The US space agency began the countdown on Tuesday for the test launch of a rocket designed to replace the aging space shuttle fleet and one day take astronauts to the Moon and Mars.
Barring bad weather, a four-hour window will begin at 8:00 am (1200 GMT) for the launch of the Ares I-X rocket from Florida's Kennedy Space Center, a key test for the future of US space exploration amid deep uncertainty about the program.
After a "call to stations" to begin the countdown at 1:00 (0500 GMT), the US space agency said some 30 team members began work at Kennedy's Launch Control Center.
But while NASA scientists said they had "no issues" with the 327-foot (100-meter) prototype, the world's largest at present, the weather could thwart the launch.
The forecast for Tuesday shows only a 40 percent chance of favorable weather. NASA needs just 15 minutes of good weather to launch.
If weather concerns cloud the test launch, the next window will be on Wednesday, when the forecast is 60 percent favorable. The flight will be delayed until next month if bad weather persists.
The flight, NASA said, will allow the agency to "test and prove hardware, models, facilities and ground operations associated with the Ares I launch vehicle," which is seen as a first step in US human space flight after the shuttle is retired.
NASA will gather data collected by more than 700 sensors placed throughout the rocket during the ascent of the integrated stack.
Only the first stage of Ares I-X -- a modified solid-fuel motor from the shuttle program -- will be tested, while the upper stage and capsule are mock-ups.
Data obtained during the two-and-a-half-minute flight will help the US space agency determine whether the prototype is safe and stable in flight before the new generation of launch vehicles is used to take astronauts into orbit.
A team of experts has projected that will not happen before 2015, leaving a five-year gap after the shuttle is retired in 2010.
The test launch comes as the White House studies a report by a high-level commission set up by President Barack Obama to review plans for post-shuttle human space flight established by his predecessor, George W. Bush.
The panel chaired by Norman Augustine, a former executive at aerospace giant Lockheed Martin, concluded that the US human space flight program "appears to be on an unsustainable trajectory" and did resources not match seeking to achieve goals.
The Ares rocket has suffered major development problems, and its hefty price tag has fueled criticism of NASA, an agency notorious for its cost overruns.
The initial budget for the Constellation program, which includes Ares rockets, was set at 28 billion dollars, but has swollen to at least 44 billion.
Augustine Commission member Ed Crawley said last week that Ares I was "not the right ship" for post-shuttle space flight.
"The question is not can we build Ares I, but should we build Ares I," Crawley said.
The National Aeronautics and Space Administration's yearly budget is about 18 billion dollars, 10 billion of which are plowed into the human space flight program, chiefly in developing the successor of the space shuttle: the Ares 1 rocket and the Orion Crew Exploration Vehicle.
The Augustine Commission said an additional three billion dollars a year are needed for NASA to meet Constellation program goals or take human space flight the next step beyond the existing International Space Station (ISS).
The commission's review of the Constellation program proposed several alternatives, including sidestepping the rocket and going straight to the Ares V family of launch vehicles, which would take astronauts back to the Moon and eventually on to Mars.