The Titan I was the first version of the Titan family of rockets, first developed in October 1955, when the U.S. Air Force awarded the then Martin Company in Denver, Colorado, a contract to build an Intercontinental ballistic missile (ICBM). It was the United States' first two-stage rocket and formed an integral part of their strategic deterrent force. In the early 1960s, the rocket was adapted to launch the Gemini capsule that carried two people at a time into space. Titan II succeeded in launching 12 Gemini spacecraft and has also helped to launch the Viking missions to Mars, Voyager 1 and 2 and most recently Cassini–Huygens to Saturn. It began as a backup ICBM project in case the Atlas was delayed. It was a two-stage rocket powered by RP-1 and LOX. The Titan I and Atlas ICBMs using RP-1/LOX fuel did not have a quick launch sequence. They took about 30 minutes to fuel up and fire. Most Titan rockets were derivatives of the Titan II ICBM. The Titan II ICBM had one W-53 warhead with a 9 megaton yield, making it the most powerful ICBM on-standby in the U.S. nuclear arsenal. The Titan III was a modified Titan II with optional solid rocket boosters. It was developed by the U.S. Air Force as a heavy-lift satellite launcher to be used mainly to launch U.S. Military payloads such as Defense Support Program (DSP) early-warning, intelligence (spy), and defense communications satellites. The Titan IV is a stretched Titan III with non-optional solid rocket boosters. It could be launched either with the Centaur upper stage, with the Inertial Upper Stage (IUS) or without any upper stage. It was almost exclusively used to launch U.S. military payloads, though it was also used to launch NASA's Cassini–Huygens probe to Saturn in 1997.