STORY: "I think it's just illustrative that this is a very complicated machine, a very complicated system, and all those things have to work," Nelson said in remarks broadcast by NASA shortly after the launch was scrubbed.

"You don't want to light the candle until it's ready to go," he added.

The countdown was halted about 40 minutes before launch time as the 32-story-tall, two-stage Space Launch System (SLS) rocket and its Orion crew capsule awaited liftoff from the Kennedy Space Center in Cape Canaveral, Florida.

The U.S. space agency cited a problem on one of the rocket's main engines, after launch teams had begun filling its core fuel tanks with super-cooled liquid oxygen and hydrogen propellants. Mission engineers struggled to properly condition that engine to the right temperature for launch, the agency said.

NASA did not give a new launch date but said its first available backup launch opportunity was set for Friday, Sept. 2.

In announcing the setback, Nelson recalled his experience as a former astronaut aboard the Space Shuttle program.

"We scrubbed four times on the pad. And the fifth try was a flawless mission. We know, had we launched on any one of those scrubs, it wouldn't have been a good day... This is just part of the space business," he said.