Artemis might not be going to the moon as quickly as planned.

The mission was scheduled for a wet dress rehearsal this weekend to allow NASA teams to practice launch procedures without actually starting the rocket.

According to NASA, Artemis I is the first in a series of increasingly complex missions aimed at making human exploration of the moon and Mars possible. NASA announced this afternoon that the craft was struggling to pressurize. That process is crucial to keeping hazardous gases from disturbing the rocket as it is pumped full of 700,000 gallons of fuel.

NASA said it was assessing next steps for the rehearsal, including a media briefing this evening and a potential second try on Monday, April 4.

As of Sunday morning, teams were approved to begin tanking the rocket and the weather was not threatening.

An official launch date for Artemis is dependent on the success of the dress rehearsal. A lot is riding on this mission, as the rocket is the most expensive ever built.

NASA will send a human crew into space on the follow-up mission Artemis II, but there are no astronauts aboard Artemis I.