Thor was the first operational ballistic missile in the arsenal of the United States, operated by the US Air Force. Thor was in height and in diameter. Named after the Norse god of Thunder, it was deployed in the UK between 1959 and September 1963 as an Intermediate Range Ballistic Missile (IRBM) with thermonuclear warheads. It was later augmented in the U.S. IRBM arsenal by the Jupiter. A large family of space launch vehicles—the Delta rockets—were derived from the Thor design. A modified version is still in use today as the first stage of the Delta II.