On the 20th anniversary of the Apollo 11 moon landing, President George H.W. Bush called for a return to the lunar surface, affirming in 1989 that “it is humanity’s destiny to strive, to seek, to find.”

Since then, other presidents have called for NASA to send astronauts to the moon or Mars with soaring rhetoric that was never matched by the resources or political resolve to make such promises come true.

Now, on the 50th anniversary of the moon landing, it is the Trump administration’s turn. Last month, Vice President Pence echoed John F. Kennedy’s “because-it-is-hard” speech, saying it is “time for us to make the next ‘giant leap’ ” and directing NASA to land humans on the moon within five years “by any means necessary.”

The announcement — which moved up a lunar landing by at least four years — caught many at NASA by surprise and left an agency starved of funding for such missions with a severe case of whiplash, scrambling to figure out how it would meet the latest White House mandate within its reduced budget.

NASA officials also face a major test of their agency’s effectiveness: Is this another empty promise by an administration nostalgic for the triumph of Apollo and looking to make a splash while in office, or can NASA somehow pull off what would be an audacious step just in time for the presidential election?

Already, there are signs that the White House’s plan is running into fierce head winds.

At a hearing Tuesday, Rep. Eddie Bernice Johnson (D-Tex.), the chair of the House Committee on Science, Space and Technology, blasted Pence’s speech for lacking any details of how NASA would achieve what she called a “crash program” or what it would cost.

“We need specifics, not rhetoric,” she said. “Because rhetoric that is not backed up by a concrete plan and believable cost estimates is just hot air. And hot air may be helpful in ballooning, but it won’t get us to the moon or Mars.”

Before Pence’s speech, NASA was hoping to get human beings to the moon by 2028, at the earliest. The White House’s own budget request, released a few weeks ago, was geared toward a crewed lunar landing by the end of the next decade, which many in NASA’s leadership considered a more reasonable goal. But the White House suddenly shifted course and decided that timeline was too long — and would fall outside of Trump’s second term, should he be reelected.

Others, too, had been critical of the 2028 date, including former NASA administrator Michael Griffin, who called a moon visit that late not “worthy of being on the table. Such a date does not demonstrate that the United States is a leader in anything.”

The White House had made clear to NASA Administrator Jim Bridenstine within the last few weeks that it wanted NASA to move faster and tasked him with making it happen by 2024. He told administration officials he could do it. “They’ve been consistent in their desire to accelerate,” he said in a brief interview after he testified before the congressional hearing Tuesday.

But in a sign of how quickly he is having to pivot, his written testimony for the hearing cited the old timeline, vowing to “land humans on the moon within a decade,” not the five years the White House wanted.

During the hearing, however, it became clear NASA is scrambling to accelerate the moon mission. Bridenstine said the plan was “in flux” and would require additional funding. He pledged to come back with an amendment to NASA’s budget request later this month, but he wouldn’t or couldn’t say how much more NASA would need.

“My concern is: What does the plan look like, and what is the reality?” Rep. Kendra Horn (D-Okla.), the chair of the subcommittee on space, said in an interview. She said she was also concerned that “schedule pressure doesn’t overcome safety, and the real issues we’re going to have to address.”

Major technical challenges also lie ahead. The rocket NASA plans to use to launch astronauts to the moon is so far behind, and so far over budget, that last month Bridenstine threatened to sideline it in its first mission in favor of commercial rockets instead.

That touched off a fury in Congress, especially from Sen. Richard C. Shelby (R-Ala.), the chairman of the appropriations committee, and the rest of the Alabama delegation, where NASA’s program office for the rocket, known as the Space Launch System (SLS), is largely based.

Bridenstine quickly backtracked, recommitting to SLS as the best option, and said that Boeing, the rocket’s main contractor, would look to dramatically speed up development.

It’s not just the rocket that needs to get back on track. NASA’s current moon mission plans call for the construction of a station that would orbit the moon on a permanent basis. Astronauts would stop at the station, known as the Gateway, before traveling on special landers to the lunar surface — an ambitious technological advance that didn’t exist when Apollo astronauts landed there a half-century ago after a three-day direct journey.

The problem for NASA is that none of this architecture has been built or is even under contract. And without a budget or assurances from Congress that the program would be funded, many fear that the White House is setting the agency up for yet another letdown.

“It does at first glance appear to be what we’ve been through before, with presidents making a bold speech without it getting backed up with resources,” said Wayne Hale, the former manager of NASA’s space shuttle program who now serves as a consultant. “Where are the resources? We’re waiting to see what gets proposed in the budget.”

Todd Harrison, a defense and space analyst at the Center for Strategic and International Studies, said now that the Trump administration has put its stamp on the program, other White Houses may be quick to discard it.

“Before you know it, there will be a change in administration, and a new NASA administrator will come in and say, ‘Whatever these guys were doing is all wrong.’ That’s what history over the past 30 years has told us is likely to happen,” he said.

There is deep skepticism within NASA, as well. To assuage concerns, Bridenstine held a town hall Monday and took questions from employees. One referenced how different presidents keep pointing NASA to different missions — the moon, then Mars, then back to the moon — while none gets achieved.

“What steps do you plan to take to reduce the programmatic whiplash that keeps us from actually accomplishing any of these grand plans?” one employee asked.

Others, though, find the new sense of urgency invigorating — and just what an aging bureaucracy like NASA needs. Trump reconstituted the National Space Council to set U.S. space policy, and he has pushed for the establishment of a Space Force, a new military branch dedicated to helping the U.S. fight adversaries in space.

Pence has dedicated more time to space than any other top White House official since the Kennedy administration. His passion is real, associates have said, and so is his belief that the agency can land humans on the moon by 2024.

In his March 26 speech, Pence cast the mission as part of a new space race against superpowers such as China and Russia, vying for the water at the moon’s south pole, which could be used not just to sustain human life but also as rocket fuel to push farther into the solar system. Water, as many have said, is the oil of the solar system.

“It’s not just competition against our adversaries,” Pence said. “We’re also racing against our worst enemy: complacency.”

Over the years, critics have lambasted NASA for losing the boldness that defined it during its early days, dampened by a pair of space shuttle disasters that killed 14 astronauts. In 1969, NASA sent men to the moon, 250,000 miles away. Today, its astronauts go to the International Space Station, 250 miles away. And because NASA hasn’t had the ability to fly humans to space since the nation’s space shuttle fleet was retired in 2011, its astronauts fly on Russian rockets at a cost of more than $80 million a seat.

China, meanwhile, has emerged as a rival in space. Earlier this year, it became the first nation to land a spacecraft on the far side of the moon. It plans to send another uncrewed mission to the moon later this year. In the process, Pence said in his speech, China has “revealed their ambition to seize the lunar strategic high ground and become the world’s preeminent space-faring nation."

Part of what’s driving the push to the moon is an unproven dream that a lucrative business could be built by mining it for precious metals just below its surface. Commerce Secretary Wilbur Ross has said that lunar mining could help fuel a trillion-dollar space economy.

Homer Hickam, the author and longtime NASA engineer who serves as an adviser to the space council, said the agency should seize the opportunity.

“It doesn’t happen that often that the vice president, or someone at that level, is so interested in NASA and spaceflight,” he said.