On October 30, 1961, aircrews on two military aircraft high above the Arctic Circle were about to make history.
But, probably much like some members of the US Enola Gay crew who dropped the world's first atomic bomb, they were likely concerned first and foremost with their own survival.
The Enola Gay's commander, General Paul Tibbets, was given cyanide pills for his crew, so that if things went wrong and they crashed behind enemy lines they could take their own lives rather than fall into the hands of the Japanese.
16 years later, it wasn't enemy capture or suicide that would have weighed on the mind of Andrei Durnovtsev. Rather, it must have been the fact that the nuclear test his crew was about to carry out would involve a weapon so incredibly powerful, that they had only been given a 50% chance of escaping the blast alive.
Their Tu-95V plane had had its bomb bay doors and fuselage fuel tanks ripped out to make room for their cargo.
It was, after all, hauling an eight-metre long by 2.1-metre wide, 27 metric ton bomb, which would have its descent slowed by an enormous 800 kg parachute, to maximise the chances of Durnovtsev's plane and the observer aircraft behind getting away in time.
Further, both aircraft were painted with special reflective white paint to reduce heat damage, and crews were given visors to stop them going blind when the bomb went off. That much at least was guaranteed, even if their survival was not.
Also maximised was the height at which the bomb was dropped, to buy the two planes more time to bank sharply away and rush headlong in the other direction after the bomb's release.
And they did just that at 11:32 Moscow time that day, after releasing the bomb from a height of 10.5 km (34,000 ft) while over the Mityushikha Bay test site.
The bomb drifted to earth then detonated 4km above the ground, destroying everything below it, and unleashing a mushroom cloud that shot 64 km (40 miles) into the air.
It broke through the stratosphere, spreading out to 40 km (25 miles) at its base and 95 km (59 miles) at its peak.
Given several names, 'Project 7000', 'Big Ivan', and 'JOE 111' by the CIA, it eventually came to be known simply as the 'Tsar Bomba'.
Its yield was 50 megatons, equivalent to around 1,570 times as powerful as the bombs dropped on Hiroshima and Nagasaki combined, 10 times all the bombs dropped in World War 2, or a quarter of the power of the 1883 Krakatoa volcanic eruption - a blast so powerful that it blew one volcanic island into three smaller islands.
The test came at the height of the Cold War, and the bomb's capacity theoretically could have been doubled with a Uranium-238 tamper to 100 megatons (a figure for the test quoted by U.S. Defense Secretary Robert McNamara in The Fog of War).
But thankfully, Soviet planners never pushed this limit, fearing that the fallout from the bomb would be too dangerous and that testing a bomb twice as powerful as 'Tsar Bomba' would doom the crew dropping it.
As it was, Major Andrei Sakharov and his two planes were able to escape that day.
Nuclear weapons power had peaked. Leaders on both sides began to see the madness inherent in their use.
In 1963, the Partial Test Ban Treaty (PTBT) was signed by the USSR, US, UK, and Mexico, forbidding any further nuclear testing anywhere in the atmosphere, space, or underwater, allowing them only to be carried out underground.
Instead, the superpowers would embark on a more productive race - getting to the moon.
Cover photo shows the mushroom cloud from "Ivy Mike," the first-ever hydrogen bomb - tested (in 1952) - which had a yield a fifth of the power of the Tsar Bomba.