The Thor rocket was the first operational ballistic missile deployed by the U.S. Air Force. Authorized in November 1955 to provide an independent Intermediate Range Ballistic Missile capability, the first production model was delivered to the U.S. Air Force in October 1956. Thor also served as the core booster for many satellite and space exploration missions, and despite its short life as a weapons system, it was a critical development in the evolution of the U.S. space program. It measured 65 feet long and 8 feet in diameter, had a range of 1,500 miles, and used liquid oxygen and kerosene at 600 pounds, or 72 gallons, per second. A direct descendant of the Thor engine is still used in Delta rockets that launch many satellites and space probes, and the Delta name came from its being the fourth variant of the original Thor, Thor-Delta.