Shooting down missiles is big business, but remains a flawed science. The US government alone plans to spend US$55 billion on missile defence over the next six years, yet many defence analysts and scientists question whether an effective shield can be built. Getting objects travelling at supersonic speeds to collide at a given point in space is no easy task at the best of times, but in a battle cluttered with decoys, friendly forces and electronic interference it becomes even more difficult. Just as missile defences are improving, so too are the missiles, making them harder to shoot down. Even the highest profile US anti-missile weapon, the Patriot, has been far from impressive since entering service in the late 1970s. In their 1991 combat debut during the Gulf war, Patriot batteries were initially hailed as success stories, with reports that they shot down 41 out of 42 missiles fired by Iraq. Later, independent assessments said that Patriots shot down four of 47 missiles fired. On average, three to four Patriots were fired to destroy each intercepted missile. The US invested US$3 billion upgrading the Patriots after the first Gulf war, and during the second attack on Iraq the Pentagon again claimed spectacular results, saying the improved missiles shot down all nine Iraqi missiles that they engaged. But critics said the missiles failed to shoot down any of the low-flying cruise missiles Iraq launched and likewise failed to engage several small unmanned aerial vehicles. The Patriots have also accidentally downed two friendly fighter aircraft. Noting that the latest version of the Patriot - the Pac-3 - cost US$3.5 million per missile, Dennis Gormley, a senior fellow at the Monterey Institute's Centre for Nonproliferation Studies, told a US House of Representatives sub-committee last year that developing an effective anti-missile system was becoming prohibitively expensive. 'A flock of cruise missiles or converted airplanes several orders of magnitude cheaper could readily saturate most economically feasible missile defence architectures,' he said. Still, with cheap and easy to operate missile technology allegedly available from North Korea and mainland China, there is a growing demand for anti-missile systems and billions of dollars are being spent to improve the defence systems. According to the Patriot's manufacturer, Raytheon, the missiles are used by the US, the Netherlands, Germany, Japan, Israel, Saudi Arabia, Kuwait, Taiwan and Greece. Many other nations - including the mainland - field similar weapons manufactured by Russia.