tion.20 The ABL’s weapons are not well suited for attacking an incoming SAM, and they would offer no defense at all for an attack beyond the field of regard of the turret for the rear 120 degrees of azimuth. Ideally, establishing air supremacy would include taking out all the air defenses like SAMs.

Establishing air supremacy before the outbreak of hostilities would be very provocative and could itself lead directly to hostilities. Thus, it is more likely that the ABL would be used only after hostilities had begun, and that it would therefore be of limited value in stopping the first attack from an enemy.

Space-Based Boost-Phase Defense21

Space-based boost-phase defense can avoid the geographic limitations of terrestrial systems and can in principle engage even ICBMs from a large country. However, it would require hundreds and perhaps several thousand interceptors stored in orbit and would be more than 10 times as expensive than any other defense alternative.

Because geography is a fundamental constraint on terrestrial-based boost-phase defenses, advocates for space-based boost-phase intercept argue that putting the interceptors on orbiting satellites eliminates that constraint. On the other hand, a spaced-based system would face two other daunting constraints. Like all other proposed boost-phase defense systems, they must get from their orbital bases to the threat missile just as fast, with the same very short decision times, and to be close enough to do so. If the interceptors are to be constantly in a position to intercept, many platforms would be needed that must obey the laws of orbital mechanics. In addition it would be very expensive to put them into orbit and sustain them there in the first place.

Figure 2-17 illustrates how SBIs fly out from their storage orbit to engage ascending missiles during their boost. The SBI circles Earth in its storage orbit at some velocity and altitude. When dispatched to intercept a missile during boost, the SBI fires, adding more velocity to reach the threat. That fly-out velocity can be added in any direction after the interceptor is dispatched at t0. While space-based interceptors do not have to deal with flying out through atmospheric drag, and therefore for similar fly-out velocities can be smaller, they have to be boosted into orbit along with their host vehicle or garage. There is therefore a very great sensitivity to each kilogram of mass in the space-based interceptor, particularly in the kill stage. Specifically, after being boosted into orbit, the kill stage must be flown out from the orbiting garage at high acceleration and greater velocity in order to maximize its reach in the optimum-sized constellation. The result is a need for hundreds of SBIs to handle just the relatively slow liquid-propellant



21Additional information and analysis can be found in the classified annex (Appendix J).

The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement