Things are getting real on the way to the Moon. Assembly of NASA's Artemis III Moon rocket has begun in earnest. Technicians  have stacked the first segments of the twin Space Launch System solid rocket boosters onto the mobile launcher inside the Vehicle Assembly Building. They started with the left-hand aft assembly.

By Saturday, July 11, both the left- and right-hand aft assemblies were secured to the launcher. This is the same mobile launch platform that supported Artemis II and has since undergone repairs after sustaining damage during that April launch.

The milestone comes less than three months after the Artemis II crew returned from their lunar flyby. The remaining booster segments arrived at Kennedy by train in June and are being inspected, coated, and processed at the Rotation, Processing, and Surge Facility. Next, they are placed on lift stands and stacked atop the aft segments. Each five-segment booster stands roughly 54 meters tall, and together the pair delivers more than 75 percent of the rocket's thrust at liftoff. That's the dominant share of SLS's power. Once the solid propellant ignites, the boosters cannot be throttled or shut down. And without them, the four RS-25 engines on the core stage could not lift the vehicle off the pad.

Work is advancing in parallel elsewhere in the VAB. Core stage processing continues after teams mated the four-fifths upper section with its engine section in May. The first two RS-25 engines arrived in June, and once all four are installed, the completed core stage will be integrated onto the mobile launcher.

The 65-metre core stage arrived from Michoud back in April. Fully assembled with its upper stage, Orion, and boosters, SLS will stand roughly 98 meters tall.

Launch teams have also begun monthly countdown simulations, rehearsing propellant loading and terminal count inside Firing Room 1. NASA Administrator Jared Isaacman has said the agency aims to complete a wet dress rehearsal before year's end, with Artemis III targeted for launch in 2027.