The leader of the world’s pre-eminent space and military power is always in position to make a mark on space; the question is whether that person has any desire to do so. In the case of U.S. President Barack Obama, the desire is clearly there, and thus he tops the list of 10 difference-makers for the second year in a row.

Shortly after taking office last year, the Obama administration moved to end two Pentagon programs — the Transformational Satellite Communications System and Kinetic Energy Interceptor — as part of a broader effort, now getting under way in earnest, to rein in the defense spending of Obama’s predecessor, former President George W. Bush. But while it was clear a year ago that Obama had reservations about Bush’s vision for NASA’s human spaceflight program, just what he was planning to do about it was still a matter of speculation — until his 2011 budget request was unveiled Feb. 1.

Many who fully expected to see changes for NASA in that request were nonetheless surprised at their magnitude: The White House not only proposed to rely on commercial-like services to ferry astronauts to and from the space station, it also targeted the entire Moon-bound Constellation program, including the Orion exploration vehicle, for termination and sought to defer work on a heavy-lift launcher in favor of developing technologies that would change the economics of exploring deep space.

Not surprisingly, Obama’s plan touched off a debate as fierce and impassioned as anything ever seen in the space community, pitting entrenched industrial interests and traditionally minded exploration advocates against those who saw opportunity in change. Obama backpedaled in April, announcing plans to retain Orion for crew rescue purposes and to select a design for a heavy-lift rocket within five years. NASA’s congressional overseers were not mollified, however; they drafted bills providing varying degrees of support to the commercial crew taxi plan but insisting that work begin immediately on a heavy-lift vehicle that takes advantage of space shuttle infrastructure and work done to date on Constellation.

How this will all be resolved remains to be seen, as Congress has yet to finalize its NASA policy and spending legislation. But there can be little doubt that the controversy surrounding the president’s human spaceflight plan has been the dominant feature of the space policy landscape since early 2010.

This is not to say that human spaceflight is the only activity to undergo dramatic change in the last year as a result of actions by the Obama administration. In missile defense, for example, the White House last September scrapped plans for a European shield featuring 10 interceptors located in Poland — these were to be two-stage versions of the interceptors installed on U.S. territory — and radar located in the Czech Republic. The new plan, called the Phased Adaptive Approach, relies initially on the existing Aegis sea-based missile defense system and later will feature ground-based variants of the Standard Missile 3 interceptor.

Another major change, which was unveiled with the 2011 budget request, was the dismantling of the National Polar-orbiting Environmental Satellite System (NPOESS), a merger of U.S. civil and military weather satellite systems that was mandated by the administration of then-U.S. President Bill Clinton back in 1994. Intended to save money by combining seemingly redundant capabilities, NPOESS was done in by technical difficulties compounded by an ineffective management structure involving three federal agencies. The program was restructured in 2006 due to severe cost growth and extensive delays, but the problems persisted, prompting this year’s White House directive to revert to separate satellites for civil and military weather forecasting.

Obama was elected to the White House on a promise of change. In the space industry, for better or for worse — this all seems to depend on one’s perspective — he has more than delivered.