Artist's impression of lasers being fired at missiles in flight. These weapons are part of the US Strategic Defense Initiative (SDI, or 'Star Wars') programme. A visible-wavelength laser is being fired from the ground, whilst an X-ray laser is fired in space (right). On the left, a satellite is firing high-energy X-ray pulses toward the target. The SDI programme aims to provide full protection against any level of attack by ballistic missiles. In addition to lasers, the programme is researching particle beam weapons, kinetic energy weapons (including rail guns) and conventional interceptor missiles.