Why were Americans fearful when Sputnik was launched?

The United States' fears of Sputnik 1 were based on the premise of a Soviet-American pending missile gap. The reasoning went that if the Soviets could launch Sputnik into orbit, the technological threshold for intercontinental ballistic missiles laden with nuclear warheads was now crossed. However, and ultimately a missile gap never came to exist between the two powers suggesting American paranoia.