Fifty years ago today, US pilot Francis Gary Powers was shot down while flying a U-2 spy plane over the Soviet Union in a dramatic episode of the Cold War that pushed the rival superpowers closer to confrontation.

Now his son has come to Moscow to tell his late father's story, help preserve Cold War history and prevent future generations of Russians and Americans from ever again facing the threat of nuclear war.

On May 1 1960, Powers was in the cockpit of the world's highest-flying plane, concentrating on keeping his course steady to film Soviet military bases far below, when he saw an orange flash all around him.

His plane had been hit by a Soviet surface-to-air missile. He parachuted to safety but was quickly captured.

In the months before Powers' plane was downed, Moscow and Washington had been moving cautiously towards a thaw, but the U-2 incident shattered these efforts.

It also humiliated US president Dwight Eisenhower, who had to admit that an initial claim by his administration that the plane was on a weather mission was a lie.

"In order to understand the world today you must understand how we got here and we got here through the Cold War," the pilot's 44-year-old son, also called Francis, said yesterday.

"And then we have to understand how this period of time developed and expanded and how close we came to nuclear war, but through diplomacy and some luck we were able to avert it during the Cuban missile crisis of 1962.

The younger Powers joined Russian military historians in speaking to soldiers and cadets at the Central Armed Forces Museum in Moscow, where the charred wreckage of his father's spy plane is on display.

His visit comes as Washington and Moscow try to improve ties, recently signing a deal on reducing their nuclear arsenals.

Mr Powers junior, who has dedicated his professional life to preserving Cold War history, has his own museum which has just found its first permanent home on a former army communications base outside Washington and he also runs spy tours of the US capital.

His father's fateful mission was the 24th overflight of the Soviet Union in a highly-secretive CIA programme considered vital for national security at a time before spy satellites.

After nearly four years of unsuccessful Soviet attempts to intercept the U-2s flying at about 70,000 feet, the CIA grew confident of the plane's immunity, but the Soviets worked desperately to develop higher-flying fighter jets and a powerful new air defence missile.

Soviet leader Nikita Khrushchev received reports about another US spy plane intrusion as he was preparing to attend a Red Square parade on May Day. His son Sergei, then a young missile designer, said he discussed this with his father that morning.

"I asked him, 'Will they shoot it down this time?'," the younger Khrushchev recalled. "And he said, 'What kind of question is that? They will if they don't let the chance slip by'."

Khrushchev was standing on Lenin's mausoleum with other officials watching the parade when the Soviet air defence chief, Marshal Sergei Biryuzov, whispered the news about downing the plane into his ear.

When no statements came immediately from the Kremlin, the CIA assumed that neither the pilot nor the spying equipment had survived and on May 3, the US claimed a high-altitude weather plane had gone missing on a flight over Turkey.

Khrushchev kept a poker face, announcing first that a US spy plane had been downed without saying a word about its pilot. The US stubbornly stuck to its cover story until the Soviet leader announced on May 7 that the pilot had been caught and had confessed to spying.

"The Americans, the US, for the first time were caught red-handed in espionage activities," Mr Powers junior said.

For Khrushchev, the incident provided a long-sought opportunity to punish the US.

"My father perceived the U-2 flights as not only damaging national security, but even more important as a sign of condescension, a demonstration by the Americans that they could do whatever they want and fly where they liked without consequences," Sergei Khrushchev said.

The scandal led to the collapse of a peace summit in Paris scheduled for mid-May and also ruined hopes for a quick agreement on a nuclear test ban.

After months of KGB interrogation, Powers was sentenced to 10 years in prison, but later exchanged for KGB spy Colonel Rudolph Abel on in 1962.

CIA officers were unwilling to believe that his plane had been shot down by a Soviet missile but he was eventually exonerated and worked as a test pilot for Lockheed until 1970, then flew a light plane as a traffic reporter and later worked as a pilot for a Los Angeles television station.

He died when his helicopter crashed on August 1 1977.

After the Soviet collapse, Russian military veterans revealed that the Soviets accidentally shot down one of their own fighter planes that had been scrambled to intercept Powers' aircraft, killing its pilot.

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