As envisioned since 1996, the U.S. National Missile Defense effort consists of three main elements: infrared early-warning satellites, ground-based radars to precisely track warheads and decoys from thousands of kilometers away, and multistage, rocket-powered homing interceptor missiles launched from underground silos. The most critical element of this defense is the roughly 1.5-meter-long “exoatmospheric kill vehicle” that the homing interceptor deploys after being launched to high speed by its rocket stages. After deployment, the kill vehicle has about a minute to identify the warheads in a cloud of decoys as it closes on the targets at high speeds. To that end, it carries its own infrared telescope and has small rocket motors that enable it to home in on its prey. The kill vehicle does not carry a warhead. Rather, it is designed to destroy its quarry by force of impact.
When an enemy missile is launched, it typically takes 30 to 60 seconds to reach altitudes where the infrared early-warning satellites can detect the hot exhaust from its engines. These satellites orbit at an altitude of 40,000 kilometers and can be kept over the same point on the earth’s surface. Once two or more detect the rocket, they can crudely track it in three dimensions by stereo-viewing. However, the satellites can only see the hot exhaust from the rocket’s engines, so their tracking ends abruptly when the engines shut down-an event that typically happens in space at between 200 and 300 kilometers in altitude.
Roughly three minutes after engine shutdown, the rocket’s upper stage and the just released warhead and decoys rise above the horizon, where they can be tracked by radar. The radar systems originally planned for this task operate on a very short wavelength (three centimeters at a frequency of 10 gigahertz), which allows them to identify objects to an accuracy of 10 to 15 centimeters from many thousands of kilometers away. This makes it possible to observe distinct reflections from different surfaces-even the seams on an object as it tumbles through space. The spacing and intensity of these signals, and the way their echoes vary as the orientation of a target object changes, can in some circumstances be used to determine which object is a warhead and which a decoy. If all goes well, this information will be used to deploy one or more interceptors within about 10 minutes of an attack’s being launched. The interceptors will fly to the defense, destroying their targets about 18 minutes after launch (see “Space-based vs. boost-phase defense”).
That, at any rate, is how the system was initially supposed to work. President Bush’s latest proposal does not include this high-resolution radar, making tracking and identification of enemy missiles harder and delaying the interception time. But even with the more advanced original system, big problems surround the scenario. For starters, an adversary could alter the reflections from decoys and warheads by covering surfaces and seams with wires, metal foil or radar-absorbing materials. These simple strategies would render the radar unable to reliably sort out warheads from their armadas of decoys.
Compounding this problem is a simple fact: in the near vacuum of space, a feather and a rock move at the same speed, since there is no air drag to cause the lighter object to slow up relative to its heavier companion. This basic vulnerability makes it even easier for an adversary to devise decoys that will look like warheads to radar or an infrared telescope observing them from long range.
What’s more, an adversary would likely deploy decoys and warheads close together and in multiple clusters. Under these conditions, even if the radar could initially identify a warhead among all the decoys, it couldn’t track it accurately enough to predict the relative locations of the different objects when the kill vehicle encountered them some eight minutes later. Consequently, the kill vehicle must be able to identify warheads and decoys without help from satellites, ground radars or other sensors. If it cannot perform this task, the defense cannot work. This is where the infrared telescope comes in-and it was really this critical part of the system that the June 1997 test was all about.