NASA is pushing the pause button to the start of its newest moonshot program.

On Monday, personnel at the agency’s Kennedy Space Center in Cape Canaveral, Florida, began to roll back the 5.75-million pound Space Launch System (SLS) mega-rocket, setting it into motion atop the modified Apollo-era rocket dolly, the 131-foot-long Crawler-Transporter 2. It began its journey back to the Vehicle Assembly Building at an estimated speed of one mile an hour at around 8 p.m. Eastern time. The short trip to the VAB will take eight to 12 hours, according to NASA’s Exploration Ground Systems.

At the VAB, engineers can investigate a handful of problems that arose during recent testing known as the wet dress rehearsal, which is the final major checklist of items that need to be in order so that SLS can launch this summer. NASA began the wet dress rehearsal at the beginning of April, but after three versions of this test on April 3, April 4, and April 14, teams are at a hardware-related impasse that requires further investigation.

The space agency developed the 322-foot-tall SLS rocket for its Artemis program. Aptly named after Apollo’s goddess sibling, the new program seeks to inch towards new space frontiers like Mars. To do that, NASA wants to first return humans to the lunar environment. And to make sure the new rocket they build for Artemis is up to the tremendous tasks at hand, it will travel on an uncrewed three-week flight around the moon and back hopefully later this year.

Its launch readiness teeters on the wet dress rehearsal, the final major pre-flight check of the ground systems and fuel loading mechanisms that helps ensure all will likely be nominal on the day of liftoff.

At the VAB, technicians will access a piece of hardware called a check valve near the top of the rocket. This part, which malfunctioned during testing, ensures that fuel flow only goes in one direction, preventing backflow.

Teams will also review a leaky propellant connection. The tail service mast umbilical, which loads liquid hydrogen into the rocket’s core stage and is connected to the mobile launcher — the bare tower structure standing up alongside the rocket — prevented the team from completing the April 14 wet dress rehearsal.