Photos: Atlas 5 rocket aims for International Space Station

In a major shakeup at NASA Headquarters, agency Administrator Jim Bridenstine said Wednesday that Bill Gerstenmaier, the widely respected director of human spaceflight, has been replaced in the midst of an ambitious push to meet the Trump administration’s directive to send astronauts back to the moon within five years.

Juggling simultaneous launch campaigns on both coasts, a team of SpaceX engineers put a Falcon 9 rocket through a customary countdown rehearsal and hold-down firing Tuesday at Vandenberg Air Force Base in California ahead of a scheduled liftoff Sunday.

Two NASA astronauts and an Italian flight engineer took shelter in the space station’s Russian segment Wednesday as mission control analyzed a possible leak of toxic ammonia coolant aboard the complex. Initial data indicated it was likely a false alarm.