In an interview, the 73-year-old founder of the Mars Society delivered a blistering critique, accusing the world's richest person of undermining the mission through divisive politics and a bleak vision of the Red Planet as an escape from Earth rather than a journey of hope.

"On one level, he's absolutely instrumental in opening up this opportunity to get humans to Mars, both through the development of Starship and also the inspiration that has caused," Zubrin told AFP, referring to Musk's prototype rocket.

"But for it to succeed, it has to go beyond these -- this initiative cannot be seen as a Musk hobbyhorse or a Trump hobbyhorse -- it must be seen, at a minimum, as America's program, or preferably the Free World's program."

Zubrin's 1996 book "The Case for Mars," since updated numerous times, laid out a practical blueprint for reaching and settling the Red Planet using existing technologies and local resources -- with the ultimate goal of transforming the atmosphere for long-term human habitation.

- Supporter turned critic -

The book won praise from Musk himself, who once posed with Zubrin at SpaceX's Starship facility in Texas and called it "worth reading."

But today, Zubrin -- who co-authored the Mars Direct plan in 1990, has published hundreds of papers, and invented several advanced propulsion concepts -- sees troubling signs.

While he described Musk as a "tremendously talented and forceful person," he said his success has bred "hubris and arrogance," comparing him to Napoleon as he thumped his fist for emphasis.

He was especially critical of Musk's embrace of Donald Trump during the 2024 election and his role as the administration's chainsaw-wielding cost slasher.

"This combination of Trump and Musk is not going to persist forever," Zubrin warned, in an interview conducted before the pair's relationship imploded Thursday in a spectacular public row.

"And if this program is identified as their deal, it will be crushed as soon as opposing forces have sufficient power."

During their fight Thursday, Trump called Musk "crazy" threatened to terminate his government contracts worth billions of dollars.

Zubrin also condemned Trump's efforts to gut NASA's space science budget -- a move he sees as fundamentally at odds with the exploratory spirit of the Mars endeavor.

The Mars Sample Return mission -- aimed at retrieving specimens collected by the Perseverance rover -- is among the biggest science projects on the chopping block.

Although the mission, developed with the European Space Agency, has suffered delays and budget overruns, Zubrin said eliminating it entirely rather than reforming it would be a mistake.

"This threatens to brand this program with the mark of Cain of original sin -- that this program is born with the blood of the murder of Space Science on it."

- Creative outpost -

Where Zubrin still sees promise is in Starship -- Musk's massive prototype rocket aimed at making life multiplanetary, though the vessel's repeated test explosions show there's a long way to go.

He diverges with Musk over how it should be used. Starship is far too large to serve as a Mars ascent vehicle, Zubrin said.

The Mars expert has proposed a vessel he calls Starboat -- a compact lander that could shuttle between planetary surfaces and orbit, using a fraction of the propellant and surface power.

But his sharpest critiques are philosophical.

He rejects Musk's portrayal of Mars as a refuge from a dying Earth -- a vision that echoes the works of science fiction writer Isaac Asimov.

"We're not going to Mars out of despair," Zubrin said. "We're going to Mars out of hope... to establish new branches of human civilization which will add their creative capacity to that of humanity as a whole."

He sees Mars not as refuge but renewal, where a campaign beginning with robotic missions in the late 2020s and culminating in human landings by 2033 could inspire bipartisan support, showcase American ingenuity and restore national purpose.

"If we do the kind of program that I advocated... we will once again, as we did in Apollo, astonish the world with what free people can do," he said. "We'll make it clear that freedom, not authoritarianism, is the future of the human race."

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