Asif Siddiqi, a historian of science and technology, describes the major decisions that shaped U.S. human spaceflight in the 20th century. (National Academy of Sciences) (National Academy of Sciences)

A sweeping review of NASA’s human spaceflight program has concluded that the agency has an unsustainable and unsafe strategy that will prevent the United States from achieving a human landing on Mars in the foreseeable future.

The 286-page National Research Council report, the culmination of an 18-month, $3.2 million investigation mandated by Congress, says that to continue on the present course under budgets that don’t keep pace with inflation “is to invite failure, disillusionment, and the loss of the longstanding international perception that human spaceflight is something the United States does best.”

The report makes a case for sending astronauts back to the moon. That had been a key element of NASA’s strategy under President George W. Bush. But President Obama and his advisers explicitly opposed another moon landing (“I just have to say pretty bluntly here: We’ve been there before,” Obama said in a speech on space policy in 2010).

A major argument against returning to the moon was that it didn’t pencil out — that there wasn’t nearly enough money dedicated to the program. Now the NRC’s Committee on Human Spaceflight has come to the same conclusion about the Obama administration’s vision for NASA. If the goal is a human landing on Mars, the current strategy won’t work.

“Absent a very fundamental change in the nation’s way of doing business, it is not realistic to believe that we can achieve the consensus goal of reaching Mars,” Mitch Daniels, the former Indiana governor and co-chair of the committee, said Wednesday morning in an interview.

A 2009 committee appointed by Obama urged NASA to keep its options open while investing in spaceflight technology and letting the commercial sector handle routine trips to low Earth orbit. But the NRC reviewers argue that NASA and its international partners should focus on the “horizon goal” of Mars and do whatever it takes to get there, step by step, avoiding changes in strategic direction.

NASA officials, aware that critics see the agency as adrift, say they have already been moving in the direction advocated by the NRC. Their strategy targets Mars, just as the NRC report now demands, they say. “All this work will eventually enable astronaut missions to Mars,” NASA Administrator Charles Bolden said in a recent NASA white paper.

“NASA’s been doing some work, and has been doing some thinking over the last six months, that is in alignment with what the NRC says the top-level goals are,” said Greg Williams, a NASA deputy associate administrator.

But Williams was cool to the suggestion of a return to the moon, saying the airless moon offers little help in developing the kind of descent and landing techniques needed on Mars, which has a thin, troublesome atmosphere.

The NRC committee probed the philosophical question of why we send people into space to begin with. The committee concluded that the purely practical, economic benefits of human spaceflight do not justify the costs but that the aspirational nature of the endeavor may make it worth the effort.

The report says the United States should pursue international collaborations that would include China — currently treated as a space rival and not as a potential partner.

The report cites three potential pathways to Mars, two involving a return to the moon. A lunar landing and habitat would help develop technologies that could later be used on a Mars mission, the report says.

The third pathway is essentially the one that the Obama administration has chosen, which includes the Asteroid Redirect Mission.

The plan, still being studied, would use a robotic spacecraft to grab a small asteroid that is orbiting the sun and passing close to the Earth and then tug the asteroid to a new orbit around the moon. Astronauts would then go to the rock and take samples. The mission architecture would take advantage of big-ticket NASA projects already underway. Most notably, it would provide a destination for the Orion capsule being developed by NASA in tandem with a heavy-lift rocket called the Space Launch System (SLS).

The asteroid mission has been politically controversial — Republicans in Congress unsuccessfully tried last year to forbid NASA to do it — and it has technical challenges, not the least of which is the difficulty identifying an asteroid that could plausibly be captured by a robotic spacecraft.

The NRC report is not bullish on the idea. The report says the mission involves the creation of a large number of “dead end” technologies that don’t get the United States closer to a Mars landing.

The committee raised a safety issue. The current schedule calls for an unmanned SLS launch in 2017, followed by a mission with a crew in 2021. The committee feared that launch teams could become rusty with long lags between missions. The program “cannot provide the flight frequency required to maintain competence and safety,” the report states.

NASA official Williams said Wednesday that the cadence of launches will speed up after 2021.

The committee did not delve deeply into what the private sector, operating commercially, might accomplish independently of the government. There are many space buffs, including SpaceX founder Elon Musk, who have said they want to land on Mars. But committee member John Sommerer said Wednesday that it is unrealistic to expect a commercial company to spend the money and take on the risk necessary to achieve human exploration on the Martian surface.

“You need to develop a very substantial armamentarium of really high-tech stuff to get humans on Mars,” he said. “Mars is very hard.”

John Logsdon, professor emeritus of George Washington University’s Space Policy Institute, said the report has a familiar ring to it.

“They go through all this negative analysis and still conclude we ought to go to Mars. No one ever says, ‘Let’s lower our ambitions.’ It’s always, ‘Increase the budget,’ not ‘Lower ambitions,’ ” he said.

As for going to Mars: “It’s a dream. It’s been a dream forever. And will remain a dream unless something changes.”