Video: Orion flight preps

The mobile service tower is retracted from the Delta 4-Heavy rocket and Orion spacecraft for Exploration Flight Test 1.

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 Trump administration is adding an additional $1.6 billion to NASA’s $21 billion 2020 budget request to kick start plans to return American astronauts to the moon in 2024, four years earlier than previously planned, NASA announced Monday.