The September 11, 2001 terrorist attacks have reshaped whole swaths of debate over U.S. foreign and national security policies. Certainly, the issue of homeland security is a case in point. In that context, it was inevitable that the various partisans and detractors of national missile defense, and those with contending views of how homeland security should be organized, would use the September 11 tragedy as evidence for their particular position. And they have. For example, those who have held the very idea of national missile defense to be a form of inanity-if not insanity-have argued that since no imaginable deployment of national missile defenses could have prevented the September 11 tragedy, this proves how bad an idea it is. This is a little like arguing that if a person has purchased homeowner's insurance, he or she has no need for auto or life or medical insurance. The dangers to the security of a nation are multiple, no less than the dangers to the security of individuals.

That said, it does not follow that all kinds of insurance policies are equally necessary or that all insurance products are equally wise and cost-effective investments. All such policies and products require study, for nations no less than for individuals. Many such studies are going forward. But while there are ongoing and fairly well-rehearsed debates afoot over homeland security organization and national missile defense, one area that has slipped through the cracks of public consciousness concerns defense against cruise missiles. The analysis that follows represents a study of the technical requirements and costs of a defense against cruise missiles.

A National Cruise Missile Defense

The United States today utterly lacks an effective cruise missile defense plan. But apart from the obvious post-September 11 concern about the hijacking of domestic flights, defending against cruise missiles has probably become the most challenging air defense problem for the United States in this era.

Groups such as the Rumsfeld Com-mission focused elsewhere but did note the possible threat of cruise missiles, including those launched from ships off U.S. coasts-and they were right to do so. Cruise missiles are prevalent around the world, with 75 countries owning a total of about 75,000. Most at present are antiship missiles, but a number of countries are working on converting some to land-attack variants. Cruise missiles are small and relatively easy to hide on ships or other vehicles that could approach U.S. territory before the missile was fired. They are hard to detect when launched or even as they approach their targets, not only because of their small size but also because of their modest infrared heat signature (especially by comparison with ballistic missiles) and their ability to fly low, using terrain for cover.

Indeed, cruise missiles are small and inexpensive enough that it may not be beyond the means of terrorists to acquire them. Reconfiguring a standard cruise missile to carry a primitive nuclear warhead, likely to weigh half a ton or more, is probably beyond the abilities of terrorists, but outfitting a cruise missile with a dispensing mechanism for distributing chemical or biological agents or radiological materials may be feasible. In this sense, the cruise missile threat to the United States should be construed as one that could be posed by terrorists as well as other states seeking a means of coercion or deterrence.

To reliably protect the country against cruise missiles is admittedly a very difficult proposition, given the multiplicity of possible launch points, approach trajectories and targets. But a system of radars, perhaps held up by aerostat balloons, together with the existing network of U.S.-based fighter aircraft and a new series of surface-to-air missile sites, could provide at least some coverage of all of the nation's borders. That network might not provide leakproof defense in all cases, but it could stop most small attacks with high confidence and deny any attacker certainty that his cruise missiles would reach U.S. territory once fired.

Technologies are being developed that could perform some of these tasks. But the pace of research is too slow, with a target date of roughly 2010 for completing a master plan on cruise missile defense for overseas battlefields. Cruise missile defense research efforts should grow to at least the cost of individual Theater Missile Defense (TMD) programs-$200 to $300 million a year above current levels.

What would be involved in eventual deployment of a cruise missile defense, something that, owing to the nature of the threat, might have to be attempted in the course of this decade on roughly the same time frame as deployment of ballistic-missile defense? Given that the United States typically spent well over $10 billion a year on air defense during the early decades of the Cold War, when it worried seriously about the Soviet bomber force, the scale of the necessary effort could be substantial. Large numbers of radars as well as widely-distributed interceptor missile bases would likely be needed to defend the vast perimeter of the United States, including non-continental states and possessions.

More specifically, the concept of operations for national cruise missile defense might look roughly like this. As an outermost layer of defense, Navy ships might intercept foreign ships known to have no legitimate business anywhere near American waters or the U.S. exclusive economic zone (which extends 200 nautical miles beyond American terra firma). However, this approach would likely work only against the vessels of certain foreign navies; it would risk violating international laws of shipping if employed wantonly beyond U.S. coastal waters (which extend out only twelve nautical miles from the coast). As a second and better layer, the Coast Guard could monitor ship traffic, using its new requirement for 96-hour notice of port visits to find commercial ships with suspicious owners, crews or cargoes. Its cutters could intercept and board such ships, generally tens of miles from land.

Clearly, even if they realized their maximum potential, such Coast Guard efforts could fail to prevent the launchings of missiles from dozens of miles off American shores. Both the Coast Guard and the Air Force would also be hard-pressed to prevent the launchings of cruise missiles from commercial aircraft secretly outfitted for such purposes. For these reasons, actual defenses against cruise missiles would be needed, too.

To provide such defenses, the United States would need to see the missiles coming, recognize them as cruise missiles and not small aircraft, and then launch nearby interceptors quickly enough to destroy the missiles before they could reach American shores. Accomplishing the detection mission requires continuous coverage of all approaches to U.S. territory by a system of radars based on land, ships, aircraft or balloon aerostats. Accomplishing the intercept mission, by contrast, requires one to estimate the distance at which enemy missiles might be launched. Clearly, if ships or planes could launch missiles arbitrarily close to U.S. territory, even a very dense network of shore-based interceptors could prove insufficient. Enemy forces could approach to within say a couple miles of their target, then fire their missile or missiles; unless interceptors were located within a few miles of any such point, they could not reach the enemy missile quickly enough to prevent it from striking its target.

So how much warning would likely be available, and from how close could enemy missiles realistically be launched? Assuming that the main threat of cruise missile attack would arise from ships, the problem may be solvable. If Coast Guard monitoring can work effectively, suspicious ships entering U.S. territorial waters and contiguous zones should certainly be identifiable. Assuming that they could be stopped at the outer edges of contiguous zones-and fired upon immediately if they did not-it should be possible to prevent the launch of cruise missiles from closer than roughly twenty nautical miles or about 25 standard miles. For a relatively simple subsonic cruise missile, that might correspond to five minutes of flight time in which interceptors could do their work before targets on land were struck.

These timelines translate into a difficult but not impossible job of protecting coastal regions. If defenses had to be deployed uniformly around American coastlines, and interceptor missiles could accelerate quickly and then travel at roughly one kilometer per second, it might suffice to have a base of several interceptors every fifty miles or so. If, however, the main goal was to protect larger towns and cities as well as key ports and infrastructure, fewer interceptors would be required along certain stretches of coastline, where spacing might be every 100 to 200 miles. Either way, given that the length of U.S. coastal zones is several thousand miles (even after one "smooths out" complex coastlines and waterways, and focuses just on the main perimeter of the country), many dozens of interceptor bases would be needed.

What about radar coverage? Here the key variables are the altitude at which the radars can be situated, together with their power and range. Radars on the ground, even if on hills, cannot see very far due to the curvature of the earth and the low altitudes at which cruise missiles customarily fly. If cruise missiles could fly as low as fifty feet, and radars often could not be located near shorelines on hills higher than 100 to 200 feet (for illustrative purposes), radars would be needed every fifteen to twenty miles all around the perimeter of the United States. By contrast, if airplanes or aerostats at several thousand feet altitude could be used to provide surveillance and targeting information, they could be spaced every 100 to 200 miles, reducing the total need to several dozen for the entire country. However, two or more such radars would probably be needed to keep one platform on continuous station.

What total costs are implied by such a cruise-missile defense architecture? Short-range missiles would typically cost perhaps $1 million to $3 million apiece; their associated ground installations would add to that tab. If ten were based at each of 100 locations, total acquisition costs for the missiles and their ground support might be $5 billion to $10 billion. Radars based in the air might cost anywhere from several tens of millions to several hundred million each, depending on their sophistication and on how successful engineers may be at finding inexpensive solutions. If 100 were needed at $100 million each, costs would be $10 billion. If, by contrast, costs could be held to those for advanced unmanned aerial vehicles such as the Global Hawk, expenses might be half as much or less. In addition, given the long endurance of that aircraft, fewer total aircraft might be needed to keep a certain number in the sky at a time, so total acquisition costs for the airborne platforms might be held to $3 or $4 billion.

Operating costs would of course be additional. For an unmanned aerial vehicle (UAV), annual operating costs might be $3 million to $5 million. For a medium-sized aircraft, costs would more likely be $5 million to $10 million each year. Assuming roughly 100 such aircraft, annual costs would thus be $300 million to $1 billion, depending on the capabilities of the airframe and the radar.

All told, a rudimentary cruise missile defense for the United States could probably cost $10 billion to $20 billion to deploy. It would cost roughly as much to operate over a twenty-year period. Overall, these figures would represent a modest investment on the scale of national ballistic missile defense, but a large number relative to most other homeland security requirements.

These numbers ignore several other factors that could change costs significantly. However, two of them would tend to raise costs, and the others would tend to lower them, so the net effect would likely be quite modest, and the above estimates might turn out to be reasonably accurate.

One factor that would raise costs would be the need for command, control and battle management infrastructure to connect radars to each other and to interceptors. As one guidepost to possible costs, command and communications facilities for the proposed midcourse ballistic missile defense system designed by the Clinton Administration are expected to cost just over $2 billion (or about 10 percent of the total). In addition, costs of $1.5 billion were expected for construction of major sites; similar demands would arise for the cruise missile defense discussed above.

Lower costs would likely result from the fact that existing Federal Aviation Administration radars might make it unnecessary to purchase and deploy radars in some parts of U.S. coastal regions. In addition, as noted above, aerostat balloons with very long endurance might be able to replace ground radars in a number of locations at considerably lower cost.

This analysis shows that a defense against ship-launched cruise missiles is desirable and achievable, but difficult and expensive. As with all other defense systems, any cruise missile defense effort would have to compete with other programs of roughly equal importance. If we rule out, as we should, both technological impossibility and technological inevitability arguments, and if we recognize that resources for defense are far more elastic in a national crisis than almost anyone thinks they are in normal times, then the question of cruise missile defense falls into the familiar and proper context of political judgments about competing needs. Such judgment, however, needs to be exercised. The United States should seriously consider the desirability and feasibility of a national defense against cruise missiles armed with chemical, biological or radiological payloads.

Michael O'Hanlon is a senior fellow at the Brookings Institution.