No human has set foot on the moon in over 50 years, but NASA’s Artemis program is reaching the final stages in the process of sending astronauts back for the first time since 1972.

On Monday, NASA fueled the massive Orion rocket for the first time during its trials and rehearsed a simulated countdown to liftoff, AP writes.

The fueling and liftoff simulation is dubbed a “wet rehearsal,” as the rocket is fueled up for the practice run. This rehearsal phase is the most recent in a series of development stages that began in 2017.

The Artemis program eventually aims to lay the groundwork for a lunar base where astronauts can conduct extended research.

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NASA’s rehearsal of the liftoff signals that the actual launch date is approaching. However, the wet rehearsals have so far been hampered by a multitude of problems, forcing NASA to push back its schedule.

The first attempt on April 3 had to be stopped due to pressurization issues within the crew’s cabin. Since then, a total of four attempts have been made, each one being stopped short after running into issues such as fuel line leaks and stuck valves.

Monday’s test almost met a similar fate when an external fuel line began to leak. However, under the authority of Launch Director Charlie Blackwell-Thompson, the simulated launch continued.

Ideally, NASA staff were hoping to get within 9 seconds of launch, the closest possible window to an actual launch. However, the test was stopped at 29 seconds to launch. NASA spokesman Derrol Nail told AP that the reason for the premature termination was unknown.

The failed tests have made it difficult for NASA to give an accurate launch date. As of now, the timeline is vague, with possible launch dates ranging from as soon as July 26 to as late as spring 2023.

Before humans can take off in NASA’s moon-bound rockets, unmanned test missions must first occur under the Artemis program.

When the liftoff rehearsals conclude, the Orion will fly unmanned around the moon to prepare for the moment astronauts finally board the ship.