Question: How do you detect an atomic bomb blast?
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The atomic bomb that was dropped on Hiroshima released the equivalent of 15 kilotons of TNT of energy. It's hard to convey how big this is, but it's about the same as if the ACT Legislative Assembly were filled with TNT.
Even this is dwarfed by the Soviet Tsar Bomba, which was around 50 megatons of explosive power. That would be enough TNT to fill a cube as high as the Eiffel Tower. It was dropped near an island north of the Arctic circle in 1961. It also went by the name "Kuzkina mat", which roughly translates to "we'll show you".
The release plane had a reflective white paint to limit heat damage, but Soviet command estimated the crew had a 50 per cent chance of surviving. It detonated at an altitude of 4000 metres, and the fireball shot nearly 10km, and the mushroom cloud went 64km into the air. It could be seen from 1000km, and 55km away, the village of Severny was flattened.
If the bomb had gone off in Canberra, the blast of heat would be enough to cause third-degree burns in Goulburn.
The shockwave travelled three times around the world, which gives a clue how a nuclear blast can be detected. Low-frequency sounds below the threshold of human hearing is called infrasound. Like whale calls, these travel extremely long distances. A network of 60 stations around the world are constantly listening. Each is arranged in a star-shaped cluster, and the timing in each node indicates direction. There's also an array of underwater acoustic monitors.
Another sign is seismic waves. There are many stations that can detect tiny movements in the Earth. These are in the nanometre scale, and analysis is needed to differentiate between an earthquake and an explosion. In an earthquake, the ground starts shaking slowly as plates slide against each other. In an explosion, the initial blast is extremely powerful then tapers off gradually.
An obvious product is radioactive chemicals in the air. In the 1949, the US used weather balloons to collect data about tests in the Soviet Union. If the bomb is underground, this probably won't work, but it still gives off clues. In 2009 North Korea detonated a bomb one kilometre below the surface, and this was detected by changes to the signals received by GPS satellites.
Response by: Rod Taylor, Fuzzy Logic
Next week: Detecting a missile launch
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