A crew of four moon-bound astronauts will remain on the ground for at least a month after NASA delayed the launch of the Artemis II mission. During critical pre-launch testing Monday, mission managers uncovered a number of issues that prevented the completion of the test.

NASA is now planning a March launch date for the four astronauts — three from the U.S. and one from Canada — on a ten-day mission to circle the moon and return to Earth, traveling farther than any humans have ventured into deep space.

Issues leading to that delay began about an hour into Monday’s test, known as the wet dress rehearsal. As the team began fueling the rocket at the Kennedy Space Center in Florida, sensors picked up a hydrogen leak. Super-chilled hydrogen is used as the fuel for the massive Space Launch System (SLS) rocket.

Hydrogen is an efficient propellant for rockets — but its molecules are so tiny and light they can escape even the tightest of seals. Launch director Charlie Blackwell-Thompson said they had troubleshooted the initial leak, but when they began to pressurize the tank, another leak surfaced.

“And so as we began that pressurization, we did see that the leak within the cavity came up pretty quick,” said Blackwell-Thompson.

(L/R) NASA Associate Administrator Amit Kshatriya, NASA Associate Administrator Lori Glaze, launch director Charlie Blackwell-Thompson, and manager of NASA’s Space Launch System Program, John Honeycutt, hold a news conference on the Artemis II mission at Kennedy Space Center in Cape Canaveral, Fla., on Tuesday.

Miguel J. Rodriguez Carrillo/AFP via Getty Images

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Miguel J. Rodriguez Carrillo/AFP via Getty Images

Hydrogen leaks plagued testing of NASA’s Artemis I mission in 2022. Blackwell-Thompson said lessons learned from that uncrewed flight were utilized for Artemis II, but there’s more investigation is needed.

The wet dress rehearsal uncovered other issues — including a problem with the Orion capsule, which will carry the crew to the moon. While no one was on board Monday, teams practiced preparing the spacecraft for its passengers. A valve that pressurizes the vehicle required additional attention and took more time to close the hatch than anticipated.

Teams also uncovered issues with cameras due to cold weather and audio dropouts across communication channels. “As always, safety remains our top priority, for our astronauts, our workforce, our systems and the public,” said NASA administrator Jared Isaacman in a post on X, and that NASA will only launch when the agency is ready.

Work now begins to fix the issues. NASA will require another wet dress rehearsal before giving the “GO” to put astronauts on board. “All in all, a very successful day for us on many fronts,” said Blackwell-Thompson. “Then, on many others, we got some work we’ve got to go do.”

The earliest launch window for another attempt is March 6. NASA has additional launch opportunities on March 7, 8, 9 and 11.

The crew of NASA astronauts Reid Wiseman, Victor Glover, Christina Koch, and Canadian Space Agency astronaut Jeremy Hansen were released from quarantine and will remain in Houston, Texas. They’ll re-enter quarantine about 14 days ahead of the next launch attempt and make the trip to the Kennedy Space Center six days before liftoff.

Artemis II is testing key systems of the Orion spacecraft, like its maneuverability and life support systems, ahead of the planned Artemis III mission that will take humans to the lunar surface. The Artemis II will mark the first time humans have returned to the moon since the final Apollo lunar mission in 1972.