Pegasus is NASA’s one of a kind barge used to transport the space vehicle hardware between NASA centers for testing and eventually to the launch site at the Kennedy Space Center in Florida.

The Pegasus has been lengthened to accommodate the SLS core stage which is longer than the Space Shuttle External Tanks it previously shipped from NASA Michoud to NASA KSC. 

The SLS core stage pathfinder is a full-scale mockup that is identical to the core stage in shape, size and weight. 

The pathfinder hardware will be used by NASA engineers and contractors to practice stacking the hardware in the VAB High Bay 3 using the same procedures needed to stack the real hardware for the Artemis I mission.

“The pathfinder, though not actual flight hardware, will provide the SLS program, Exploration Ground Systems (EGS) team with the opportunity to practice stacking maneuvers and certify the new system inside the Vehicle Assembly Building (VAB) High Bay 3 before Artemis flight hardware arrives next year,” say NASA officials. 

“Over the next several months, pathfinder will be used to validate ground support equipment and demonstrate how the core stage will be integrated in the VAB – the same process the actual core stage will undergo when being processed for Artemis I.”

NASA will conduct a full duration ‘green run’ engine fire test of the completed core stage at Stennis to fully confirm its readiness for flight. But that test will require six months of intense effort work.

After months of deliberating back and forth NASA Administrator Jim Bridenstine announced July 25 that he has ‘greenlighted’ the “Green Run” full duration engine test of the agency’s core stage for the mammoth Space Launch System (SLS) moon rocket next year at NASA’s Stennis Space Center – ahead of the upcoming Artemis 1 lunar mission launch.

Bridenstine had considered curtailing and even cancelling the 8 minute long full duration core stage engine test until recently in favor of a much shorter duration static test fire lasting only a few seconds on the launch pad at the Kennedy Space Center – but ultimately cited astronaut safety and rocket reliability as the top reasons for his decision to give the ‘Go Ahead’ to the ‘Green Run’

The ”Green Run” test involves conducting a full duration eight minute long static fire test of the 212 foot long (64.6 m) SLS core stage at the B-2 Test Stand at NASA’s Stennis Space Flight Center near Bay St. Louis, Mississippi.

The test would be run at some point next year and pave the way to the first Artemis lunar mission set to blastoff on the uncrewed Artemis-1 test flight perhaps by late 2020 or more likely in 2021.