In a surprising tweet, President Trump said NASA "should [not] be talking about going to the Moon," but should focus on "much bigger things."

"For all of the money we are spending, NASA should NOT be talking about going to the Moon - We did that 50 years ago," Trump wrote in a tweet aboard Air Force One. "They should be focused on the much bigger things we are doing, including Mars (of which the Moon is a part), Defense and Science!"

It's unclear what prompted Trump's tweet and exactly what he meant when he said the Moon is a part of Mars. In May, Trump tweeted that under his administration, NASA would return to the Moon and ultimately, Mars, in an effort to "return to space in a big way!"

CURIOSITY ROVER SPOTS 'SHINY' OBJECT ON MARS AND NASA ISNT' SURE WHAT IT IS

The White House and NASA have not yet responded to requests for comment for this story.

NASA, for its part, has made several discoveries since its Curiosity rover landed on Mars in August 2012, including, most recently, evidence of an ancient lake. In November, Curiosity was joined on the Red Planet by NASA’s Insight Mars Lander.

In November 2018, NASA announced that it has selected the location where its Mars 2020 rover will land on the Red Planet. The rover is expected to land on Mars Feb. 18, 2021.

Return to the Moon

In April, Vice President Pence said that the U.S. would return to the Moon by 2024, with or without the help of NASA, as part of the Trump's administration directive to focus on space exploration and defense.

"It's time to redouble our effort," Pence said during a meeting of the National Space Council in Huntsville, Ala. "It can happen, but it will not happen unless we increase the pace."

Pence warned that if NASA can't put astronauts on the moon by 2024, "we need to change the organization, not the mission." He cautioned that the space agency needed to transform into a more efficient organization, or else it would be replaced by private industry.

“We’re not committed to one contractor. If our current contractors can’t meet this objective, then we’ll find ones that will. If American industry can provide critical commercial services without government development then we’ll buy them. And if commercial rockets are the only way to get American astronauts to the moon in the next five years, then commercial rockets it will be," Pence said.

NASA Administrator Jim Bridenstine assured Pence that NASA would work hard to meet the deadline, expressing confidence that the SLS, or Space Launch System, would be ready for the job.

The government space agency, which announced plans to commercialize the International Space Station on Friday in an effort to focus its efforts and resources on deep space exploration, including the Moon and Mars, has been under pressure to return to the Moon.

In recent months, Bridenstine and NASA have been grilled by Congress to know why their plan to return to the Moon by 2024 is not ready yet. The space agency has said it is continuing to work with the Trump administration on a budget request that would make that happen, although no official figures have been publicly discussed.

Last month, the U.S. Government Accountability Office (GAO) reported that NASA’s major projects are more than 27 percent over baseline costs and the average launch delay is 13 months. That’s the largest schedule delay since the GAO began assessing NASA’s major projects 10 years ago.

The still-in-development James Webb Space Telescope is the major offender. The projected launch date for this advanced successor to the Hubble Space Telescope is now 2021, with an estimated $9.6 billion price tag, the GAO noted. Its original target launch date was 2007, with initial cost estimates as low as $1 billion.

NASA’s yet-to-fly mega rocket, the Space Launch System, also faces big cost overruns because of production challenges and, likely, even more launch delays.

On the bright side, the Parker Solar Probe, looping ever closer around the sun, launched on time last summer and came in millions under budget.

Fox News' Bradford Betz and The Associated Press contributed to this report.

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