Deep Space Atomic Clock (DSAC) orbital test, illustration. At upper right is the satellite that is scheduled to be launched on 25 June 2019 as part of the US Air Force Space Technology Program (STP-2) mission. At lower left is an expanded view of one of its payloads, the Deep Space Atomic Clock (DSAC), forming part of the General Atomics Orbital Test Bed spacecraft. DSAC is a prototype low-mass atomic clock (using mercury-ion trap technology) that aims to provide accurate timekeeping on spacecraft during future NASA missions. It will be able to keep time to an accuracy of better than 2 nanoseconds. This accuracy is needed for more efficient spacecraft control, independent of signals from Earth. Image published in 2018.