Assembly, Test, and Launch Operation (ATLO) concentrates people, tools, and authority while components first exchange real traffic across flight interfaces. Test controllers, EGSE, simulators, flatsats, loaders, and data recorders connect to the same buses and command paths that will exist on orbit. Threat actors exploit this density and dynamism: compromised laptops or transient cyber assets push images and tables; lab networks bridge otherwise separate enclaves; vendor support accounts move software between staging and flight hardware; and “golden” artifacts created or modified in ATLO propagate into the as-flown baseline. Malware can traverse shared storage and scripting environments, ride update/checklist execution, or piggyback on protocol translators and gateways used to stimulate subsystems. Because ATLO often introduces late firmware loads, key/counter initialization, configuration freezes, and full-system rehearsals, a single well-placed change can yield first execution on multiple devices and persist into LEOP.