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The Sentinel project, which began in 1967, was a two-tiered anti-ballistic missile system designed to protect U.S. cities against a limited intercontinental ballistic missile (ICBM) attack from either China or the Soviet Union. Although never deployed, Sentinel’s successor, Safeguard, was deployed in North Dakota in 1975.(1)

By 1966, it had become evident that the Soviet Union was deploying its own anti-ballistic missile shield, System A-35. Pressures on the Johnson administration to deploy Nike-X, its current missile defense project, intensified. The Joint Chiefs of Staff recommended that the U.S. start building its own limited area defense of the U.S., initially to defend 25 cities, and later to protect 52 cities. The system was intended to reduce total casualties in the event of a nuclear war with the Soviet Union.(2)

In June 1967, however, China exploded its first nuclear device, demonstrating that it too had entered the nuclear arena. The Johnson administration decided to reprioritize its anti-ballistic missile efforts away from the threat of large scale Soviet ICBMs, and towards the emerging threat of a smaller Chinese attack.(3) On September 18, 1967, Secretary of Defense McNamara announced that existing Nike-X technology would be used as the basis for a new system known as Sentinel, intended to provide a thin defense of the U.S. population against the threat of a Chinese attack.(4)

A two-tiered system, Sentinel would deploy two new interceptor missiles, the long-range Spartan and the short-range Sprint. The Spartan, which had been under development since 1965, was designed to detonate a nuclear warhead above the atmosphere, where it would generate intense X-rays that would be able to knock out several incoming reentry vehicles simultaneously. It had an effective range of 600 to 800 km, and could intercept incoming ballistic missiles at altitudes above 100 km during the last few minutes of their midcourse phase.

The Sprint, by contrast, was designed to destroy incoming missiles during their terminal phase, i.e. within the earth’s atmosphere during the last moments of descent. An offshoot of the Nike-X, Sprint’s main advantage was that it could use the atmosphere to filter out countermeasures and decoys. Its disadvantage was that its battle space was compressed into the last few seconds before the incoming missile’s impact. The Sprint interceptor, like the Spartan, was equipped with its own nuclear warhead.

The Spartan interceptors were to be supported by the Perimeter Acquisition Radar (PAR), a phased-array radar capable of detecting incoming missiles outside the atmosphere and determine their trajectory within three seconds. The Sprint interceptors were to be supported by the Missile Site Radar (MSR), designed to track the incoming warheads and guide the missiles to their assigned targets. Combined, the PAR and the MSR were to form a layered radar shield, through which no enemy missile could penetrate.(5)

Despite its advantages, the Sentinel project was widely criticized. The principle objection, as noted by Gregory H. Canavan, was that the long-range Spartans would not work, and therefore the short-range Sprint interceptors would be negated by large numbers of incoming missiles, as well as decoys and countermeasures. Thus, the logic went, it was impossible for Sentinel to provide an adequate defense of the U.S. population, and for these reasons it was not worth risking an escalation in the Cold War.(6) Opponents of ABM technology were able to mobilize opposition from groups living near the proposed deployment areas.(7)

The debate over Sentinel continued through 1969, when the Nixon administration took over. In January of that year, Secretary of Defense Melvin Laird suspended the Sentinel deployment and ordered a review of the program. In March, President Nixon announced his intention to deploy a new system known as Safeguard, which was designed not to defend U.S. cities, but rather the ICBM silos in North Dakota.(8)

Footnotes

1. Donald R. Baucom, The Origins of SDI, 1944-1983 (Lawrence, KS: University Press of Kansas, 1992), 34-50; Bradley Graham, Hit to Kill: The New Battle Over Shielding America from Missile Attack (New York: Public Affairs, 2001), 7-8.
2. Stephen P. Moeller, “Vigilant and Invincible,” Air Defense Artillery Magazine (May-June 1995).
3. Council on Foreign Relations, “The Missile Defense Debate, A-to-Z Encyclopedia of National Missile Defense,” available at http://www.cfr.org/virtualbooks/reference/glossary/glossary_s_z.html, accessed on January 10, 2005.
4. Council on Foreign Relations, “The Missile Defense Debate, Chronology of National Missile Defense Programs,” available at http://www.cfr.org/virtualbooks/reference/timeline.html, accessed on January 10, 2005.
5. Moeller, “Vigilant and Invincible.”
6. Gregory H. Canavan, Missile Defense for the 21st Century (Washington, DC: The Heritage Foundation, 2003).
7. Baucom, 208; U.S. Congress, Office of Technology Assessment, Ballistic Missile Defense Technologies, OTA-ISC-254, (Washington, DC: U.S. Government Printing Office, September 1985). 
8. Council on Foreign Relations, “The Missile Defense Debate, Chronology of National Missile Defense Programs.”