During the fueling process, which got underway at around 12:30 p.m. ET on Monday, mission managers twice paused proceedings to investigate leaking hydrogen fuel at the tail end of the rocket.

Despite eventually pressing on and conducting tests of the Orion spacecraft, which sits atop the rocket, the hydrogen leaks cropped up again in the final minutes of the simulated launch countdown.

NASA said that systems aboard the rocket that are designed to take over control of the booster in the final minutes before liftoff “automatically stopped the countdown due to a spike in the liquid hydrogen leak rate.”

Additionally, engineers will investigate several audio issues in communications channels used by ground teams that occurred during the wet dress rehearsal.

The four astronauts who were set to fly on the Artemis II flight — NASA astronauts Reid Wiseman, Christina Koch and Victor Glover and Canadian astronaut Jeremy Hansen — were set to arrive at Kennedy Space Center Tuesday afternoon. They had been in quarantine in Houston since Jan. 21 to protect against preflight exposure to germs.

But NASA said the astronauts will now be released from quarantine and will not travel to Florida as planned.

Instead, the crew will enter quarantine again roughly two weeks before the next targeted launch opportunity, according to the agency.

Artemis II will be the second trip to space for NASA’s Space Launch System rocket and Orion capsule, but the first time they carry humans.

The much-anticipated launch is expected to represent a crucial step toward realizing NASA’s goal of returning astronauts to the lunar surface.

A previous uncrewed Artemis I flight around the moon in 2022 was delayed six months because of hydrogen leaks identified during its first wet dress rehearsal.

With humans set to fly aboard the Space Launch System rocket and Orion capsule for the first time, the stakes for the Artemis II flight are high.

“As always, safety remains our top priority, for our astronauts, our workforce, our systems, and the public,” Isaacman said on X, adding that NASA “will only launch when we believe we are ready to undertake this historic mission.”