Such a move has been on the cards for some time. In 2001, a committee headed by Secretary of Defense Donald Rumsfeld warned that the US faces a potential "Pearl Harbor in space" unless it develops weapons to protect its space hardware. And the US air force has incorporated "fighting in space" into its mission statement, and speaks openly of achieving "space superiority".

Last month, analysts at two Washington-based think tanks, the Center for Defense Information (CDI) and the Henry L. Stimson Center, reported that the Pentagon's convoluted $439 billion budget plans for 2007 include almost a billion dollars for developing and testing space weapons systems. While these have not been explicitly listed as such, they are "hidden in plain sight" within Missile Defense Agency and air force projects, says CDI's director Theresa Hitchens.

At least three types of space weapon are in the Pentagon pipeline. The Multiple Kill Vehicles project will look at using -spacecraft to launch missiles that would carry a clutch of impacting projectiles designed to damage other spacecraft. The Missile Defense Agency's MicroSat project will investigate whether a satellite could sense the position of an enemy satellite and ram it. Finally, part of air force's laser weapons programme will be. aimed at using ground-based lasers to knock out a spacecraft.

The CDI fears that the Pentagon might surreptitiously build up a weapons infrastructure by sending systems like these into space, ostensibly as prototypes for testing, and simply leaving them there. This would allow space weapons to become a fait accompli without congressional or public debate. "Space weapons are extremely destabilising politically so the DoD is getting around any debate by developing systems it says are just designed to test their capabilities," says CDI analyst Victoria Samson. "The Pentagon simply finds it easier to ask for forgiveness later rather than for permission now."

It is not just the political aspect of weaponising space that is causing concern. Near-Earth space could suffer serious pollution if anti-satellite (ASAT) weapons are used. An international relations conference in San Diego, California, heard on 25 March that destroying satellites would create large amounts of orbiting debris that could have a devastating effect on other spacecraft. "The potential for debris due to space weapons use was a big issue," says David Webb of Leeds Metropolitan University in the UK. "Even some in the American military were against using anti-satellite weapons owing to the debris issue."

The risk of damage to spacecraft by conventional debris is considerable. On 29 March a piece of space junk slammed into a Russian broadcasting satellite, puncturing it and venting a jet of liquid coolant that sent it spinning. The crippled craft had to be pushed into high orbit and deactivated. Incidents like this could become increasingly common if satellites become targets for attack, rendering near Earth space almost a no-go area for spacecraft, Webb says.

Despite such risks, the economic and strategic importance of communications, surveillance and navigation satellites is now so great that the US considers it imperative to develop weapons to defend them. Much everyday technology, such a as multi-channel TV and in-car satellite navigation, relies on satellite links, so attacks on commercial spacecraft could have severe economic consequences. The military importance of satellites was seen during the invasion of Iraq in 2003, for instance, when high-resolution imaging satellites showed where Iraqi troops were, while GPS was used to guide munitions and robotic aircraft towards them. As a result, the US military now believes it needs space weapons to deter other countries from attacking its satellites.

According to the Department of Defense, the US is not the only country developing space weapons. In its report to Congress on China's military power last year, it states: "China is working on, and plans to field, ASAT systems." These are thought to include ground-based lasers, including low-power systems to blind or dazzle spy satellites and higher-power lasers to destroy spacecraft. In 2004, the DoD reported that China had developed a micro-satellite ASAT weapon that attaches itself to larger spacecraft and destroys them. Webb, however, says the information came from an "unreliable" military enthusiast's website in Hong Kong.

Meanwhile, the Israeli parliament's Foreign Affairs and Defense Committee last year urged its government to develop ASAT technology. The country is already an enthusiastic partner in the US National Missile Defense (NMD) system, which uses

surface-based radar to detect incoming missile-launched warheads, with a view to launching more missiles to intercept them out in space.

Death rays for real

The development of anti-satellite weapons is being stimulated by advances in powerful laser and microwave beams, which would form the basis of so-called "directed energy" weapons. "Advances in power density from chemical and free-electron lasers have been possible in the lab for a decade," says Doug Beason, a directed energy specialist at the Los Alamos National Laboratory in New Mexico and a science adviser to presidents George Bush senior and Bill Clinton. "Being able to do it in a portable way, so it can be used on the battlefield, is now becoming possible."

It was a lack of portable lasers that pulled the rug from under the Strategic Defense Initiative ("Star Wars") proposed in the 1980s by President Ronald Reagan. Now improvements in technologies such as supercapacitors, which can discharge large quantities of energy into a free-electron laser, have made them possible.

A key advance is the chemical oxygen iodine laser (COIL) that will form the heart of the Missile Defence Agency's huge Airborne Laser - a Boeing 747 designed to shoot down nuclear missiles as they launch hundreds of kilometres away. The chemical components that produce the excited oxygen atoms which donate energy to iodine molecules, causing them to generate laser light, have been shrunk enough to make the idea workable. Yet despite the $7 billion spent on it so far, the Airborne Laser has yet to be fired.

The UN's 1967 Outer Space Treaty forbids the stationing of weapons of mass destruction in space. There is no treaty preventing other weapons being stationed there, however, and a proposed treaty to prevent an arms race in outer space (PAROS) has been consistently blocked by the US and Israel. PAROS has been on the agenda of the UN Conference on Disarmament in Geneva, Switzerland, since 1998, but no framework for considering the issue has yet been agreed, despite attempts by Russia and China to promote debate.

Even without a ban on space weapons, it is not clear why the Pentagon's ASAT research is needed. Technologies already in development could do the job just as well, many analysts say. For example, steerable mirrors in space could be used together with a ground-based or airborne laser to knock out spacecraft, Beason says in E-Bomb, his recent book on directed energy weapons. And

Michael O'Hanlon, a strategic arms analyst at the Brookings Institution in Washington DC, says the interceptor missiles used by the NMD could be adapted to attack satellites.

According to O'Hanlon, the political cost of testing new ASAT weapons in peacetime ought to rule them out. "To develop new ASATs now would reinforce the image of an America out of control," he says.