Nation: THE ABM: A NUCLEAR WATERSHED

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issue of a missile gap, Johnson and McNamara capitulated to a certain extent. Relying on some of the components developed for Nike-X, the Administration approved the thin Sentinel system that had as its main mission the defense of the U.S. in the 1970s against attack from China. As side benefits, Sentinel could also be used to protect Minuteman sites and would guard against a stray incoming missile that might be fired—probably accidentally—by any nation possessing such weapons in the future.

The basic rationale for Sentinel is the expectation that a potential Chinese attack would be light in number (20 to 30 ICBMs) and use far less sophisticated weaponry than a Russian strike. Hence, it could be repelled. Proponents of a full ABM network, however, have viewed Sentinel from the beginning as merely the first step toward a full-scale anti-Soviet network. It was a development that McNamara fully expected when he warned: "The danger in deploying this relatively light and reliable Chinese-oriented system is going to be that pres sures will develop to expand it into a heavy Soviet-oriented ABM." McNamara was right. Last month, after Defense Secretary Laird halted actual in stallation of the Sentinel pending the Administration's review of the program, President Nixon said at a press conference: "I do not buy the assumption that the thin Sentinel was simply for the purpose of protecting ourselves from attack from Communist China." He added that a thick ABM system "adds to our overall defense capability."

How It Works

Regardless of whose missiles Sentinel guards against and in what depth it is employed, the system's technical concept is unchanged. Aimed over the Arctic (the shortest route), an attacking projectile from Russia or China would take less than 45 minutes to reach, say, New York or Chicago. For planning purposes, strategists calculate the flight time at 30 minutes. PAR, Sentinel's long-range eye, with a vision of some 1,500 miles, could be expected to detect the attackers 10 to 15 minutes after launch, leaving a warning time of 15 minutes. Unlike the old Zeus concept and its lim ited coverage, a single Sentinel site can sweep a broad area. Each radar-missile complex covers a bulbous "footprint" on the map, and only 15 overlapping tracks are necessary to provide a thin blanket for the U.S., although a full defense would require expanded facilities.

It would, of course, take a human decision that an attack is indeed under way — and authorization from the President — to trigger Sentinel. (An elaborate communications net follows the President everywhere to permit instant access to him.) Spartan is the first weapon to go. Carrying a warhead of approximately two megatons (equivalent in power to 2,000,000 tons of TNT) and propelled by a solid-fuel, three-stage engine, Spartan seeks its target where a thermonuclear explosion can do the least harm: beyond the earth's immediate atmosphere (more than 75 miles up) and up to about 400 miles away from Spartan's underground launch cylinder. Coming in for the kill at such a height substantially eliminates the danger of concentrated fallout, since radioactivity would be diffused. Also, the aim is not to blow up the incoming warhead, which would be nearly impossible. Rather, the airless environment would facilitate the transmission of X rays from