The Exoatmospheric Kill Vehicle (EKV) has a dramatic name and a mundane appearance. It resembles a conventional space satellite with rocket boosters pointing sideways and has a brass-coloured funnel at its front end. This funnel contains the technology that guides the EKV towards its target at 15,000mph.

The EKV is the final stage of the US National Missile Defence (NMD) project. This "tip of the sword" as Raytheon, the US defence contractor, characterises it is intended to punch incoming ballistic missiles out of their orbit at the outer edge of the earth's atmosphere.

Raytheon built the Patriot missiles that protected Israel and Saudi Arabia during the Gulf War. Today, it is a big contractor for NMD and is responsible for building the EKV.

Charles La Due, EKV program manager at Raytheon's plant in Tucson, Arizona, illustrates the challenge involved in arranging a collision between two objects each travelling at incomprehensible speeds. "The EKV could travel from central London to Heathrow airport in four and half seconds."

To intercept a small warhead at this velocity the EKV must compute information in a flash. But the technology deployed on board this space missile is commonplace. "We presently employ four processors, three from Motorola and one from Intel, inside the EKV," says Mr La Due. "This is quite old technology. The processor that sits inside your desktop PC would rival these."

Software does not feature in this system. The time constraints and information throughput from detection systems mean that only hardwired systems working to a pre-ordained routine can fly the EKV.

Cameras and sensors on the vehicle will try to isolate real warheads against the background of decoys launched by an enemy.

The practice of packing an Intercontinental Ballistic Missile (ICBM) with decoys resembling traffic cones that emit the same radar signal as a real warhead is established among western and Russian ICBM forces. But these decoys may not share the infrared signature of the genuine target. And against the backdrop of space, an incoming missile emits a terrific heat signature.

Ground-based computers will load the EKV with the characteristics it has to look for in a target. "Ten years ago we could not get that type of process onto one chip. Now we can buy it in the commercial marketplace," says Mr La Due.

Protecting the circuitry

The gamma rays emitted during a nuclear explosion destroy circuitry, so many computer chips in military applications are specially shielded. Not so the EKV processors: "The approach we have taken is that they have to work after a nuclear warhead has detonated in space, so we have a way of switching them off during a nuclear explosion."

Mr La Due refers to a hostile warhead exploding as "a nuclear event" and explains that his chips will switch back on in the instant the explosion - which would have come from a warhead other than the one the EKV is heading for - dies down.

If production of the EKV goes ahead, Raytheon will be assembling a relatively small number of interceptors at Tucson, says Mr La Due. "We would not be building a lot of these, less than 200, I expect."

So assembly in a plant employing around 500 staff would be tightly managed. "We will go for paperless planning, giving each factory operator a CD loaded with instructions on how to put their part of the EKV together."

Computer-aided design software is currently used at Tucson to keep NMD running to a tight schedule. "This is not your classic engineering, manufacturing and development program. Here we go straight through flight test and prepare for production."

Boeing is the central contractor for NMD. It manages the $2.4bn the Pentagon has allocated to testing a ballistic missile interceptor. The full NMD system has been given an estimated price tag of $60bn by the Congressional Budget Office. So it is hardly surprising that President Clinton has pushed a final decision on NMD's future back beyond this year's presidential elections.

But the testing program will continue, with the terrestrial computing resources supplied by familiar commercial companies. Josh Edwards, a former US Navy helicopter pilot who is now director of business development for NMD at Boeing in Anaheim, California, describes how Boeing uses commercial workstations and servers to power much of NMD.

"The future architecture of the program requires easy upgrades and replacements of these components. Plug and play equipment is a mandated design feature."

The need to move quickly and control costs in a program that is already under intense political scrutiny dictates that NMD is built on standard equipment from companies like Motorola and Intel that power home PCs.

The pace of change in IT means that NMD will not be using leading-edge computers. Unless the program formalises IT specifications at certain stages, it will become mired in constant, distracting updates.

As Lt Col Rick Lehner with the Pentagon NMD office puts it: "A lot of commercial stuff has to be used for NMD. Obviously, computing capacity has increased 1,000-fold in recent years. So we probably do not have the latest generation of computers in place. This technology moves so fast you have to lock it down at one point."

Critics of NMD point to the fact that in two of the three NMD test interceptions so far the EKV has not hit its target. But NMD supporters say that the two unsuccessful tests threw up engineering problems rather than fundamental flaws in the system. As Raytheon's Mr La Due puts it: "The science all works."

Both France and Italy are studying the application of existing missile technology to protect against medium-range missiles. Whatever the technical hurdles faced by the EKV, the threat it is aimed at is being taken very seriously today.