The final countdown: Artemis 1 completes a major test to finally reach the Moon

Hundreds of thousands of gallons of frigid rocket fuel were pumped into NASA’s Artemis 1 on Monday — but it’s all just a practice run for the Moon rocket’s eventual launch. But like three previous attempts, it wasn’t without some hurdles.

Update on Tuesday: The team told reporters on Tuesday that Monday’s wet dress rehearsal was a success, and remain optimistic about the factors that the test did not verify during the countdown sequence. NASA will select an updated Artemis 1 launch target date after the team reviews data from yesterday’s performance.

Monday’s wet dress rehearsal is supposed to be the final test Artemis 1 needs to ace in order to get greenlit for launch. Artemis 1’s flight will officially begin NASA’s newest endeavor to return humans to the Moon. But on Monday, a hydrogen leak held up the wet dress rehearsal, with NASA eventually deciding to temporarily ignore the issue and press on with other parts of the rehearsal.

During the rehearsal, researchers scrutinized how super-cold liquid oxygen (LOX) and liquid hydrogen (LH2) funnel into Artemis 1. They also observed how well the fuel drains out of the spacecraft in the event that NASA needs to cancel its launch with moments to spare before liftoff.

Despite the setbacks — including a computer reading raising a flag at T-minus 29 seconds for reasons NASA is still analyzing — the agency was able to complete the fourth wet dress rehearsal, with the NASA Exploration Ground System’s Twitter account posting a statement from launch director Charlie Blackwell-Thompson calling it a "great day for our team,” adding that she was, "really proud of the team and all they've done to get us here."

Fourth time’s a charm

The wet dress rehearsal takes approximately two days to complete.

“The rehearsal will run the Artemis 1 launch team through operations to load propellant into the rocket’s tanks, conduct a full launch countdown, demonstrate the ability to recycle the countdown clock, and also drain the tanks to give them an opportunity to practice the timelines and procedures they will use for launch,” says the NASA Artemis blog that publishes regular updates.

After three failed wet dress rehearsals in April and a six-week-long hiatus for repairs, Artemis 1 has already had a long road. But Monday could be that momentous day at Kennedy Space Center (KSC) in Cape Canaveral, Florida when it finally succeeds at this crucial performance evaluation. NASA might then conduct one additional wet dress rehearsal just to be sure: Space agency leaders previously stated they’d like to perform at least two successful tests before scheduling the flight.

Artemis 1 is stationed at Launch Complex 39B, a historic spot where Apollo 10 launched in 1969 to rehearse the first moon landing. The vehicle is made up of NASA’s ultra-powerful but unproven Space Launch System (SLS) and topped with an uncrewed Orion capsule. Monday marks the fourth wet dress rehearsal’s last leg. NASA is expected to announce soon if they will need another wet dress rehearsal after assessing issues with this one.

Teams have been waiting for this day since June 6, when NASA re-released Artemis 1 from KSC’s rocket garage known as the Vehicle Assembly Building. The fully stacked lunar launcher actually first rolled out on March 17, but failed three wet dress rehearsals in April. Several anomalies appeared after those tests. Two in particular — a leaking fuel line and a malfunctioning valve — required service at the building.

June 20 Wet Dress Rehearsal

On Saturday at 5:00 p.m. Eastern, the wet dress rehearsal officially started with a “call to stations” asking launch team members to get to their consoles inside KSC’s Launch Control Center.

Overnight from Saturday to Sunday, engineers powered up SLS’s core stage and the Orion spacecraft. Teams also “configured several systems on the ground, rocket, and spacecraft” and prepared the cables that connect the rocket to the mobile launcher. These lines, called umbilicals, “are used to provide power, communications, coolant, and propellant” to Artemis 1.

Over the weekend the team felt that Monday’s weather forecast would be good for fuel loading, so they proceeded with the numerous checks along the way that lead up to actual fueling.

Early Monday morning, the mission management team gave a “go” to begin Artemis 1’s tanking process. First, they chilled down the umbilicals to cool the liquid oxygen (LOX) and liquid hydrogen (LH2) fuel. Then this material slowly flowed into four tanks on Artemis 1. Two of these tanks — one LOX and one LH2 — belong to the SLS core stage towards the bottom of the rocket.

The other two tanks are part of Artemis 1’s ICPS, or interim cryogenic propulsion stage. Sandwiched between SLS and the Orion capsule, ICPS is designed to pick up the proverbial baton where SLS leaves off on the race to the Moon. After SLS catapults Artemis 1 into space, ICPS ignites to produce more than 24,000 lbs of thrust, powering the Orion capsule beyond the Moon.

Initially things were going fine during Monday’s test. The slow-phase fueling began at 10:43 a.m. Eastern, according to a tweet by NASA’s Exploration Ground Systems at KSC. By around 1:30 p.m. Eastern, the core SLS tanks were full, according to another tweet. Once a tank is filled, they top them off, because the Floridian environment causes some of the very cold propellant to evaporate.

The first issue appeared around 2:00 p.m. Eastern. NASA KSC commentator Derrol Nail announced an issue with the liquid hydrogen. “The team is currently evaluating that situation. Of course, any leak of liquid hydrogen is something to be concerned about for the launch team, as it is a hazardous gas and they want to understand the situation fully before they take any action.”

As he later explained, there is an apparatus at the base of the rocket called the tail service mast umbilical. It has many flex hoses, which naturally droop downward, and they funnel liquid hydrogen propellant into Artemis 1. They are designed to separate from the rocket at liftoff.

“When the rocket launches, those flex hoses which are connected to the rocket by disconnect lines, they separate,” Nail says. “One half stays with the rocket, and the other with the flex hoses that are connected to the tail service mast.”

The issue was that the quick disconnect at the base of the rocket is leaking. After several hours of troubleshooting, the launch team identified the problem as a sealing issue with the hydrogen valve. After finding themselves unable to fix the issue, the team decided around 6:04 p.m. Eastern to continue the terminal count, and opted at 6:34 p.m. to leave bleed lines open to prevent pressure from rising. The bleed valve was closed ay 7:22 p.m., allowing the team to proceed.

At 7:28 p.m. Eastern, the countdown clock was finally on, getting all the way from T-minus 10 minutes to T-minus 29 seconds before an internal computer raised a flag and stopped the count. At 7:50 p.m., the dress rehearsal wrapped, with the Artemis team gathering the next few days to assess the issues encountered in the rehearsal.

Depending on the outcome, NASA may opt for a fifth wet dress rehearsal, or it may be able to move toward confirming a launch date for Artemis 1, reopening an era of human lunar exploration.

Science editor John Wenz contributed to this report.