The US Air Force developed a top-secret plan to detonate a nuclear bomb on the moon as a display of military might at the height of the Cold War.

In an exclusive interview with The Observer, Dr Leonard Reiffel, 73, the physicist who fronted the project in the late Fifties at the US military-backed Armour Research Foundation, revealed America's extraordinary lunar plan.

'It was clear the main aim of the proposed detonation was a PR exercise and a show of one-upmanship. The Air Force wanted a mushroom cloud so large it would be visible on earth,' he said yesterday. 'The US was lagging behind in the space race.'

'The explosion would obviously be best on the dark side of the moon and the theory was that if the bomb exploded on the edge of the moon, the mushroom cloud would be illuminated by the sun.' The bomb would have been at least as large as the one used on Hiroshima at the end of World War II.

'I made it clear at the time there would be a huge cost to science of destroying a pristine lunar environment, but the US Air Force were mainly concerned about how the nuclear explosion would play on earth,' said Reiffel.

Although he believes the blast would have had little environmental impact on Earth, its crater may have ruined the face of the 'man in the moon'.

Reiffel would not reveal how the explosion would have taken place. But he confirmed it was 'certainly technically feasible' and that at the time an intercontinental ballistic nuclear missile would have been capable of hitting a target on the moon with an accuracy of within two miles.

Reiffel was approached by senior US Air Force officers in 1958, who asked him to 'fast-track' a project to investigate the visibility and effects of a nuclear explosion on the moon. The top-secret Project A119, was entitled 'A Study of Lunar Research Flights'.

'Had the project been made public there would have been an outcry,' said Reiffel.

Many Cold War documents are still classified in the US, but details of Project A119 emerged after a biography of celebrated US scientist and astronomer Carl Sagan was published there last year.

Sagan, who died in 1996, was famous for popularising science in the US and pioneering the study of potential life on other planets. At the Armour Foundation in Chicago - now called the Illinois Institute of Technology Research - he was hired by Reiffel to undertake mathematical modelling on the expansion of an exploding dust cloud in the space around the moon. This was key to calculating the visibility of such a cloud from the Earth.

At the time scientists still believed there might be microbial life on the moon and Sagan had suggested a nuclear explosion might be used to detect organisms.

Despite the highly classified nature of the work, Sagan's biographer, Keay Davidson, discovered that he had disclosed details of it when he applied for the prestigious Miller Institute graduate fellowship to Berkeley.

Yet, until today, the full nature of Project A119 has never been revealed. Friends of Sagan believe he never would have wilfully revealed classified information, but Reiffel has come forward to put the 'historical record straight'.

Reiffel continued: 'It was well known that the existence of this project was top secret. Had Sagan wanted to make any disclosures to any party, as his boss at the time, I would have had to take forward any such request and Air Force permission would have been extremely unlikely in those very tense times.'

In a letter to the science magazine Nature, Reiffel said: 'Fortunately for the future of lunar science, a one or two horse race to detonate a nuclear explosion never occurred. But in my opinion Sagan breached security in March, 1959.'

Reiffel produced eight reports between May 1958 and January 1959 on the feasibility of the plan, all of which were destroyed in 1987 by the foundation. Reiffel would not discuss details of these reports, believing they were still classified, but it was clear the conclusion was that the explosion would have been visible from Earth

He does not know why the plans were scrapped, but said: 'Thankfully, the thinking changed. I am horrified that such a gesture to sway public opinion was ever considered.'

Dr David Lowry, a British nuclear historian, said: 'It is obscene. To think that the first contact human beings would have had with another world would have been to explode a nuclear bomb. Had they gone ahead, we would never have had the romantic image of Neil Armstrong taking "one giant step for mankind".'

Lowry believes Project A119 has relevance today with the US proposing a missile defence system in space. He said: 'The US has always wanted to militarise space and some of the fanciful ideas currently being put forward will seem as incredible as the idea of nuking the moon in the Fifties seems today.'

A Pentagon spokesman would not confirm or deny the plans.

 antony.barnett@observer.co.uk