Workers in the Vertical Processing Facility observe the lower end of the Inertial Upper Stage (IUS) that will be mated with the Chandra x-ray Observatory (out of sight above it). After the two components are mated, they will undergo testing to validate the IUS/Chandra connections and to check the orbiter avionics interfaces. Following that, an end-to-end test (ETE) will be conducted to verify the communications path to Chandra, commanding it as if it were in space.