Photos: Predawn launch of Atlas 5

The countdown for a planned launch of a SpaceX Falcon 9 rocket from NASA’s Kennedy Space Center in Florida automatically aborted in the last second before liftoff Sunday after an on-board computer detected unexpected data during an engine power check.

Near the end of a ground-shaking test firing of Northrop Grumman’s solid-propellant Castor 600 rocket motor — the first stage of a new booster being designed to launch spy satellites and other national security payloads — a major part of the rocket’s big nozzle appeared to break apart, littering the test site with shards of debris.

SpaceX is preparing for liftoff Tuesday of its first mission for the U.S. Space Force, a launch from Cape Canaveral that will deploy a new GPS navigation satellite using a redesigned profile to allow the Falcon 9 rocket’s first stage booster to reserve enough propellant for landing on SpaceX’s drone ship in the Atlantic Ocean.