Ares is meant to be the rocket that will launch a new era of lunar exploration. Instead it is in danger of crashing into its own launch tower or of shaking its astronauts to death.

Nasa has strongly defended the $20billion (£12billion) back-to-the-Moon programme after claims from its own engineers that its rocket design could be dangerously flawed. One senior engineer resigned from his post, complaining of “catastrophic-level risks”, while others are moonlighting on a rival design project, codenamed Jupiter, convinced that they can get man to the Moon quicker, safer and more cheaply than the apparently troubled Ares.

“Nasa has a big reality check coming and I can't begin to guess how it will all turn out,” Jeff Finckenor, a structural design engineer at the Nasa Marshall Space Flight Centre in Alabama, said in a memo to colleagues explaining his departure.

The space agency admits that in certain conditions Ares could blast off into its own launch tower, and that other potential problems include the rocket vibrating so violently that its astronauts could die before they reach orbit.

Managers say that the problems are “growing pains” to be expected during the developmental stage and that they can be fixed in time to get America back into space by 2014 and then on to the Moon by 2020.

The agency has turned to technicians who worked on the Apollo/Saturn programme that put man on the Moon in the 1960s to inject some fighting spirit. “These are really exciting times,” Steve Cook, Ares project manager, said. “The workforce we have today has never experienced anything quite like this before ... because of this, we have brought in some of the veterans of the Apollo/Saturn era who are just giants at this.”

Apollo represented Nasa's golden era of spaceflight, setting Neil Armstrong and Buzz Aldrin on the Moon in 1969, followed by ten more astronauts over the next three years, At the time, Nasa was driven by President John F. Kennedy's desire to beat the Soviet Union in the space race, set out in a 1961 speech in which he told America: “We choose to go to the Moon in this decade and do the other things, not because they are easy but because they are hard.”

Work is under way to launch Ares 1-X, a prototype, from Florida in July. Once perfected, the rocket will be topped with a crew-carrying capsule called Orion — also under development — which is intended to carry astronauts to the International Space Station about 250 miles (400km) above Earth, and on longer-haul trips to the Moon and even to Mars.

Some people believe that the programme, which is known collectively as Constellation and began in 2005, needs to go back to the drawing board. “With catastrophic-level risks accumulating across the programme and a steadfast refusal to accept reality, it's become clear to me that as bad as things are they are going to have to get a whole lot worse before the pieces can be picked up,” Mr Finckenor said.

Jeff Hanley, manager of the Constellation project, said that experts who worked on Apollo and the space shuttle “know what it took to get something done”.

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