They warn us of approaching storms. They allow us to make emergency phone calls on mobile phones. They're the digital conveyor belt for global commerce. They help police and ambulances reach their destinations when every minute counts. And the Pentagon relies on them to provide US forces with intelligence, communications and targeting information.

Satellites, it seems, have become our lifelines. Still, it's easy to take satellites for granted - easy, that is, until the People's Liberation Army (PLA) crashes a missile into one of China's ageing meteorological satellites, as it did last month. It was a crude show of strength, which the PLA will do on occasion when it wants to make a point. In 1995, for example, Beijing sent a fusillade of missiles in Taiwan's direction, a blunt reminder to think twice about independence. This time around, the PLA's message seemed directed at the Bush administration and the Air Force, which has adopted a "space control'' doctrine that endorses the use of weapons in, from and through space.

The debris from China's missile blast could travel in space for much more than a quarter-century before burning up in the Earth's atmosphere. That's a long time, but not longer than the debate over weapons in space has raged, beginning with the launch of Sputnik in 1957. Having prying eyes overhead is unsettling enough, but it is not nearly so worrisome as weapons circling overhead, ready to fire.

What the Air Force euphemistically calls "offensive counter-space'' capabilities - use of the terms "space weapons'' and "space dominance'' is verboten - does not have a broad constituency of support in the Pentagon or on Capitol Hill. The notion of turning space into one more war zone offends many sensibilities, from those of devout believers who don't think the heavens should be sullied by weapons, to those of pragmatic soldiers who realise that, if satellites become fair game in warfare, their other missions will become even harder.

President Ronald Reagan couldn't dent these concerns with the Strategic Defence Initiative, his 1983 proposal to use space-based weapons as a shield against nuclear attack. Journalist Frances FitzGerald offers a sceptical account of this period in Way Out There in the Blue, which treats Reagan's scheme as part fantasy, part public relations and part device to kill arms control. Mikhail Gorbachev is the hero of FitzGerald's narrative, while Reagan's contributions towards devaluing nuclear weapons are short-changed.

Astronomer Robert Jastrow makes the moral case for Reagan's vision of space-based defences in his 1985 book How to Make Nuclear Weapons Obsolete.

Demonstrations

So far, the Bush administration's testing in space appears limited to demonstrations of multipurpose technologies: For example, a recent test manoeuvred a small satellite to make close passes at US space objects. These techniques could ultimately be used to help with satellite docking or monitoring.

The Air Force's new doctrine and the Bush administration's refusal to discuss, let alone negotiate, anything that could limit US freedom of action in space - along with the traditional secrecy surrounding military space programmes - has gotten China's attention.

Last September, press reports indicated that China had "painted'' a US satellite with a laser. It is unclear how often this has occurred or whether the United States has carried out similar practices against Chinese satellites. (Shining lasers on satellites can be used for space tracking and monitoring, as well as for temporarily blinding a satellite, among other uses.) Now that Beijing has in turn gained Washington's attention, the competition in space is likely to heat up. An old US-Soviet-style space race seems unlikely - after all, we live in an era of asymmetric warfare - but it doesn't take an arms race to mess up space, as the PLA just proved.

These days, "lasing'' and jamming are the preferred Pentagon means for dealing with satellites that could threaten US combat forces. Initially, however, the Pentagon considered nuclear detonations as a way to destroy satellites, even deploying (but never launching) two nuclear-tipped rockets for this assignment after the Cuban missile crisis. The Kennedy administration learned that this was a bad idea after one particularly powerful atmospheric nuclear test in 1962 damaged every US satellite - and one Soviet satellite - in low Earth orbit.

The United States and the Soviet Union turned next to space weapons that killed on contact, as detailed in Paul Stares's 1985 book, The Militarisation of Space. The US military conducted dozens of such tests, but only one, in 1985, was like the recent Chinese test, with the Air Force blowing up an ageing meteorological satellite.

Debris

Fourteen years later, a piece of debris from this test came within one mile of the international space station. It took three additional years for this lethal hazard to clear out of low Earth orbit. (The recent Chinese test has produced a much larger debris field at a higher altitude, meaning the resulting hazard to spaceflight will be much worse.)

Political interest in space weapons is usually linked to spikes in public anxiety. During the Reagan administration, many were concerned that the Kremlin had achieved strategic and military superiority and might exploit its advantages - including the use of futuristic space weapons.

Now, the focus is squarely on China.

It is perhaps fitting that some of the best analysis on weapons in space would be found in, well, cyberspace. And among the bloggers, I would recommend Leonard David of Space.com, as well as Jeffrey Lewis, who keeps perhaps the leading blog on nuclear proliferation and arms control, AviationWeek.com - where he broke the Chinese anti-satellite test story on January 17.

Michael Krepon, co-founder of the Henry L. Stimson Centre, lectures on nuclear proliferation at the University of Virginia.