Lightning, inability to pressurize the mobile launcher, and now a stuck valve. The wet rehearsal of NASA's massive Moon rocket, the Space Launch System (SLS), which will launch Artemis I, has encountered several problems in the last few days. A test launch that was originally scheduled to be carried out on April 2 got pushed to April 4 due to the aforementioned lightning etc. However, it got scrubbed again after a stuck valve stalled NASA's second attempt to fuel the rocket. It's now unclear when the next attempt will be.

Of course, while a smooth sailing rehearsal would be preferable, this practice run’s job is to work out exactly what is going to happen and have a solution for every possible problem that might arise. A lot is riding on this rocket launch system, so any kinks need to be worked out now.

A "wet rehearsal" is a test conducted as a scrub, the name for an aborted launch, with the countdown going down to just 10 seconds before lift-off. The rocket, which weighs a whopping 2.6 million kilograms (5.75 million pounds) when fuelled, will receive almost 3.2 million liters (700,000 gallons) of cryogenic propellant and will be tested as if it really was about to launch. This will guarantee that all is good for the real launch in May.

The April 4 wet rehearsal saw the team being able to fill the rocket’s tank with liquid oxygen up to 50 percent. However, a stuck vent valve about 50 meters up the mobile launcher structure that supports the huge rocket forced the space agency to scrub the test, officials said.

“During chilldown of the lines in preparation for loading the liquid hydrogen, the teams encountered an issue with a panel on the mobile launcher that controls the core stage vent valve. The purpose of the vent valve is to relieve pressure from the core stage during tanking,” the Artemis I team wrote in a blog post. “Given the time to resolve the issue as teams were nearing the end of their shifts, the launch director made the call to stop the test for the day.”

The next step will be to have a team investigate the issue right at the launch pad and work out what is the best way forward to solve it.

Artemis I is the first of a series of missions that will take humans back to the Moon. Launching some time in May, it will be an uncrewed mission and will last just short of a month testing crucial technology for human exploration of deep space.