Technicians with NASA’s Exploration Ground Systems prepare to lower a mock-up, or pathfinder, of the Space Launch System’s (SLS) center booster segment onto an aft pathfinder segment inside the Vehicle Assembly Building (VAB) at the agency’s Kennedy Space Center in Florida on Sept. 14, 2020. Teams rehearsed stacking both pathfinder segments on top of the mobile launcher in High Bay 3 of the VAB in preparation for the Artemis I launch. Stacking of the actual SLS booster segments will occur later this year, when the rocket’s core stage arrives at Kennedy. 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.