The explosive yield of atomic bombs is measured in kilotons, each unit of which equals the explosive force of 1,000 tons of TNT. The explosive power of hydrogen bombs, by contrast, is frequently expressed in megatons, each unit of which equals the explosive force of 1,000,000 tons of TNT. The biggest bomb in the world is 50 megatons.  Thermonuclear bombs can be made small enough (a few feet long) to fit in the warheads of intercontinental ballistic missiles; these missiles can travel almost halfway across the globe in 20 or 25 minutes and have computerized guidance systems so accurate that they can land within a few hundred yards of a designated target.