After the Columbia space shuttle disaster in 2003, former president George W. Bush decided to phase out the shuttle flights by 2003 and set a more ambitious mandate for America in space.

Launched in 2004, the so-called Constellation program aims to take Americans back to the moon by 2020 to use as a launch pad for manned voyages to Mars.

Without renouncing those objectives, President Barack Obama has named a commission of experts to review the US manned space flight program and make recommendations by the end of August.

The space shuttles, which have carried crews of astronauts into space since 1981, were conceived as reusable vehicles to transport heavy, bulky equipment into Earth's orbit, primarily for the construction of the International Space Station.

But the shuttle has kept the United States stuck in a low orbit for too long at a time when other countries like China are emerging as rivals in space, argues Michael Griffin, the former NASA chief who championed the Constellation program.

"I think we must return to the moon because it's the next step. It's a few days from home," he said. "Mars is only a few months from Earth."

In unveiling the Constellation program in 2004 to the Congress, Griffin said: "The single overarching goal of human space flight is the human settlement of the solar system, and eventually beyond."

"In the long run, human populations must diversify if it wishes to survive," he said in an interview with AFP last year.

But NASA's budget is not big enough to cover the cost of Constellation's Orion capsule, a more advanced and spacious version of the Apollo lunar module, and the Ares 1 and Ares V launchers needed to put it in orbit.

Constellation is projected to cost about 150 billion dollars, but estimates for the Ares 1 have skyrocketed from 26 billion dollars in 2006 to 44 billion dollars last year.

With a space exploration budget of six billion dollars in 2009, Senator Bill Nelson of Florida said: "NASA simply can't do the job it's been given -- the president's goal of being on the moon by 2020."

Nelson, a former astronaut, deplored that between 2020 and 2015 the United States will have no way of transporting its astronauts to the ISS except aboard Russian Soyuz space craft.

Meanwhile, a group of active and retired NASA engineers, who are critical of the Constellation project, have been working in their spare time on a parallel project dubbed Jupiter Direct.

It envisions using the Orion capsule but replacing the Ares launchers with a family of launchers with common components based on existing shuttle technology.

Proposals presented to Obama's commission on human space flight estimate Jupiter's cost at 14 billion dollars, half the original estimate for the Ares 1.

The commission chairman, respected former Lockheed Martin chief executive Norman Augustine, said it comes down to money.

"With a few exceptions, we have the technology or the knowledge that we could go to Mars if we wanted with humans. We could put a telescope on the moon if we wanted," he said.

"The technology is by and large there. It boils down to what can we afford?"