Weather delays have forced NASA to postpone by 24 hours the launch of a prototype rocket designed to return astronauts to the Moon, and perhaps one day to Mars.

Standing 328 feet tall, the Ares I-X is the longest rocket ever built and when lift-off goes ahead it will be the first spacecraft the Kennedy Space Centre has launched other than a shuttle in 30 years.

Tuesday, however, proved frustrating for space enthusiasts as first the weather, then a ship at sea, and then clouds conspired to delay the long-awaited launch at Cape Canaveral in Florida.

NASA said it would try again in a four-hour window from 8am on Wednesday. "Overall, the weather will improve," a space agency official said.

The Ares I-X is a prototype of the Ares I, which NASA hopes to have in service by 2015 to replace its aging shuttle fleet that is due to be mothballed next year.

During the two-minute test flight NASA aims to gather data from more than 700 sensors placed along the rocket that should enable engineers to fine-tune its design.

The Ares craft is intended to launch mission crews into orbit in the Orion spacecraft, which is still under development.

Ares and Orion are part of Constellation, NASA's grand program to send astronauts back to the Moon by 2020, and then on to Mars and other destinations.

The test flight is crucial as the White House is considering a report ordered by President Barack Obama's administration that raises concerns about Ares and the whole Constellation project.

A commission, chaired by Norman Augustine, a former chief executive at aerospace giant Lockheed Martin, concluded that the program "appears to be on an unsustainable trajectory" and seeks to achieve goals not matched by resources.

The Ares rocket has suffered major development problems and its hefty price tag has fuelled criticism of NASA, an agency notorious for its cost overruns.

The initial budget for the Constellation program was set at $US28 billion, but has swollen to at least $US44 billion.

Ed Crawley, a member of the Augustine commission set up by Obama to review the space plans of his predecessor George W Bush, said 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.

But Doug Cooke, associate administrator of NASA's Exploration Systems Mission Directorate, was upbeat ahead of the launch.

"This test is very important," he told reporters last week. "The data is important to us, regardless of what comes next."

NASA's yearly budget is about $US18 billion, $US10 billion of which is ploughed into the human space flight program, chiefly in developing the Ares I rocket and the Orion Crew Exploration Vehicle.

The Augustine commission said an additional $US3 billion 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).

If weather delays the test flight again on Wednesday, the team has a four-hour launch window each day until Friday.

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