WASHINGTON – NASA Administrator Jim Bridenstine said NASA won't be able to meet a 2024 deadline to land astronauts on the moon unless Congress approves the Trump administration's request for an additional $1.6 billion in next year's budget.

"It takes this off the table," Bridenstine told USA TODAY in an exclusive interview Thursday. Without the extra funding, "we're going to be back looking at 2028."

It was the NASA chief's strongest statement to date that the fate of a lunar return five years from now – ballyhooed by President Donald Trump – depends almost entirely on congressional action in the coming months. Humans last walked on the moon in 1972.

In March, Vice President Mike Pence announced an ambitious plan to accelerate a return to the lunar surface from 2028 to 2024, the last year of Trump's presidency if he wins a second term next year. Last month, Bridenstine unveiled a proposal to fund the first year of the program.

As part of the newly dubbed "Artemis" program, the agency proposed raising its original 2020 budget request from $21 billion to $22.6 billion. But the proposal outraged education activists and some Democratic lawmakers because the funding would come from Pell Grants, which help low-income students pay for college.

"We already have students forgoing meals & sleeping in cars because they can't afford the rising costs of college," said California Sen. Kamala Harris, who is running for the Democratic presidential nomination, last month. "This is backwards."

The administration said the transfer wouldn’t affect students currently benefiting from Pell Grants and that the program is expected to have “sufficient discretionary funds until 2023,” according to language in the budget amendment.

The Pell Grant program is running a surplus of nearly $9 billion, according to The Associated Press.

Bridenstine, a former GOP congressman from Oklahoma, has been meeting with lawmakers and was optimistic he'd get bipartisan buy-in by October when the 2020 federal budget year begins.

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As for concerns about using Pell Grant money to help pay for space exploration, the NASA administrator said there are many places to find the money.

"I've been in the House. I've seen how it works," he said. "If people like the program but they don't like where it's being funded from, they can take it from another account."

Congress approved $21.5 billion in the current budget, or about $1.1 billion less than what NASA is seeking for 2020.

Contributing: Chris Quintana