Snapshots from Atlas 5 launch

A critical test firing Saturday of the four main engines powering the first stage of NASA’s gargantuan Space Launch System moon rocket is the final major hurdle before the fully assembled booster’s costly, oft-delayed launch late this year on an unpiloted test flight.

The first piloted test flight of Boeing’s CST-100 Starliner commercial crew capsule has been delayed to early 2018 as designers wrestle with the mass of the spacecraft, aerodynamic loads the vehicle will see during launch, and new software requirements levied by NASA, company officials said.

The step-by-step stacking of United Launch Alliance’s final Delta 2 rocket was completed in June at Vandenberg Air Force Base in California for liftoff Sept. 15 with an Earth science satellite for NASA.