The White House is asking Congress for an additional $1.6 billion for NASA’s budget next year as the space agency attempts to return humans to the moon by 2024.

The announcement comes about six weeks after Vice President Pence called for an accelerated program to return humans to the lunar surface for the first time since the last Apollo lunar landing in 1972. But since the White House issued that bold mandate, NASA has released few details about how it would achieve it or what the program would cost.

In a tweet Monday evening, President Trump wrote: “Under my Administration, we are restoring @NASA to greatness and we are going back to the Moon, then Mars. I am updating my budget to include an additional $1.6 billion so that we can return to Space in a BIG WAY!”

The money would come from a surplus in Pell Grant money, a federal program used to help students pay for college, according to the Associated Press.

It was unclear what the reaction in Congress would be. Democrats in the House have been particularly critical of the White House’s plan, calling it a political gambit timed to align with the elections calendar.

“The lack of planning evident so far is no way to run our nation’s human space exploration program,” Rep. Kendra Horn (D-Okla.), chairwoman of a subcommittee that oversees NASA, said at a recent hearing. “The 2024 missive left NASA in a tizzy — scrambling to develop a plan and hastening to pull together a budget amendment that still have not been delivered to Congress; and upending groundwork with international partners on future exploration goals.”

Pence’s announcement took many inside NASA by surprise. The agency had been aiming to get humans to the lunar surface by 2028. And in March, the White House submitted a budget request totaling just over $21 billion for NASA, a nearly $500 million cut from what the agency received this year.

Since the White House called for NASA to speed up the timeline, the space agency has been scrambling to figure out how it could pull off such a complicated mission in a short amount of time. At the moment, NASA does not have the ability to fly astronauts anywhere in space. Instead, it pays Russia more than $80 million a seat to fly its astronauts to the International Space Station.

As part of the amended budget request, the White House included $1 billion to begin the development of a lander capable of getting astronauts to the surface of the moon. An additional $651 million would go to help speed development of NASA’s massive Space Launch System (SLS) rocket, built largely by Boeing, and the Orion spacecraft, built by Lockheed Martin.

It would also boost technologies to help the agency explore the lunar poles with robots ahead of a human mission, using solar energy as a propulsion source and converting ice found under the moon’s crust into water.

Use of the Pell Grant money to fund the moon mission, dubbed Aretmis, would "cut any spending for for Pell Grant programs as the budget continues to ensure all students will get their full Pell Grant and keeps the program on sound fiscal footing,” Office of Management and Budget spokesman Wesley Denton said in a statement, according to the AP.

After Trump’s announcement Monday, NASA Administrator Jim Bridenstine tweeted: “This is the boost @NASA needs to move forward with putting the next man and the first woman on the Moon.”

But NASA officials warned that the agency would need additional money in the future to meet the goal.

“This additional investment is a down payment on NASA’s effort to land humans on the moon by 2024,” Bridenstine said in a call with reporters Monday evening. “In the coming years, we will need additional funds. This is a good amount that gets us out of the gate in a very strong fashion and sets us up for the future.”

Instead of going directly to the surface of the moon, as it did during the Apollo missions of the late 1960s and early ’70s, NASA is instead looking to build a more permanent presence by building an orbiting outpost known as the Gateway.

While that is a more ambitious endeavor, Bridenstine has said the agency wants to go back to the moon — this time to stay. Still, to meet the five-year goal, NASA has said it would scale back the scope of the Gateway, at least initially. To accommodate getting humans to the surface quickly, the budget amendment proposes cutting $321 million from that project and shifting “potential Gateway capabilities into the future.”

It is not clear what the reaction in Congress would be. Many would likely bristle against the notion that a popular student aide program is being used. Bridenstine said that he had been contacting key congressional leaders but that it was “not easy to get a hold of everyone” on short notice.

“We will see various reactions” from Congress, he said. “But I will tell you, when we talk about what NASA is trying to achieve, there is a lot of excitement.”

NASA plans to launch its massive SLS rocket for the first time as early as next year, without astronauts. That would be followed by another mission, with astronauts in the Orion crew capsule, for a trip around the moon. The third mission, to come in 2024, would send a pair of astronauts, one woman and one man, to the Gateway and then to the lunar surface.

Shortly after Pence’s announcement, in which he said astronauts would get to the lunar surface “by any means necessary,” there was talk that the White House would ask for a huge amount of additional funding, as much as $8 billion a year.

But at a recent congressional hearing, Bridenstine disputed that.

The White House’s amended budget request comes a week after Jeff Bezos unveiled a lunar lander that his space company, Blue Origin, has pitched NASA to get cargo and eventually people to the lunar surface. Many other companies, such as Boeing, Lockheed Martin and the Sierra Nevada Corporation, are also vying to build various parts of the Gateway. (Bezos owns The Washington Post.)