In 1967, a solar storm almost caused World War III.
At the time the Cold War with the Soviet Union was in full swing. Meanwhile, radar sensors meant to detect missile attacks against the United States were disrupted by a huge electromagnetic storm on the sun.
This Youtube video by SciShow Space explains how a potential misinterpretation of signals could have led to war, and how the crisis was ultimately averted.
To begin, it's important to understand how solar storms work. The sun is made of plasma, very hot charged particles. On the sun's surface, called the photosphere, all these electric charges lead to powerful magnetic fields. When energy builds up from the magnetic fields, it gets released in the form of a solar storm, which ejects clouds of plasma ions and radiation into space.
When the radiation gets close to Earth, it's pulled toward the north and south poles by the planet's magnetic field. That's how we get phenomenons like the Northern Lights: when air molecules in the upper atmosphere absorb energy from charged particles in the plasma and emit a glow.
However, if the storm is very powerful and more electromagnetic radiation than usual hits earth, the air molecules can't absorb all the energy, and it arrives at Earth's surface. That's what happened on May 18, 1967, when the United States Air Force saw a large group of sunspots—cooler, magnetized regions of the photosphere—and gathered that a storm was happening.
Then five days later on May 23, the sun released one of the largest bursts of radio waves in recorded history. They had a frequency of 440 megahertz, which was the same frequency used by United States and allied radar stations meant to look out for Soviet nuclear missiles. Using the Ballistic Missile Early Warning System (BMEWS), the US would have fifteen minutes upon learning of a nuclear strike to launch a nuclear counter attack.
So during the solar storm, the North American Air Defense Command, was picking up weird signals. The sun's radio waves overwhelmed the detectors at BMEWS, making it seem as if the stations were jammed, which ordinarily would be interpreted as an act of war and that missiles were en route.
Air crews would have been given immediate instructions to retaliate and drop missiles on the Soviets, which would have in turn actually caused them to attack the United States for real. It would have been world wide nuclear war.
However, the American solar forecasters reconciled readings from BMEWS with the air weather service, determining that in fact the odd frequencies were caused by the solar storm. There was no attack, and in 1967, we avoided World War III thanks to people who understand science.
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