In 1820, the U.S. Congress passed the Missouri Compromise, which admitted Missouri as a slave state and Maine as a free state, maintaining the balance between slave and free states. In 1923, the Great Kantō earthquake struck Japan, devastating Tokyo and Yokohama and leading to significant urban redevelopment. In 1961, during the Cold War, a B-52 carrying two nuclear bombs broke up in mid-air over North Carolina, and one of the bombs nearly detonated. In 1986, the Soviet Union launched a space probe to Halley's Comet, marking an international effort to study the celestial phenomenon.