After three previous aborted attempts, NASA successfully fueled its new massive deep-space rocket, the Space Launch System, for the first time on Monday — completing a critical milestone ahead of the vehicle’s first flight. However, there was a shadow over the achievement. The fueling was part of an elaborate dress rehearsal that ended 20 seconds earlier than NASA had planned, and it’s unclear if the agency got all the data and practice it needed to proceed with the rocket’s debut launch.

The Space Launch System, or SLS, is a key piece of NASA’s flagship Artemis program — an elaborate effort to send the first woman and the first person of color to the surface of the Moon. But first, SLS needs to actually fly, and before that can happen, NASA wanted to go through all of the intricate steps that lead up to an actual launch — except for the part where the rocket takes off.

With the SLS standing upright on its launchpad in Kennedy Space Center in Florida, NASA engineers and flight controllers filled the vehicle with its ultra-cold propellants on Monday, just as they would on a launch day. With all the SLS tanks full, the flight team counted down to a simulated liftoff time, with the plan to stop the countdown at roughly T-minus 9 seconds. Instead, the team stopped the countdown short at T-minus 29 seconds due to a hydrogen leak. NASA says it was able to complete most of its objectives for the test, primarily loading the vehicle with propellant — but that there are still a handful they weren’t able to get to with the premature cutoff.

“I would say that the majority of our objectives were met,” Charlie Blackwell-Thompson, the Artemis launch director at NASA, said during a press conference after the test. “There were maybe small pieces within that one primary objective that we came up a little short on.”

NASA has attempted this dress rehearsal three times before, and all of those attempts ended before flight controllers could fully load the rocket with propellant. After the third failed attempt, NASA rolled the SLS into the agency’s massive Vehicle Assembly Building to perform various repairs and upgrades before rolling the rocket back out onto the pad on June 6th.

Three of the biggest goals of the dress rehearsal included demonstrating that the flight team could load the vehicle up with propellant, stopping the countdown, and then draining the SLS of fuel — all of which were performed on Monday. Additionally, NASA was able to get into terminal countdown, the final phase of the countdown that begins T-minus 10 minutes prior to launch.

NASA/Ben Smegelsky

One of the big things they weren’t able to test was a recycle attempt. The team had originally planned to get down to T-minus 33 seconds, stop the count, and then go back to T-minus 10 minutes. That goal was to simulate an unexpected hold on launch day and an attempt to try again, which can sometimes occur. The team would then have gotten down to T-minus 9.3 seconds — the moment just before the sequence begins to ignite the main engines at the base of the rocket.

Plans changed during the test. The simulated liftoff was delayed by hours due to a number of issues that flight controllers worked on throughout the day, including a hydrogen leak. Ultimately, flight controllers opted to skip the stop and turnaround at T-minus 33 seconds and just proceed down to T-minus 9.3 seconds, according to CBS. However, they knew that the hydrogen leak would likely trigger an abort within the flight computers before reaching that final countdown time.

During a press conference after the test, NASA personnel stressed that most of the objectives of the dress rehearsal were met. “I would say we’re in the 90th percentile in terms of, you know, where we where we need to be overall,” Mike Sarafin, the Artemis mission manager at NASA, said. But they were vague about the items that weren’t completed. One of the objectives they didn’t meet included demonstrating something known as “bleed flow” — a way to maintain proper temperatures of the propellant — because of the hydrogen leak. There was also some old hardware on the solid rocket boosters that didn’t get a chance to fire as planned.

Now, NASA says it’s looking at the data it did collect and will determine next steps. “I think we’ll take a couple of days and get through that, and then we’ll make a decision on what’s the best path forward,” Tom Whitmeyer, NASA’s deputy associate administrator for common exploration systems, said. It’s possible that the agency will opt to do another type of fueling test, and NASA’s Jim Free, NASA’s associate administrator for exploration systems development, did say prior to this attempt that NASA wanted a thorough test before flying. “This is the first time we’re flying this vehicle, and I think we need to understand everything we can before we commit to launch,” Free said last week. But Whitmeyer did note there is “relative risk to continuing to exercise the hardware at the pad.”

As for how this might affect the timeline for the SLS’s first flight, NASA won’t say. SLS is slated to make its debut during a flight called Artemis I, which will see the rocket launch an empty crew capsule called Orion around the Moon on a weeks-long trip. Ahead of this dress rehearsal, NASA noted that the earliest likely attempt for a launch would be during a window that opened in late August. Now with this test, NASA isn’t setting any firm dates. “I don’t think we know yet,” Whitmeyer said. “We need to really sit down and do everything we just talked about: look at the objectives, see what we accomplished, and see what additional work might be required.”