President Donald Trump signed the 'White House Space Policy Directive 1' on Monday, pressing NASA to put people on the moon for the first time in 45 years.

The directive is a shift from President Obama's plans for NASA, which called for crewed missions onto asteroids by 2025.

Funding for the agency has dropped from nearly 4.5% of the federal budget in 1966 to less than 0.5% today.

President Trump wants to put US astronauts back on the moon for the first time in 45 years.

The White House ‘Space Policy Directive 1,’ which Trump signed on Monday, calls for NASA to work with "private sector partners" to return US astronauts to the surface of the moon.

"This time we will not only plant our flag and leave our footprint," President Trump said, "we will establish a foundation for an eventual mission to Mars, and perhaps, some day, to many worlds beyond.”

The idea of putting people on the moon is a bit of a shift for the space agency. In 2010, President Obama said he wanted NASA to be orbiting Mars by the 2030s, and to send humans to an asteroid by 2025, with crews charting destinations "beyond the moon."

Simply getting back to the moon presents less of a logistical challenge for NASA, as they already sent 12 men to the lunar surface in the late 60s and early 70s.

But where the agency will get the cash to return remains an open question.

NASA told Business Insider that the specifics of when the moon landing will happen and how much it will cost will need to be worked out in discussions about next year's budget, which has to be approved by Congress. Currently, US spending on space as a percentage of total federal dollars is close to an all-time low. The roughly $18-19 billion federal dollars the agency gets every year is less than half of one percent of all federal money. Trump's proposed 2018 budget calls for that total to decrease by roughly $200 million, while Congress has agreed to give the space agency $19.5 billion next year.

Either way, that's drastically less than the agency was working with the last time Americans went to the moon. In 1966, three years before NASA first landed there, the federal government spent nearly 4.5% of the national budget to beat the Russians to the moon. Adjusted for inflation, that number today would be roughly $45 billion — more than twice what the US spends on NASA now. Astronauts say projects often languish for years or get canceled before completion, since NASA funding and goals aren't consistent from year to year or administration to administration.

Astronaut Scott Kelly, who recently spent a year in space, hinted at the problem during a January 2016 Reddit Ask Me Anything session (before President Trump took office). Kelly wrote, "I would like the next president to support a budget that allows us to accomplish the mission that we are asked to perform, whatever that mission may be."

A new space race may be taking shape

Astronaut and geologist Harrison Schmitt — one of the last people to touch down on the moon — visited the White House Monday to support the announcement, as did Buzz Aldrin and Peggy Whitson, who logged a record-breaking 665 days in space.

Since Schmitt's trip in 1972, only the Chinese have visited the lunar surface. The Chinese 'Yutu' rover scooted around on the moon in 2013, sans humans. Despite being banned from inclusion on the International Space Station, China has an ambitious plan to send people to the moon by 2036. The Indian space program is also planning its own unmanned mission to the moon in the first quarter of 2018, on a very skinny budget.

Trump said landing on the moon again will inch Americans closer to the ultimate goal of Martian exploration. He added that such a mission would be good for jobs and military research, too. But another country could easily beat the US space agency at its own game.