NASA doesn't know how much its accelerated mission to get Americans on the moon again will cost or where the money will come from, the leader of the space agency told Congress on Wednesday.

"We are not in a position right now to say what that budget number is, or, necessarily, where the administration is interested in that money coming from," NASA Administrator Jim Bridenstine said.

Lawmakers called a hearing to discuss NASA's fiscal 2020 budget request, which was drafted before Vice President Mike Pence called on the agency to return to the moon in 2024, four years earlier than previously planned.

Despite promises to deliver by mid-April, NASA has not yet submitted an updated request to Congress. The agency is expected to ask for a significant funding increase to account for the mission's new timeline.

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"Think of this as, in essence, a surge of funding for the purpose of getting to the moon within the next five years," Bridenstine told a Senate Appropriations subcommittee.

A recent report put the mission's cost as high as $8 billion each year for the next five years. The actual cost of the mission, which will include the first female astronaut to walk on the moon , will be "no where close to that amount," according to Bridenstine.

Bridenstine said that NASA is prepared to meet the new timeline, noting that technological challenges will not be a problem.

But "the longer the program goes, the more difficult it becomes to achieve the end state because of the political risk," he said, noting that priorities and budgets change as administrations turn over.