1. Why build a 100-megaton bomb
By 1961 the Cold War arms race had become a contest of numbers. The United States had tested a 15-megaton weapon in 1954. Nikita Khrushchev wanted a demonstration nobody could ignore, and in July 1961 he ordered Soviet weapon scientists, led by Andrei Sakharov, to produce a 100-megaton device in time for the Communist Party congress in October. They had sixteen weeks.
The bomb they built, officially RDS-220 and later nicknamed Tsar Bomba, "the king of bombs", was a three-stage thermonuclear weapon: a fission primary, a fusion secondary, and an enormous fusion tertiary. It was 8 metres long, 2 metres across and weighed 27 tonnes, so heavy that the Tu-95 bomber that carried it had its bomb bay doors and fuel tanks removed and its skin painted white to reflect the flash.
2. Cut in half at the last minute
The full design would have used uranium casings around the fusion stages to double the yield through a final round of fission. That would also have produced a colossal amount of fallout, spread across the Soviet Union and beyond, and the fallout would have reached the bomber's crew. Sakharov argued for replacing the uranium with lead. The yield fell to roughly 50 megatons, and the test became one of the "cleanest" ever conducted relative to its size: about 97 percent of the energy came from fusion.
Even so, the crew were told they had only a 50 percent chance of surviving. The bomb was dropped by parachute to give the plane time to fly 45 km away before it detonated.
3. The explosion
At 11:32 on 30 October 1961 the bomb detonated 4 km above Severny Island in the Novaya Zemlya archipelago, inside the Arctic Circle.
- The fireball reached about 8 km across. It never touched the ground, because its own shock wave, reflected from the surface, pushed it back up.
- The flash was visible 1,000 km away. Observers 270 km away felt the heat on their faces.
- The mushroom cloud rose to about 67 km, well into the stratosphere, and spread 95 km wide.
- The shock wave broke windows in Norway and Finland, 900 km away, and was recorded circling the planet three times.
- The abandoned village of Severny, 55 km from ground zero, was flattened. Wooden houses hundreds of kilometres away lost their roofs.
- The bomber, 45 km away by the time of the blast, dropped a kilometre in the shock wave before the pilot regained control.
Seismic stations around the world registered the blast as a magnitude 5 to 5.25 earthquake.
4. What it would do to a city
Tsar Bomba was never a practical weapon: no missile could carry it, and one bomber could not be expected to reach a defended city. But the numbers give a sense of scale. Using the same scaling model as the simulator, a 50-megaton airburst over a city would:
- vaporise everything within a fireball about 4 km in radius,
- collapse most residential buildings out to about 18 km,
- cause third-degree burns to exposed skin out to more than 50 km,
- break windows and injure people beyond 45 km.
Over a dense city like New York, London or Tokyo that means a death toll in the millions and a destroyed area larger than the city itself. You can see it drawn on the map by choosing Tsar Bomba in the simulator.
5. Why nobody built another
Tsar Bomba was the peak of an arms race that had already turned a corner. Missiles were replacing bombers, and a missile can carry only so much weight. Military planners discovered that several warheads of a few hundred kilotons, spread across an area, destroy more than one giant one, because destruction grows with the cube root of yield. Within a decade both superpowers were building smaller warheads and more of them.
Two years after the test the United States, the United Kingdom and the Soviet Union signed the Partial Test Ban Treaty, ending nuclear tests in the atmosphere. Tsar Bomba remains the largest man-made explosion in history, and almost certainly always will be.