The Thor intermediate-range ballistic missile (IRBM) provided nuclear deterrence before intercontinental ballistic missiles were ready. It passed its capability tests in 1958. It was deactivated in 1966, as the U.S. began to rely on larger, more advanced ICBMs. Thor missiles later were configured as space launch vehicles, Thor-Able-Star and Thor-Agena, for the Air Force and NASA, providing technology for current launch systems. A fourth modification of Thor, the Thor-Delta, has evolved into the world's most dependable and most frequently used commercial space launch vehicle, the Delta rocket. The Rocketdyne division of North American provided the propulsion system for the Thor missile and subsequent space launch vehicles.