NASA’s rehearsal for Artemis II marks one of the final steps before four astronauts get launched into deep space for the first time since the Apollo program ended.

NASA’s wet dress rehearsal—a crucial test of the agency’s Artemis II mission to the moon—hit a snag on Monday. Engineers had been fueling the mission’s Space Launch System (SLS) rocket up with liquid hydrogen and liquid oxygen propellant and planned to initiate a countdown sequence to simulate the launch.

Before NASA can decide a rocket launch date for Artemis 2, the giant SLS rocket has to ace a critical fueling test known as a wet dress rehearsal.