During a training exercise, technicians with NASAs Exploration Ground Systems lower a mock-up of the Space Launch System (SLS) aft booster segment, referred to as a pathfinder, onto the mobile launcher in Kennedy Space Centers Vehicle Assembly Building (VAB) on Sept. 11, 2020. The rehearsal involved teams preparing the aft pathfinder segment in High Bay 4 of the VAB, lifting and moving it over to High Bay 3, and placing it on the mobile launcher in preparation for Artemis I. Stacking of the actual booster segments will occur later this year, before the SLS core stage arrives at the Florida spaceport. Artemis I is the first in a series of increasingly complex missions that will test SLS and the Orion spacecraft as an integrated system prior to crewed flights to the Moon.