The spacecraft used a small nuclear generator for power and scientists have speculated that it would be capable of travelling at up to 54,000mph — three times the speed of conventional craft — although Nasa officials said yesterday that this was still theoretical.

The generator would not power the craft’s initial take-off — this would still require conventional rockets — but would be switched on after it left the Earth’s atmosphere.

As well as providing electricity for propulsion, the generator would power on-board scientific experiments and communications with Houston.

The development of space-based nuclear power is unlikely to go down well with other nations, who may fear that the United States will use the project — unofficially named Prometheus, after the mythological Greek figure who stole fire from the gods — for military purposes. China is the most likely to retaliate with its own space-based nuclear programme, which would prompt comparisons with the space race between the US and the Soviet Union during the Cold War.

Scientists are excited about the prospect of nuclear- powered space travel. It could lead not only to human missions to Mars but also to the construction of a permanent lunar base and the exploration of Jupiter’s icy satellite, Europa. It would also allow an unmanned craft to be sent beyond the Earth’s solar system, in a mission that could last for more than a decade.

Nasa also hopes that the ambitious project will inspire American schoolchildren to take a greater interest in science and technology. The US is already beginning to suffer from a shortage of young engineers.

Project Prometheus will involve dusting down a programme that many regarded as a relic of the Cold War. It was last backed by President Bush Sr more than a decade ago, before the first Gulf War. It was dropped after opposition from Congress and a lack of public interest.

It is not clear how the American public will react this time, especially given the recession in the United States and the estimated $200 billion (£125 billion) cost of going to war with Iraq. The state of the economy, however, has not always been a factor in big Nasa space projects. Analysts have pointed out that President Nixon started the Space Shuttle programme during a recession.

With this in mind, Nasa is thought to be pushing for a significant increase in funding for Project Prometheus, which has already been allocated $1 billion (£620 million) of funding for the next five years.

Speculation that the President will make an announcement on Nasa funding during his State of the Union address to the American people on January 28 was being played down yesterday.

“We’re talking about doing something on a very aggressive schedule to not only develop the capability for nuclear propulsion and power generation but to have a mission using the new technology within this decade,” Sean O’Keefe, the Nasa administrator, said.

The move is a significant shift in strategy for Nasa, which, since the end of the space race with the former Soviet Union, has been forced to request funding on a mission-by-mission basis. Now it wants the freedom to develop so-called “enabling technologies” that could be put to any number of future uses.

“The laws of physics are the only things controlling how fast we go anywhere, what we do and whether we can survive the experience,” Mr O’Keefe said. “So until we beat the technical limitations, you basically end up arguing about fantasy missions. We’ve been restricted to the same speed for 40 years. With the new technology, where we go next will only be limited by our imaginations.”

Nuclear-powered spacecraft have long been seen as the only serious way to give manned spacecraft enough power to explore beyond the Moon. Existing spacecraft use the momentum of the launch to simply “coast” through space.

Even before that, however, the United States had spent billions trying to build a space-based nuclear reactor and several prototypes had been tested in the Nevada desert. One of the reactors was launched in 1965, operated for only 43 days and remains in orbit to this day. The nuclear project was eventually discontinued.

There are still concerns over the safety of using a nuclear-powered craft in space. The protection of astronauts would have to be improved, for example.

Astronauts return from space with a 30 per cent decrease in muscle mass and a 10 per cent loss of bone mass. The radiation in a spacecraft is the equivalent of getting eight chest X-rays every day.

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