Throughout the 1950s and 1960s, the United States Project Nike air defense program focused initially on targeting hostile bombers before shifting focus to targeting ballistic missiles. In the 1950s, the first United States anti-ballistic missile system was the Nike Hercules, which had the ability to intercept incoming short-range ballistic missiles, but not intermediate-range ballistic missiles (IRBMs) or ICBMs. This was followed by the Nike Zeus, which was capable of intercepting ICBMs by using a nuclear warhead, upgraded radar systems, faster computers, and control systems that were more effective in the upper atmosphere. However it was feared the missile's electronics may be vulnerable to x-rays from a nuclear detonation in space. A program was started to devise methods of hardening weapons from radiation damage. By the early 1960s the Nike Zeus was the first anti-ballistic missile to achieve hit-to-kill (physically colliding with the incoming warhead).