Phase III: Midcourse Intercept
After phases I and II are completed, the midcourse phase, lasting some twenty minutes, begins. During this phase the individual RVs (perhaps originally amounting to ten thousand, but perhaps greatly reduced by the interceptions effected during the previous phases) ineluctably follow ballistic trajectories toward the targets. They still emit infrared radiation, but at longer wavelengths and at far lower power levels than during the previous phases. Indeed, the individual RVs are about the same size and temperature as a human being. One may therefore understand the problem by comparing it to the task of detecting a human being just by the heat the body radiates using a device located thousands of miles away.
As in phases II and IV, perhaps the most effective countermeasure in Phase III is the deployment of decoys. As long as the RVs remain in outer space, the decoys may be of very low density—in the form, for example, of metallic balloons. There is no drag, and only the surface characteristics of the decoys—shape, reflectivity, emissivity—need to mimic the real thing. The decoys are usually thought of as weighing one one-hundredth as much as the RV itself, so there could be about a hundred times as many of them.
Those who are optimistic about SDI's prospects generally believe that detection and analysis of the emissions from the RVs and the decoys will enable discrimination. To distinguish the RVs from the decoys, they propose the use of various means of "active" or "interactive" discrimination. One such technique involves the use of laser beams that would
transfer enough momentum to a lightweight balloon to change its velocity by an amount that could be detected by a Doppler laser radar. Another involves the use of a beam of high-energy, neutral hydrogen atoms that penetrate deeply into even the heavy RVs and interact very differently with balloons and decoys. It is also assumed that interceptors similar to those employed in the initial phases would also be used here. For both detectors and interceptors, however, the task is much harder in Phase III. As a result, less attention has been devoted to interactive discrimination, and none of the near-term deployment schemes focuses on it. An elaborate experiment, code-named "Delta 181," was conducted in February 1988, at a cost of $250 million, to collect information on how objects look in space, including potential decoys. Some important elements of this experiment failed, but large amounts of data were produced, which have yet to be fully analyzed.