WASHINGTON – President Donald Trump used an Oval Office photo op Friday commemorating the 50th anniversary of the Apollo 11 lunar landing to grouse about NASA Administrator Jim Bridenstine and question the space agency's latest efforts to go back to the moon.

As Apollo 11 astronauts Buzz Aldrin and Michael Collins stood next to the president's desk, Trump turned to Bridenstine and noted the former GOP Oklahoma congressman (who supported Texas Sen. Ted Cruz for president in 2016) wasn't always supportive of him.

"It was not that easy of a decision for me, but that's OK," the president said, referring to Bridenstine's nomination in 2017. "You know what I'm talking about."

A bit later, Trump expressed confidence in his NASA administrator.

"You've done a fantastic job," Trump said, a view that was seconded by Vice President Mike Pence as he stood nearby.

Pence, who chairs the National Space Council, was instrumental in defending the choice of Bridenstine during a months-long nomination fight that ended in May 2018 when the Senate narrowly confirmed him.

"Sir, I appreciate that," Bridenstine responded to Trump.

Later, Trump appeared to question NASA's decision to send humans back to the moon by 2024 – a mission Trump has both touted and trashed – that would serve as a stepping stone for an astronaut mission to Mars.

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At one point, the president turned to Aldrin, a longtime friend and the second person to walk on the lunar surface, and asked him for an assessment of the space program. Aldrin said it was deflating to watch U.S. dominance in space slipping away after the tremendous gains NASA made during the 1960s.

"We have the number one rocket in the U.S., and we have the number one rocket. And they cannot get into lunar orbit with significant maneuver capability," he told Trump. "That's a great disappointment to me."

The president immediately turned to Bridenstine.

"How do you feel about that, Jim?" he asked.

“We’re working on it,” the NASA administrator responded, noting that work is being done now on the Orion crew capsule.

"Well, I'd like to have you also listen to the other side," Trump told Bridenstein. "Because some people would like to do it a different way. So you'll listen to Buzz and some of the other people. I know this has been going on for a little while, and we're so advanced, but I would like (you) to listen to other side, OK?"

"Yes, sir," Bridenstine said.