Given how far NASA lies down the food chain of White House priorities, it's always welcome when a president engages in a discussion of space policy. And that's what President Obama did on Tuesday when he authored an op-ed that appeared on CNN.com and called for America to take a "giant leap" by sending humans to Mars in the 2030s.

There wasn't much new in the president's call to action, as it wasn't all that different from a space policy speech he delivered in 2010. The president said then at Kennedy Space Center, "by the mid-2030s, I believe we can send humans to orbit Mars and return them safely to Earth. And a landing on Mars will follow." The similarity of his words, spoken six years apart, gives us a chance to judge his administration's space policy, and the verdict is pretty straightforward: Obama has set NASA and the United States on a course to Mars.

It is an easy thing to say "We are going to Mars," however, and a far more difficult thing to do it. In reality, Obama has put NASA on an unsustainable pathway to Mars given NASA's current resources and approach, and he is leaving the hard work of actually getting to Mars to his successors. In other words, right now, NASA is on a journey to Mars in name only.

Obama sought change early on

Back in 2009, before Obama first articulated a Mars plan, the president's space advisers recognized that NASA's human exploration programs were in trouble. These confidants included Lori Garver who, although she was installed as deputy administrator of NASA, in reality ran the show. Garver and her leadership team felt that NASA's Apollo-like approach toward human exploration, with big government rockets using chemical propulsion, was no longer affordable. So in early 2010 they rolled out a new kind of budget, one that called for the cancellation of the big-ticket programs to build the Ares I and V rockets, as well as the Orion space capsule.

Instead, Garver and others sought to make space exploration sustainable in the 21st century. NASA would eventually need more powerful rockets, Garver and her allies realized, but they didn't have them back in 2010 when NASA had no immediate plans to go beyond low-Earth orbit. Rather, they felt NASA needed reliable, lower-cost vehicles to replace the space shuttle and faster, more efficient propulsion systems. So they continued the commercial crew program initiated by President George W. Bush to replace the shuttle, and they called for investment into new propulsion technologies that would zip astronauts through the Solar System at higher velocity, thereby reducing the risk of health problems due to microgravity. By the end of this decade, when NASA might begin to need big rockets, Obama's advisers believed the private sector—either United Launch Alliance, SpaceX, or someone else—would be ready to build them.

But Congress hated this plan. It would further diminish the roles of the ten NASA field centers and cede more power and money to upstart companies like SpaceX. Congress fought back and forced the president to accede to building the costly Space Launch System rocket as well as continuing development of the Orion space capsule. Congress got its big, Apollo-like space program back.

At this point, in 2010 and 2011, Obama largely retreated from space policy. His advisers battled for commercial crew and Earth science funding with some degree of efficacy, but they largely abandoned efforts at game-changing propulsion and other space technology investments. Congress, meanwhile, assumed control of the nation's human exploration program. By 2013, Garver had left the agency and, feeling more free to speak, characterized Congress' insistence on government-developed rockets as a "socialist" approach toward exploration.

Ars has delved deeply into the flaws with NASA's current approach to Mars exploration, including publication of this feature-length article, Make Mars Great Again. Given these problems and where the space program is today, there are arguably two paths forward to a human Mars exploration program that the next presidential administration could take. That is, if a president really wants humans to walk on Mars in the 2030s, here is how she or he might get there from here. Unfortunately neither path is politically easy, nor likely to be realized.

Hard choice no. 1: Double the budget

The first approach is simple: double NASA's exploration budget. The space agency presently spends a little more than $4 billion annually to develop the Space Launch System rocket, Orion spacecraft, and related ground systems. But if Washington was really serious about sending humans to Mars in the 2030s, it would spend double that to research habitation modules for the trip to Mars, landing systems, surface habitats, surface power systems, redundant life support systems in deep space, radiation mitigation, and so much more. Instead, NASA's costly rocket and spacecraft eat up so much of the budget there is precious little money left over for anything else. When pressed about this, NASA doesn't have good answers.

The next president could find those funds in several places. Most simply, she or he could double the space agency's exploration budget by adding $4 billion to NASA's bottom line. But this seems highly unlikely given that there is no national imperative to explore Mars. It is an aspirational goal, to be sure, but it doesn't advance the cause of beating ISIS or addressing rising health care costs. Another alternative would be to take this money from the International Space Station program, which receives about $4 billion from NASA's annual budget to support the station and deliver supplies and astronauts to the orbiting laboratory. However, NASA has firm commitments to its international partners to fly the station through 2024, and the organization wants to continue the program through 2029.

Absent an infusion of funds, NASA appears likely to plod along and try to make incremental steps toward Mars in the 2020s and 2030s. A blue-ribbon review panel in 2014, led by the National Academies, found in its "Pathways to Exploration" report that such an approach was both risky and unlikely to succeed for a number of reasons, particularly because it would be so hard to sustain such a long-range vision across multiple presidential administrations. Are the next two or three presidents really going to be excited about sending humans to cislunar space (near the Moon) while China is landing humans there?

In short, the only way to have a real Mars program using the SLS rocket and Orion spacecraft is to find $4 billion more or so annually for NASA. Good luck with that, President Clinton or Trump.