"B" followed, without a hyphen, by the sequential number of the "physics package" it contains. The "B61", for example, was the primary bomb in the US arsenal for decades. Various air-dropping techniques exist, including toss bombing, parachute-retarded delivery, and laydown modes, intended to give the dropping aircraft time to escape the ensuing blast. The earliest gravity nuclear bombs (Little Boy and Fat Man) of the United States could only be carried, during the era of their creation, by the special Silverplate limited production (65 airframes by 1947) version of the B-29 Superfortress. The next generation of weapons were still so big and heavy that they could only be carried by bombers such as the six/ten-engined, seventy-meter wingspan B-36 Peacemaker, the eight jet-engined B-52 Stratofortress, and jet-powered British RAF V bombers, but by the mid-1950s smaller weapons had been developed that could be carried and deployed by fighter-bombers. Ballistic missile Missiles using a ballistic trajectory usually deliver a warhead over the horizon, at distances of thousands of kilometers, as in the case of intercontinental ballistic missiles (ICBMs) and submarine-launched ballistic missiles (SLBMs). Most ballistic missiles exit the Earth's atmosphere and re-enter it in their sub-orbital spaceflight. Placement of nuclear missiles on the low Earth orbit has been banned by the Outer Space Treaty as early as 1967. Also, the eventual Soviet Fractional Orbital Bombardment System (FOBS) that served a similar purpose—it was just deliberately designed to deorbit before completing a full circle—was phased out in January 1983 in compliance with the SALT II treaty. An ICBM is more than 20 times as fast as a bomber and more than 10 times as fast as a fighter plane, and also flying at a much higher altitude, and therefore more difficult to defend against. ICBMs can also be fired quickly in the event of a surprise attack. Early ballistic missiles carried a single warhead, often of megaton-range yield. Because of the limited accuracy of the missiles, this kind of high yield was considered necessary in order to ensure a particular target's destruction. Since the 1970s modern ballistic weapons have seen the development of far more accurate targeting technologies, particularly due to improvements in inertial guidance systems. This set the stage for smaller warheads in the hundreds-of-kilotons-range yield, and consequently for ICBMs having multiple independently targetable reentry vehicles (MIRV). Advances in technology have enabled a single missile to launch a payload containing several warheads.