For the last five years or so, NASA has sold the public on a Journey to Mars, a grand voyage by which the agency will land humans on the red planet during the 2030s. With just budgetary increases for inflation, the agency said, it had the resources for humanity's next great step, to land crews safely on Mars, and to bring them home. The agency's new rocket, the Space Launch System, and spacecraft, Orion, were sold by NASA administrator Charles Bolden as the vehicles that would get the job done.

There were plenty of naysayers. For example, a National Research Council report cautioned that the agency had too much work, and too little funds, to accomplish these goals in the 2030s with the SLS rocket—and that sustaining a "Mars program" into the 2040s would be a tremendous challenge. NASA's remarkable response to this critical report was that it validated the Journey to Mars.

Can't land

Now, finally, the agency appears to have bended toward reality. During a propulsion meeting of the American Institute for Aeronautics and Astronautics on Wednesday, NASA's chief of human spaceflight acknowledged that the agency doesn't really have the funding it needs to reach Mars (see video).

"I can't put a date on humans on Mars, and the reason really is the other piece is, at the budget levels we described, this roughly 2 percent increase, we don’t have the surface systems available for Mars," said NASA's William H. Gerstenmaier, responding to a question about when NASA will send humans to the surface of Mars. "And that entry, descent and landing is a huge challenge for us for Mars."

This seems like a fairly common sense statement, but it's something that NASA officials have largely glossed over—at least in public—during the agency's promotion of a Journey to Mars. The reality is that the SLS rocket and Orion spacecraft have cost a lot to build, and therefore NASA hasn't been able to begin designing vehicles to land on Mars or ascend from the surface.

Agency officials have also been loath to mention the possibility of NASA astronauts landing on the Moon, because George W. Bush had an initiative to return to the Moon that President Obama canceled. However, Gerstenmaier opened the door to this possibility Wednesday.

Maybe the Moon?

"If we find out there’s water on the Moon, and we want to do more extensive operations on the Moon to go explore that, we have the ability with Deep Space Gateway to support an extensive Moon surface program," he said. "If we want to stay focused more toward Mars we can keep that." Further Reading Why we’re going back to the Moon—with or without NASA

It has been a long time since NASA, especially its chief human spaceflight official, talked openly about an "extensive Moon surface program." However after six months of a new presidential administration, the agency realizes that its destination may well change. Therefore its leadership is keeping the decision about destinations open, be it the surface of the Moon or Mars.

The reality is that NASA may not be able to go either place unless something changes. The agency doesn't have the funding to build a large lunar outpost if it must rely on the Space Launch System—which will only fly about once a year, at a cost of more than $1 billion. Mars landings, clearly, would cost even more with the big, expendable rocket approach requiring five or more launches per mission.

Another, less costly option is having the freedom to rely much more heavily on partly or completely reusable launch and in-space transports systems being built by SpaceX, Blue Origin, and United Launch Alliance. Politically, so far any reliance on commercial companies for deep space exploration has been a non-starter in Congress. But that could change, as Vice President Mike Pence has been making some noise about increasing commercial partnerships at NASA. "The truth is that American business is on the cutting edge of space technology," he recently said.