High-profile critics fear President Barack Obama's commercial overhaul of human spaceflight is going nowhere and could mark the end of half a century of US supremacy among the stars and planets.

"We will have no American access to, and return from, low Earth orbit and the International Space Station for an unpredictable length of time in the future," Neil Armstrong, the first man to set foot on the Moon, warned lawmakers at a recent hearing.

The end of the space shuttle era has left America's human spaceflight program in an "embarrassing" state, Armstrong said, arguing that NASA needs a stronger vision for the future and should focus on returning humans to the Moon and to the International Space Station.

With the US space shuttle program now mothballed after its last flight in July, the United States is forced to depend on Russia's Soyuz capsules to ferry astronauts to the orbiting research laboratory until at least 2015.

Obama canceled the Constellation program that aimed to return humans to the Moon by 2020 and called on NASA to instead focus on new, deep-space capabilities to carry people to an asteroid by 2025 and Mars by 2030.

NASA is counting on the private sector to develop a shuttle alternative at the least possible cost within the next five years.

But many experts doubt that the firms, most of which have little space experience, can step up to the challenge.

"I don't think any of the ISS partners looks at what we are doing in the US with commercial cargo and crew and feels very confident," Space Policy Institute director Scott Pace told AFP.

"So there is a great gap between the aspirations of the policy and the actual capabilities that exist now."

A ticket on the Soyuz capsules to the ISS costs global space agencies between $50 million and $60 million each.

Former astronaut Eugene Cernan, who commanded the Apollo 17 flight and was the last man to walk on the Moon in 1972, said Constellation has been replaced by a "mission to nowhere" and urged NASA to return to the Moon.

Under intense congressional pressure from both his fellow Democrats and rival Republicans, the White House has agreed to develop sooner than planned a heavy-lift launch vehicle for deep human space exploration dubbed the Space Launch System. But financing and other details remain vague.

NASA is focusing especially on deploying the SLS to explore asteroids around 2025, remaining vague on plans to visit Mars and mute on a return to the Moon.

Worried about the course taken by NASA, Cernan said that "today, we are on a path of decay. We are seeing the book close on five decades of accomplishment as the leader in human space exploration."

Republican Representative Ralph Hall, the chairman of the House Science, Space and Technology Committee, agreed.

"If NASA doesn't move out quickly, more and more of our industrial base, skilled engineers and technicians, and hard-won capabilities are at risk of withering away," Hall said.

The 2012 budget request for human exploration through 2016 is only 38 percent that requested for 2007, or $50 billion less.

"The current administrations view of our nations future in space offers no dream, no vision, no plan, no budget, and no remorse," said former NASA administrator Michael Griffin.

"The resulting turmoil when this is plainly seen by all will, without doubt, further impede progress in human spaceflight, and poses a major risk for this nation."

NASA has consistently rejected such criticism, arguing, like Obama, that the Constellation plan was over budget, behind schedule and lacking in innovation.

Spokesman David Weaver described the vision laid out by the president at the Kennedy Space Center in April 2010 as "bold" and said it would "one day allow the first astronauts to set foot on Mars."