On December 11th, President Trump signed Space Directive #1, formally implementing as policy what Vice President Pence had announced at the first meeting of the National Space Council in October: that NASA will focus its human spaceflight efforts on a return to the Moon, and then onto Mars.
What changed, exactly
The Presidential memorandum replaces a single paragraph in the current national space policy guidelines released in 2010 by President Obama. The original paragraph stated that "The NASA Administrator Shall…":
"Set far-reaching exploration milestones. By 2025, begin crewed missions beyond the moon, including sending humans to an asteroid. By the mid-2030s, send humans to orbit Mars and return them safely to Earth"
Trump's Space Directive #1 replaces it with:
"Lead an innovative and sustainable program of exploration with commercial and international partners to enable human expansion across the solar system and to bring back to Earth new knowledge and opportunities. Beginning with missions beyond low-Earth orbit, the United States will lead the return of humans to the Moon for long-term exploration and utilization, followed by human missions to Mars and other destinations;".
You'll notice a few major changes. First, no more asteroids. An asteroid mission was originally discussed as a means to extend human presence into deep space while avoiding the complexities (and expense) of landing in a large gravity well. Its presence in Obama's space policy was an consequence of the final report issued by the aptly-named Review of U.S. Human Spaceflight Plans Committee of 2009, which highlighted a "flexible path" to Mars via stepwise missions pushing humans deeper into space, including a visit to a near-Earth asteroid.
NASA never really pursued a human mission to a deep-space asteroid, opting instead for a somewhat easier pursuit in line with available resources. When the Obama White House cancelled Constellation, the Moon program developed under George W. Bush, Congress passed the NASA Authorization Act of 2010 that dictated the creation of the Space Launch System rocket and continuation of the Constellation-era Orion crew capsule. These multi-billion dollar programs, combined with the spending sequester that squeezed all federal discretionary spending, severely hampered NASA's ability to develop hardware necessary for a 2025 rendezvous with a deep space asteroid.
To solve this conundrum, the Obama Administration proposed the Asteroid Redirect Mission—bringing an asteroid to lunar orbit to dock with astronauts in Orion—as a way to achieve the literal meaning of the National Space Policy directive. The proposal was never popular with congress or the scientific community, and it limped along until Trump's budget request killed the effort in the Spring of 2017.
That said, the extent of the changes made by the new Space Directive amounts to one paragraph. We have no new information on how the U.S. will "lead the return of humans to the Moon", what budget NASA will have to implement it, and what timeline they have to achieve it (or even, honestly, what "it" means—a lunar base? Mining? A deep space lunar gateway?)
But if this is your point of contention, you're missing the point.
What changed, really
For most of 2017, it was official U.S. policy that astronauts should explore asteroids. Starting in December and going forward for at least three more years, it will be that humans will return to the Moon.
High-level policies like the Space Directive #1 signed by President Trump will have trickle-down implications for the entire government and private civil space sector. The whole apparatus of NASA's human spaceflight bureaucracy will now direct its efforts toward lunar exploration, to the extent they ever departed from it. NASA is finalizing a study to propose ways to achieve this broad policy goal, and the coming 2019 fiscal year budget request for NASA will likely contain prioritized funding related to lunar exploration
In anticipation of this funding, a whole ecosystem of aerospace contractors and subcontractors, private space companies, researchers, and strategic planning entities will orient themselves to these lunar efforts and jockey for ways to provide their services to NASA and others. NASA's technology development roadmaps may be redesigned to better serve immediate needs for lunar exploration vs. long-duration human spaceflight to Mars. International space agencies will propose ways in which they can work with NASA to provide supporting or mission-critical partnerships for moon-related programs.
Whole aspects of NASA's services may end up being reorganized into a human return-to-the-moon effort, while others may fall in priority or disappear altogether. Some programs may get repackaged and rebranded to appeal specifically to this new policy in order to stay relevant to the powers-that-be.
These changes may not be as fundamental as you would think. The Space Launch System and Orion programs can continue as planned and have just as much of a relevancy to this new directive as they did previously. Their initial missions of sending humans around the Moon will satisfy the new space policy just as well as the old one. The Deep Space Gateway concept may fit nicely within a return to the Moon as well.
What to look for
Our first hint of these changes will come in the FY 2019 President's Budget Request, set to be released in February. We will see the first indications of what the White House is willing to put money behind, and see plans for the initial steps of getting humans beyond low-Earth orbit. We will see how serious these plans will be, particularly in the near term, with the new focus on the Moon. And we will see whether Congress supports this new direction for human space flight.
Additionally, I keep going back to the name of this presidential memorandum: Space Directive #1. The numeral right there in the title suggests to me that we may see additional policy declarations in the near future.