Stacked on the mobile launcher and mounted on the crawler-transporter, the 322-foot tall, 3.5-million-pound SLS made the four-mile trek from the Vehicle Assembly Building to Launch Pad 39B over a total of 10 hours and 28 minutes.

“Rolling out of the Vehicle Assembly Building is an iconic moment for this rocket and spacecraft, and this is a key milestone for NASA,” said Tom Whitmeyer, deputy associate administrator for Common Exploration Systems Development at NASA Headquarters in Washington. “Now at the pad for the first time, we will use the integrated systems to practice the launch countdown and load the rocket with the propellants it needs to send Orion on a lunar journey in preparation for launch.”

The upcoming final test, known as the wet dress rehearsal, will run the Artemis I launch team through operations to load propellant into the rocket’s tanks, conduct a full launch countdown, demonstrate the ability to recycle the countdown clock, and then drain the tanks to give the team an opportunity to practice the timelines and procedures they will use for launch.

During the approximately two-day test, teams will begin by activating the launch facilities and formally beginning the countdown sequence. NASA staff at the Launch Control Center at Kennedy will connect with staff in the Mission Control Center at NASA’s Johnson Space Center in Houston, the Space Force Eastern Range, and the SLS Engineering Support Center at Huntsville’s Marshall Space Flight Center. Launch controllers will power on the different rocket and spacecraft systems along with ground support equipment.

Teams will then load over 700,000 gallons of cryogenic propellants including liquid hydrogen and liquid oxygen into the rocket at the launch pad as it sits on the mobile launcher according to the detailed timeline they will use on the actual launch day. They will practice every phase of the countdown, including weather briefings, pre-planned holds on the countdown, conditioning and replenishing the vehicle’s propellants as needed, and validation checks.

Once launch controllers reach the point just before the rocket’s RS-25 engines will ignite on launch day, they will recycle back to the T-10 minute point, then resume the countdown once more after a hold. The countdown will then be deliberately halted at about 10 seconds before the simulated liftoff to demonstrate stopping a launch and draining the propellants from the rocket.

This last step simulates a “scrub,” a halt to the launch that takes place when launch controllers decide not to proceed with a launch due to technical or weather issues that arise during or prior to the countdown.

Several days after the wet dress rehearsal, the integrated rocket and spacecraft will return to the Vehicle Assembly Building, where sensors specifically used for monitoring during the wet dress rehearsal will be removed. Technicians will also charge Orion and other system batteries, stow late-load cargo into Orion in preparation for the launch, run final checkouts on several elements, and perform other necessary tasks. Orion and SLS will roll to the launch pad for a final time about a week before launch.

NASA will review data from the rehearsal before setting a specific launch date for the Artemis I launch.