NASA's new megarocket will launch without astronauts on its test flight, now delayed until 2019 because of technical challenges, the space agency's top leaders said Friday.

In February at the request of the White House, NASA began studying the possibility of putting a crew on the first flight. After reviewing the costs and risks, the space agency decided against it. The White House took part in the decision, said NASA's Acting Administrator Robert Lightfoot.

Still in development, the super-sized Space Launch rocket is meant to eventually send astronauts to Mars. The first launch had been targeted for next year, but now won't happen until 2019 when NASA will send an unoccupied Orion capsule to the vicinity of the moon for a three-week trial run.

The second flight will have astronauts on board. It is scheduled for 2021, but will almost certainly be later because of all the delays with the first flight.

It would have cost a lot more money and time to add life-support systems and other human-required equipment for the test flight, said Bill Gerstenmaier, head of NASA's human explorations and operations.

"The best plan we have is actually the plan we're on right now," he said.