceflight, after which the United States will rely on Russia to send astronauts to space until a replacement US capsule can be built.

"The beginning of the end of the space shuttle era is this morning," said NASA spokesman Allard Beutel.

At least 750,000 people have descended on Florida, with tourists eager to grab a glimpse of the final launch, but nostalgia has mingled with bitterness among the thousands of NASA employees set to lose their jobs.

"It is a sad time," said NASA astronaut Terry Virts, reflecting on what he called the "passion" of many of his coworkers. "The sad part about it is that we won't have an American ability to launch astronauts anymore."

Virts, who piloted the shuttle Endeavour's mission to the orbiting lab in February 2010, said the absence of a project to replace the shuttle has left many people reeling.

"It is tough to end something without having a follow-on," Virts said.

The Constellation program aimed at returning astronauts to the moon was axed by President Barack Obama last year in favor of focusing on deep space missions that could see Americans explore an asteroid and potentially Mars in the coming years.

NASA has salvaged plans for the Orion space capsule, developed as part of Constellation, with the prospect of using them as the basis for a multipurpose crew vehicle that can venture into deep space.

Private companies like SpaceX, Boeing and Sierra Nevada are competing to become the first to build a next-generation space capsule that can take astronauts and cargo to the orbiting research lab.

Those plans are not likely to come to fruition before 2015 at the earliest.

"All of us are addicted to going to space," said astronaut Cady Coleman, who recently returned from a six-month stint aboard the International Space Station.

"We always want more. We would go all the time if we could," she told AFP.

Once opportunities for space travel are limited to catching rides aboard the Russian Soyuz at a cost of $51 million per ticket, astronauts will just need to draw on the patience they have always needed to have, she said.

"If it (space travel) is the only thing you want to do... this is not the right job," she said. "The job of an astronaut is much, much broader than going into space."

Earlier this week, Obama praised the shuttle for its long legacy in space exploration but said it was time to focus on new projects.

"Let's start stretching the boundaries so we're not doing the same things over and over again. But rather, let's start thinking about what's the next horizon, what's the next frontier out there," he said.

"In order to do that, we'll need some technological breakthroughs that we don't have yet."

Of the six US space shuttles, the prototype Enterprise never flew in space, Challenger exploded after liftoff in 1986 and Columbia disintegrated on its return to Earth in 2003. Fourteen crew members died in the two disasters.

NASA plans to send the remaining three shuttles in the fleet -- Discovery, Endeavour and Atlantis -- to museums across the country to go on permanent display.

Meanwhile space fans jockeyed for spots along the "Space Coast" in the hopes of witnessing history.

"My whole life I grew up with the space shuttle," said Nicole Solomon, 35, a producer who is visiting from California, and who plans to follow every detail of the shuttle's last adventure in space.

"I'll watch the mission and I'll watch the landing and probably shed a few tears."

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