The report said this could be countered by simply increasing the thrust of the missile. Existing missiles fired for about three to four minutes, with at least half of that taking place outside the atmosphere. They showed it was possible to reduce this to about a minute, timing things so the motor was burning out just as the missile was reaching the upper atmosphere. If the warheads were quickly separated at that point, the defense would have to shoot at the individual warheads, thus facing the same poor cost-exchange ratios that had made the earlier ABM systems effectively useless. And once the rocket had stopped firing, tracking would be far more difficult.