An engine problem forced Nasa on Monday to postpone for at least four days the debut launch of the colossal rocketship it hopes will one day fly astronauts back to the moon, more than a half-century after Apollo's last lunar mission.
The United States space agency cited a problem on one of the Space Launch System (SLS) rocket's main engines, as launch teams began a test that would have cooled the engines for liftoff. One of them would not cool as expected.
The delay was called at 8.35am EDT, two minutes after the targeted launch time, as the 32-storey tall rocket and its Orion capsule awaited liftoff from the Kennedy Space Centre in Cape Canaveral, Florida.
The mission, dubbed Artemis I, calls for a six-week, uncrewed test flight of Orion around the moon and back to Earth for a splashdown in the Pacific.
Nasa did not give a new launch date for the two-stage rocket but said its first available opportunity was Friday, Sept 2.
Whether the agency sticks with that date depends on how quickly engineers can resolve the engine issue. The subsequent launch opportunity is Monday, Sept 5.
Late-hour launch delays are routine in the space business, and Monday's postponement was not an indication of a major setback for Nasa or its primary contractors, Boeing Co for SLS and Lockheed Martin Corp for Orion.
"We don't launch until it's right," Nasa chief Bill Nelson said in a webcast interview just after the liftoff was scrubbed.
"It's just illustrative that this is a very complicated machine, a very complicated system, and all those things have to work. And you don't want to light the candle until it's ready to go."
Still, the delay was a disappointment to thousands of spectators who had gathered on the shores around Cape Canaveral, with binoculars in hand.
US Vice President Kamala Harris had just arrived at the space centre, joining a throng of dignitaries and invited guests attending the event, shortly before the scrub was called.
The SLS-Orion rocket's first journey will mark the kickoff of Nasa's highly vaunted moon-to-Mars Artemis programme, the successor to the Apollo lunar missions of the 1960s and 70s.
The voyage is intended to put the 5.75-million-pound vehicle through its paces in a rigorous demonstration flight, pushing its design limits, before Nasa deems it reliable enough to carry astronauts in a subsequent flight targeted for 2024.
Billed as the most powerful, complex rocket in the world, the SLS represents the biggest new vertical launch system the US space agency has built since the Saturn V rocket flown during Apollo, which grew out of the US-Soviet space race of the Cold War era.
If the first two Artemis missions succeed, Nasa is aiming to land astronauts back on the moon, including the first woman to set foot on the lunar surface, as early as 2025, though many experts believe that time frame is likely to slip by a few years.
The last humans to walk on the moon were the two-man descent team of Apollo 17 in 1972, following in the footsteps of 10 other astronauts during five earlier missions beginning with Apollo 11 in 1969.
The Artemis programme seeks to eventually establish a long-term lunar base as a stepping stone to even more ambitious astronaut voyages to Mars, a goal that Nasa officials have said will probably take until at least the late 2030s to achieve.
The programme was named for the goddess who was Apollo's twin sister in ancient Greek mythology.
SLS has been under development for more than a decade, with years of delays and cost overruns. But the Artemis programme also has generated tens of thousands of jobs and billions of dollars in commerce.
One issue cited by Nasa officials last week as a potential show stopper for Monday's launch was any sign during rocket fuelling that a newly repaired hydrogen line fitting had failed to hold. Nasa officials said on Sunday they were also eyeing a potential, but minor, helium leak in launch pad equipment.
Although no humans will be aboard, Orion will be carrying a simulated crew of three — one male and two female mannequins — fitted with sensors to measure radiation levels and other stresses that real-life astronauts would experience.