kb/data/en.wikipedia.org/wiki/Brilliant_Pebbles-7.md

title	chunk	source	category	tags	date_saved	instance
Brilliant Pebbles	8/9	https://en.wikipedia.org/wiki/Brilliant_Pebbles	reference	science, encyclopedia	2026-05-05T13:24:19.440513+00:00	kb-cron

=== Promising cost-exchange === Earlier anti-ballistic missile (ABM) systems like Nike Zeus had the problem that they cost more than the ICBMs they were designed to shoot down; the United States would have to purchase $20 worth of interceptors for every $1 the Soviets spent on new ICBMs. In such a situation, the Soviets could defeat any possible ABM deployment simply by building more missiles. This was a major argument against ABM systems in the 1960s and 70s, and became known as the cost-exchange ratio. This led Paul Nitze to propose what became known as the Nitze criteria; to be successful, the marginal cost of adding to the defense had to be less than the cost of adding to the offense. If this is not true, then the simplest response to any new defense system is simply to build more offensive missiles. But if the defense is cheaper, this will not work, and the enemy will have to explore other solutions to address the imbalance. Ideally, they too would build defenses, ultimately rendering the offence impotent. Compared to earlier ground-based systems, the Smart Rocks interceptor missiles were relatively simple and low-cost. This meant the US could afford to launch several for every Soviet ICBM. However, they suffered the major flaw that they were dependent on support from the garage, and thus a single anti-satellite weapon could render all of the interceptors inoperative. Smart Rocks thus failed the Nitze criterion, as it was less expensive for the Soviets to attack the system than it would be for the US to build it. In contrast, the Pebbles interceptors flew independently, and thus to attack them the Soviets would have to launch an ASAT for each one. This would mean that developing countermeasures to the system would be on the same order of cost as the pebbles themselves, something the Soviets' weaker economy could not afford. This appeared to meet the Nitze criterion; they could not afford to build their way out of the problem either using ASATs or new ICBMs.

=== Absentee ratio === Critics noted a key flaw in this comparison; since it was only a pebble in the right place at the right time that could attack the ICBM, adding a single ICBM did not require one more pebble, but many more in order to fill out the orbit so one would be in the right area. In the case of Pebbles, this "absentee ratio" was on the order of 10-to-1, meaning that adding a single ICBM would require ten new pebbles, driving the cost much closer to parity. As the Union of Concerned Scientists pointed out early in the SDI program, any system that relied on boost-phase attacks had to be able to reach the target while the missile's motor was still firing. With existing Soviet ICBMs like the SS-18, this period lasted as long as six minutes. The US's Minuteman missile fleet only burned for four minutes, and the new MX missile was even less. The report went on to explore the ultimate end of such a "fast burn" approach, concluding that it was possible to build a missile that would launch and disperse its warheads in as little as one minute. Such a missile would require many dozens of pebbles for each one so that at least one of them was close enough to catch it, making the defenses much more expensive than the ICBM. The SDIO argued that such a response by the Soviets would be welcome; while the Soviets were deploying their fleet of new missiles to counter Pebbles, SDI would be on its way to deploying new systems based on directed energy weapons that could defeat those missiles. Critics noted that this meant the SDIO was arguing that Pebbles would lead to an offensive arms buildup, precisely the opposite of what they had previously claimed was the point of the entire SDI concept, and contrary to the Nitze criterion.

=== Other problems === Another issue raised was that the existing Soviet A-135 anti-ballistic missile system could be fired at the Pebbles. By timing such an attack moments before an ICBM launch, the A-135 system's 100 missiles could destroy those pebbles approaching the USSR and temporarily "punch a hole" for their ICBMs to fly through. Because of the absentee ratio, 1,000 additional pebbles would have to be added to the fleet to counter this possibility, not 100. This sort of attack would cost the Soviets very little. Finally, there was another overarching technical issue that affected all of the space-based weapons. Since the late 1970s, the Soviets had used ground-based lasers to "paint" United States satellites on a number of occasions, in some cases temporarily blinding them. The APS report noted that the amount of energy needed to do this was very low, far less than the amount of energy needed to destroy a missile. This meant that while it was still unknown whether one could ever build a useful directed-energy anti-ICBM weapon, it was already possible to build an anti-SDI weapon that would blind such a system's sensors. One commentator went so far as to note that protecting the optics was "impossible". Such a system could be used against the pebbles in a fashion similar to the A-135, rendering them useless for a critical period while ICBMs were launched.

== See also == Multiple Kill Vehicle

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