As NASA shifts the focus of its space exploration effort to the moon, the advocates of Mars exploration and settlement have a message for future lunar explorers: Don’t get too comfortable.

“I do think the moon should be included in the plan for human expansion into space,” Robert Zubrin, president of the Mars Society and author of a new book titled “The Case for Space,” told GeekWire. “But we don’t want it to become an obstacle for further human expansion into space.”

Chris Carberry, executive director of Explore Mars, takes a similar stance.

“If we spend years and years and years getting there, and then we decide we’re going to stay there for a long time, it could delay Mars by decades,” he said.

Future Mars exploration will be grabbing a share of the spotlight once more this week at the annual Human to Mars Summit, sponsored by Carberry’s nonprofit group in Washington, D.C. Among the speakers on the agenda are NASA Administrator Jim Bridenstine, other officials from NASA and the European Space Agency who are planning Mars missions, and Paul Wooster, one of the engineers leading SpaceX’s charge to the Red Planet.

The three-day conference will be live-streamed from start to finish, starting at 8:30 a.m. ET (5:30 a.m. PT) on Tuesday.

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Bridenstine has been talking a lot more about the moon than Mars lately. “This time, when we go to the moon, we’re actually going to stay,” he told reporters in February.

Jeff Bezos, who founded Amazon as well as the privately held Blue Origin space venture, has been saying something similar for years — most recently last week, when he laid out his latest roadmap for Blue Moon missions to the lunar surface.

SpaceX founder and CEO Elon Musk is also on the moon bandwagon, even though Mars settlement remains his long-term goal. “We should have a lunar base by now,” Musk said in 2017. “What the hell is going on?”

The moon looms larger thanks to a push from the White House and Vice President Mike Pence, who heads the recently reconstituted National Space Council. The Trump administration’s timetable calls for putting American astronauts on the moon by the end of 2024, which represents a significant speed-up in NASA’s plans.

NASA is still working on an estimate for how much extra an accelerated moon program will cost, and it’s not yet clear how a request for more money will be received by Congress. (Update for 3:20 p.m. ET May 13: President Donald Trump tweeted that an extra $1.6 billion would be sought for the 2020 fiscal year, which is on the low side of what’s thought to be required.)

Why the rush? On one level, it’s an effort to demonstrate America’s continuing dominance in space, amid challenges from China. On another level, planting the U.S. flag on the moon once more would guarantee a historic achievement for what the White House hopes will be President Donald Trump’s second term.

But in the big picture, the moon serves as a proving ground for farther-out space odysseys. The fact that it’s a mere 240,000 miles away, rather than the tens of millions of miles for Mars, reduces the risk and expense for crewed missions.

Mars advocates such as Zubrin and Carberry agree with the argument up to that point. But they’re wary about how NASA intends to execute the plan.

“The question for Trump and Pence is, are they willing to do what has to be done?” Zubrin said.

In Zubrin’s view, NASA’s current approach to moon missions is the wrong way to do it. The mission architecture calls for the construction of an outpost in lunar orbit, known as the Gateway, which would be the base of operations for trips going down to the surface.

Zubrin sees the Gateway as a “Lunar Orbit Tollbooth” that’s an unnecessary waste of money. In his book, he outlines an alternate architecture that uses SpaceX’s Falcon Heavy and Falcon 9 rockets, plus a yet-to-be-designed Lunar Excursion Vehicle, to transport payloads and people to the moon without a layover in lunar orbit.

If it’s adopted, the Moon Direct architecture could arguably eliminate the need for NASA’s heavy-lift rocket, the Space Launch System.

That argument is sure to spark pushback in Congress, which has already funded billions of dollars in development costs for SLS and NASA’s Orion deep-space crew capsule. But the way Zubrin sees it, relying instead on commercial ventures such as SpaceX and Blue Origin is exactly what has to be done.

“The thing that could enable the moon and Mars in the coming decade is this entrepreneurial space revolution,” he said. “NASA has only barely begun to make use of it.”

Officials at NASA have come around to the view that a permanent presence on the moon is a necessary stepping stone in humanity’s outward push. But Carberry and Zubrin aren’t so sure. They worry that the costs of settling the moon and setting up the infrastructure that’s necessary to extract water ice and other resources could siphon away the funds and political will for Mars missions.

More about Moon Direct: Mars maverick lays out low-cost plan for lunar bases

“If we’re building a base on the moon, then I can’t imagine going to Mars anytime soon,” Carberry said.

Zubrin, meanwhile, took aim at NASA’s proposal to send astronauts to Mars via the lunar Gateway, on a future spaceship called the Deep Space Transport.

“That’s not feasible, and furthermore, it’s not even attractive,” he said.

For decades, Zubrin has advocated a Mars Direct plan that, like Moon Direct, calls for a series of robotic and crewed missions to the Red Planet without side trips — and he argues that the idea is getting traction thanks to SpaceX’s efforts.

“Musk wants to go to Mars,” Zubrin noted. “You don’t see him talking about building a lunar orbiting space station to enable his plan. No one who actually wants to go to Mars would insert such requirements into their mission plan.”

For evidence that NASA’s plan needs to go through a paradigm shift, Zubrin points to an independent study from the Institute for Defense Analyses’ Science and Technology Institute, which concludes that the space agency can’t meet a 2033 timetable for sending humans to Mars. A more realistic date would be 2037, the study said.

The study, commissioned by NASA in response to a congressional mandate, attributes the extra delays to the technological risks involved in developing the Deep Space Transport.

Space exploration costs are estimated at more than $217 billion through 2037 — including $120.6 billion specifically for the elements that’d be needed to get to Mars. The rest of the money would go toward development of hardware for Mars surface missions, plus operations in low Earth orbit and on the moon.

Meanwhile, Musk is talking about sending SpaceX’s first crewed mission to Mars in the mid-2020s. Even Musk admits that his timetables are often overly optimistic, but Zubrin says the success of SpaceX and other commercial ventures is likely to force policymakers to “take a second look” at their plans for future giant leaps.

“We’ve had a shot heard round the world here. … This is going to enable human exploration and settlement of the solar system,” he said. “It will enable it sooner if NASA embraces it fully.”