The Space Launch System rocket that will dispatch four astronauts on the first Moon mission in more than 50 years passed a major milestone Wednesday.

NASA said ground teams inside the Vehicle Assembly Building (VAB) at Kennedy Space Center in Florida lifted the aft assembly of the rocket's left booster onto the mobile launch platform. Using an overhead crane, teams hoisted the left aft booster assembly -- already filled with pre-packed solid propellant -- from the VAB transfer aisle, over a catwalk dozens of stories high and then down onto mounting posts on the mobile launcher.

This marks the start of stacking for the second SLS rocket earmarked to launch NASA's Artemis II mission and slated to send NASA astronauts Reid Wiseman, Victor Glover, Christina Koch, and Canadian astronaut Jeremy Hansen on a 10-day flight around the far side of the Moon. Artemis II will be the first crewed flight of NASA's Artemis program and the first time people fly on the SLS rocket and Orion spacecraft.

The two SLS solid rocket boosters will stand 177 feet (54 meters) tall when fully stacked. Each of the boosters, produced by Northrop Grumman, are made up of five segments and a nose cone. After liftoff, they will burn for more than two minutes and provide the lion's share of the rocket's 8.8 million pounds of thrust, firing in unison with four liquid-fueled engines on the SLS core stage.

The booster segments assigned to Artemis II have previously flown on multiple space shuttle missions, when NASA recovered and reused them. For SLS launches, the boosters will jettison and won't be recovered.

A NASA spokesperson told Ars it should take around four months to fully stack the SLS rocket for Artemis II. First, teams will stack the two solid-fueled boosters piece-by-piece, then place the core stage in between the boosters. Then, technicians will install a cone-shaped adapter on top of the core stage and finally hoist the interim cryogenic propulsion stage, or upper stage, to complete the assembly.

Once the Orion spacecraft is ready, NASA will transfer it from a nearby processing facility to the VAB and stack it on top of the SLS rocket. Fully stacked, the entire vehicle will stand 322 feet (98 meters) tall.