By Gerry Northam BBC Radio 4

Colonel Howard Richardson ditched the bomb off Tybee Island

More than 50 years after a 7,600lb (3,500kg) nuclear bomb was dropped in US waters following a mid-air military collision, the question of whether the missing weapon still poses a threat remains.

In his own mind, retired 87-year-old Colonel Howard Richardson is a hero responsible for one of the most extraordinary displays of aeronautic skill in the history of the US Air Force.

His view carries a lot of weight and he has a large number of supporters - including the Air Force itself which honoured his feat with a Distinguished Flying Cross.

But to others, he is little short of a villain: the man who 50 years ago dropped a nuclear bomb in US waters, a bomb nobody has been able to find and make safe.

'Top-secret flight'

Shortly after midnight on 5 February 1958, Howard Richardson was on a top-secret training flight for the US Strategic Air Command.

It was the height of the Cold War and the young Major Richardson's mission was to practise long-distance flights in his B-47 bomber in case he was ordered to fly from Homestead Air Force Base in Florida to any one of the targets the US had identified in Russia.

We thought maybe it was something from outer space, but it could only be another plane Colonel Howard Richardson

The training was to be as realistic as possible, so on board was a single massive H-bomb - the nuclear weapon he might one day be instructed to drop to start World War III.

As he cruised at 38,000 feet over North Carolina and Georgia, his plane was hit by another military aircraft, gouging a huge hole in the wing and knocking an engine almost off its mountings, leaving it hanging at a perilous angle.

At his home in Mississippi, Colonel Richardson said: "All of a sudden we felt a heavy jolt and a burst of flame out to the right.

"We didn't know what it was.

"We thought maybe it was something from outer space, but it could only be another plane."

The colonel thought his number was up. His bomber started plummeting to earth and he struggled with the flight deck to get any kind of response.

"We had ejection seats - I told 'em: 'Don't hit the ejection seats just yet. I'm gonna see if we can fly.'"

As he dropped to 20,000 feet, he somehow got the damaged craft under control and levelled out.

He and his co-pilot then made a fateful decision which probably saved both their lives and the lives of countless people on the ground.

The B-47's engine was left hanging from the plane

Colonel Richardson told me that the decision was instantaneous - and he still has no doubt it was the right thing to do.

They would ditch their nuclear payload as soon as possible in order to lighten the aircraft for an emergency landing and also to eliminate the danger of an enormous explosion when they made their unsteady arrival at the nearest available runway.

"The tactical doctrine for Strategic Air Command gave me the authority to get rid of it (the bomb) for the safety of the crew - that was the number one priority," Colonel Richardson said.

He managed to direct the B-47 a mile or two off the coast of Savannah and opened the bomb doors, dropping the bomb somewhere into the shallow waters and light sand near Tybee Island.

He then managed a perfectly executed descent from which he and his crew walked away unscathed.

The pilot of the other aircraft, an F-86 fighter jet, also survived, after his ejector seat shot him clear of his aircraft.

I've been living with it now for 51 years Colonel Howard Richardson

Immediately after the crash, a search was set up to find the unexploded nuclear weapon, buried somewhere too close for comfort to the US's second-largest seaport and one of its most beautiful cities.

Numerous other searches have followed, both official and unofficial, and each of them has also proved unsuccessful.

So the bomb remains tucked away on the sea-bed, in an area which is frequently dredged by shrimp fishermen, any one of whom could suddenly find that they have netted something a touch larger and scarier than a crustacean.

How dangerous the bomb is after all these years is a matter of hot debate.

The US Air Force insists it is safest to leave it wherever it is, and Colonel Richardson is adamant that it is incapable of a nuclear explosion because it lacks the vital plutonium trigger.

'Practice mission'

He said these were routinely left out of the bombs used on training flights.

"This was just a practice mission. We were continually working out any problems, that's why we had to practise - we wanted to be perfect," he said.

But his case has been vigorously contested by opponents who raise apocalyptic fears of a thermonuclear explosion which could destroy much of the US eastern seaboard.

Fears have also been expressed that the bomb could be located and recovered by a terrorist group, and there are even some who believe that may already have happened.

For Colonel Richardson, the event which shaped his life has not ended quite the way he thought it would.

"I've been living with it now for 51 years.

"We had an accident and I landed the aircraft safely... I did get a Distinguished Flying Cross from a general for that.

"I thought that would be the story. That's not the story - everything's about the nuclear weapon."

Hear more from Gerry Northam on Broken Arrow, broadcast on Radio 4 at 2000 BST, Monday 22 June