Technicians work underneath the European Service Module for NASA's Orion spacecraft, September 2018.
ESA's European service module will provide power, water, air and electricity to NASA's Orion exploration spacecraft that will eventually fly beyond the Moon with astronauts. The European Service Module is now complete for Orion's first mission that will do a lunar fly-by without astronauts to demonstrate the spacecraft's capabilities.
Much like closing the bonnet on a car, with the radiators in place technicians can no longer access the internals of the European service module, symbolically ending the assembly and integration of the module that will fly further into our Solar System than any other human-rated spacecraft has ever flown before.