During a typical intercept attempt, the closing speed between the kill vehicle and targets is around 10 kilometers per second. If targets can be detected from a distance of 600 kilometers, that doesn’t leave much time-a minute or less-to distinguish between warheads and decoys and maneuver to ram into the right target. The resolving power of the kill vehicle’s telescope is quite limited, so all objects look like points of light. Still, the distinction can be made-by measuring the brightness of each object, and to some extent its wavelength or “color,” which in turn can give clues to its infrared temperature.
If, for instance, one object is a tumbling, featureless sphere, no orientation will look different from any other, and its signal will be steady. However, if another object is of a different shape, the different faces it presents to the kill vehicle will show varying degrees of brightness as it tumbles end over end through space; a rod, for example, will be brighter when its more luminous side area is exposed to the telescope than when viewed end-on and will appear to the kill vehicle as a distant point of light that increases and decreases in brightness twice during each complete rotation. So if there is prior knowledge that one target is a tumbling rod and the other is a featureless sphere, it will be clear which is which.
That’s the theory. The truth is more complicated. For one thing, measuring temperature with this infrared equipment is not possible when objects in space are observed close to the earth, because their signals are routinely contaminated by reflected infrared radiation from the planet’s surface; they are further confused by such factors as the amount of cloud cover, time of year and which part of the earth the target is over.