The Delta II rockets were based on the Thor rockets built in the 1950's. These were used to carry Inter Continental Ballistic Missiles. The first Thor was launched on January 26, 1957. The Delta II rocket was first launched on 14 February 1989. The rocket was developed after the explosion of the in 1986. The US Air Force needed another way to launch its GPS satellites. As of 2018, there had been 156 Delta II launches from both Cape Canaveral Air Force Station and the Vandenberg Air Force Base. Only two launches were unsuccessful. On 5 August 1995, a rocket motor failure meant the Koreasat 1 was put into a lower orbit. On 17 January 1997, Delta 241 carrying the GPS IIR-1 exploded 13 seconds after taking off.