Delta rockets have been built and launched since 1960. Delta's origins go back to the Thor intermediate-range ballistic missile, which was developed in the mid-1950s for the U.S. Air Force. The Thor, a single-stage, liquid-fueled rocket, was modified to become the Delta launch vehicle, which later evolved into the Delta II.
Known as the "workhorse" of the launch industry, the Delta II comprises a group of expendable rockets that can be configured as two- or three-stage vehicles and with three, four or nine strap-on graphite epoxy motors (GEMs) depending on mission needs.
Delta IV was developed in partnership with the U.S. Air Force EELV program and is the most advanced family of Delta rockets. Delta IV blends advanced and proven technology to launch virtually any size medium-to-heavy class payload to space.