A rocket developed by the US Air Force as an intermediate-range ballistic missile (IRBM). Its first launch was on 20 September 1957. A year later, it was combined with the second stage of a Vanguard rocket to form the Thor-Able launch vehicle used to launch the Pioneer probe to the Moon. In 1959 it was combined with an upper stage Agena rocket, and launched a Discoverer satellite. A year later, the first Thor Delta was launched and became known as the Delta, flying more than 200 times until the Delta III was introduced in 1998.
Subjects: Astronomy and Astrophysics.