Pre-flight operations were called off about three hours before the scheduled liftoff time targeted for the 32-storey-tall Space Launch System (SLS) rocket and its Orion capsule from Cape Canaveral, Florida, on Saturday.
The uncrewed test flight, aimed at launching the capsule out to the moon and back, was to have marked the inaugural voyage of both the SLS and Orion a half century after the last lunar mission of Apollo, forerunner of the Artemis program.
The countdown was scrubbed after Kennedy Space Center technicians made several failed attempts to fix a "large" leak of supercooled liquid hydrogen propellant being pumped into the rocket's core-stage fuel tanks, agency officials said.
The initial launch try on Monday was likewise foiled by technical problems, including a different leaky fuel line, a faulty temperature sensor and cracks found in insulation foam.
Mission managers proceeded with a second launch attempt on Saturday once the earlier issues had been resolved to their satisfaction. NASA had reserved another backup launch time, for either Monday or Tuesday, in case a third try was needed.
But after a review of data from the latest difficulties, NASA concluded the hydrogen leak was too tricky and time-consuming to finish troubleshooting and fix on the launch pad before the current launch period allotted to the mission expires on Tuesday.
The delay means the earliest opportunity to try flying the rocket again would come during the next launch period that runs September 19-30, or during a subsequent October window.
NASA chief Bill Nelson said earlier in the day that a rollback would postpone the next launch attempt at least until mid-October, in part to avoid a scheduling conflict with the next International Space Station crew due for launch early that month.
Launch-day delays and technical snags are not uncommon in the space business, especially for new rockets such as NASA's Space Launch System. On average, the odds of scrubbing a launch on any given day for any reason, including foul weather, are about one-in-three.
"This is part of our space program - be ready for scrubs," Nelson said on NASA TV.
The last-minute setbacks on the launch pad come at the tail end of a development program more than a decade in the making, with years of delays and billions of dollars in cost overruns under NASA's respective SLS and Orion contracts with Boeing Co and Lockheed Martin Corp.
Apart from its technical challenges, Artemis I signals a major turning point for NASA's post-Apollo human spaceflight program, after decades focused on low-Earth orbit with space shuttles and the International Space Station.
Named for the goddess who was Apollo's twin sister in ancient Greek mythology, Artemis aims to return astronauts to the moon's surface as early as 2025, though many experts believe that time frame will likely slip.
Twelve astronauts walked on the moon during six Apollo missions from 1969 to 1972, the only spaceflights yet to place humans on the lunar surface.
