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 dependant 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, but critics were quick to point out that is not really the case. At first glance, the (original) $100,000 price of the basic pebble is considerably less than the price of an ICBM. 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