NASA has had a big problem since the agency triumphantly landed humans on the Moon nearly half a century ago. Namely, after the Apollo landings delivered a solid US victory in the Cold War, human exploration has no longer aligned with the strategic national interest. In other words, sending humans into space has represented a nice projection of soft power, but it has not been essential to America's domestic and foreign policy aims.

As a result, NASA's share of the federal budget has declined from just shy of five percent at the height of the Apollo program to less than 0.5 percent today. At the same time, NASA's mandate has grown to encompass a broad array of Earth science, planetary science, and other missions that consume more than half of the agency's budget.

With less buying power for human exploration, NASA has had to scale back its ambitions; and as a result, astronauts have not ventured more than a few hundred miles from Earth since 1972. Twice before, presidents have attempted to break free of low-Earth orbit by proposing a human return to the Moon, with eventual missions to Mars. President George H.W. Bush did so with the Space Exploration Initiative in 1989, on the 20th anniversary of the Apollo 11 Moon landing. And George W. Bush did so in 2004, with the Vision for Space Exploration. Neither of these were bad concepts—indeed, both offered bold, ambitious goals for the space agency—but they died due to a lack of commitment and funding.

On Monday, a third president, Donald Trump, announced that NASA will go back to the Moon and eventually send humans to Mars. In signing "Space Policy Directive-1," Trump committed NASA to send humans back to the Moon and then on to Mars. "This is a giant step," the president said during a ceremony that featured two astronauts who had walked on the Moon, Buzz Aldrin and Harrison Schmitt.

Monday's ceremony offered few details about how this policy will be implemented, when humans might go to the Moon, and how NASA will afford it. Presumably, those details will be worked out in the coming months, during future meetings of the National Space Council and in concert with NASA, which is still awaiting Senate confirmation of a new administrator. The White House has nominated Oklahoma Congressman Jim Bridenstine.

What happens next?

There are essentially three courses the National Space Council could take from here. The following is a brief summary of each pathway and where it might lead:

Status quo: This seems the most likely scenario, because it offers the path of least resistance. NASA continues to spend $3 to $4 billion annually to develop the Space Launch System (SLS) rocket and Orion spacecraft, and the agency eventually gets enough funding to build its Deep Space Gateway facility in orbit around the Moon. This probably won't happen until the latter half of the 2020s. Under such a scenario, it is difficult to see NASA or its partner astronauts landing on the Moon before 2030, presuming the next president sticks with this plan. The big winners here would be contractors who support NASA's existing exploration plans, particularly Lockheed Martin and Boeing.

Big investment: Let's say President Trump really wants to send humans to the Moon. This is possible with the SLS rocket and Orion spacecraft—it's just not something NASA can afford right now with the budget limitations mentioned above. If the president were to add, say, $4 billion to NASA's exploration budget annually, the agency could expedite work on the Deep Space Gateway (or cancel it altogether) and move on to building lunar landers and surface habitats. There are private companies, such as Bigelow Aerospace and United Launch Alliance, already working on innovative concepts for human activities on the Moon. But, right now, those activities are beyond the scope of NASA's existing budget.

Go commercial: The use of fixed-price contracting has provided significant savings for NASA as it has sought to service the International Space Station. Companies such as SpaceX, Blue Origin, United Launch Alliance, and more are developing rockets and/or spacecraft capable of operating in and around the Moon for a significantly lower cost than the SLS rocket and Orion spacecraft. If NASA were to build its lunar program with fixed-price commercial contracts instead of developing its own rocket and spacecraft, the agency could probably afford lunar landings within the next decade. But, in all likelihood, the political will doesn't exist to push such a radically commercial program through Congress.

Devil in the details

Frankly, it's too early to pre-judge this announcement. President Trump has been in office for less than a year, and, by all accounts, Vice President Mike Pence has taken his leadership of the National Space Council seriously. His office has met with many of the key constituencies of the US aerospace industry, and Pence has visited each of the three key human spaceflight centers in Florida, Alabama, and Texas.

Space policy is not made overnight. We probably won't know for another six to 12 months whether this administration will make meaningful changes to space policy, or, aside from adding lunar landings on the way to Mars, more or less endorse the status quo. But if Trump is to make any significant changes in space policy, those changes can no longer come before the fiscal year 2019 budget, which would affect only the last two years of his presidency.

In short, unless this administration is taking a second term for granted, time is running out for it to meaningfully change the course of the US spaceflight program. Saying "we're going to the Moon!" is one thing. But if you're not building landers or investing heavily in Moon-centric hardware, the next president could just as easily switch course away from the Moon.