On its fourth attempt on Monday, NASA mostly completed a practice countdown for the rocket that is to take astronauts to the moon.
But agency officials said it was too early to know whether the rehearsal would be enough to give the go-ahead for the rocket, the Space Launch System, to launch the Orion capsule on a test flight around the moon with no astronauts aboard.
Even if the practice countdown had gone perfectly, that mission, Artemis 1, would have been unlikely to blast off earlier than late August. That flight is to be the starting point for the United States to return astronauts to the lunar surface more than a half century after the Apollo 17 mission.
Sitting on a launchpad at Kennedy Space Center in Florida, the rocket’s propellant tanks were for the first time fully filled with 196,000 gallons of liquid oxygen and 537,000 gallons of liquid hydrogen into the rocket’s propellant tanks. Problems that had occurred during three earlier attempts in April were solved.
“I think it is a very successful day and again was a majority of the objectives,” Charlie Blackwell-Thompson, the launch director, said during a news conference of Tuesday.
However, a new problem — hydrogen leaking in a fuel line connector — cropped up. By warming up and then cooling the connector again, engineers hoped that the seal would shift enough to stop the leaking. That did not work.
During a real launch, that problem would have been the end of the countdown for the 322-foot tall rocket.
But the exercise on Monday was what NASA calls a wet dress rehearsal — wet because of the flowing of actual fuel into the propellant tanks — that is designed to work out glitches and procedures without the excitement of engines igniting and rocket rising to space.
With the countdown clock paused at T-10 minutes, engineers worked out a plan where a valve would be closed to stop the leak and errors would be suppressed to allow the countdown to proceed to test other rocket components and launch procedures.
Ms. Blackwell-Thompson approved the plans, and the countdown continued until, as expected, the countdown terminated with 29 seconds left. The liquid oxygen and liquid hydrogen were then drained from the rocket.
On Tuesday, NASA officials said they needed to seat through the data to see what they might still need to do before they feel they are ready to launch the rocket. The Space Launch System and Orion, the capsule on top where astronauts will sit, are essential components for Artemis, NASA’s program to send astronauts back to the moon.
Still NASA officials were ecstatic with the progress.
Thomas Whitmeyer, NASA’s deputy administrator for common exploration systems development, said that filling the propellant tanks and counting down so close to zero were major milestones.
“We’re looking at the pieces of the puzzle to decide what are the pieces that we didn’t get,” Mr. Whitmeyer said. “But we also got an awful lot of pieces to the puzzle put together, and we have a pretty good idea of what the puzzle looks like at this point.”
In April, three tries to conduct a wet dress rehearsal all ended early because of a variety of problems.
The rocket was rolled back to a giant garage called the Vehicle Assembly Building where technicians could more readily diagnose the problems and make repairs. The hiatus also allowed time for an off-site vendor to upgrade its facility that provided nitrogen gas — used to purge hazardous gases — to the Kennedy Space Center. During two of the rehearsal attempts, disruptions in the nitrogen supply delayed the countdowns.
NASA could decide to perform another wet dress rehearsal, or it could decide it has enough data and roll the rocket back to the Vehicle Assembly Building one last time for final preparations for launch, which include installing the self-destruct mechanism to destroy the rocket in case something goes wrong during flight.
For Artemis 1, the rocket would launch and send the Orion capsule on a lengthy trip around the moon. It would then circle back on its way to re-entering Earth’s atmosphere and splashing down for an ocean landing.
The second Artemis flight, scheduled for 2024, would have astronauts on board for a similar trip, without landing on the moon. Artemis 3 is to be the first lunar landing by astronauts since 1972. NASA has proposed a 2025 date for that crewed trip, but it could face more postponements.