Missile Defense: How Vulnerable Is Israel to Iran’s Attacks?

Missile defense has reemerged as a topic of great international interest following Iran’s historic, direct attacks against Israel in April and October this year. Israel and its close military partners, including the United States, effectively neutralized both strikes, but their success belies the technical complexity of such a multinational missile defense effort, which could get more difficult if hostilities escalate. The United States, meanwhile, will have to weigh potential security trade-offs in other parts of the world if its forces remain committed in the Middle East.   

How does missile defense work?

In responding to the April and October attacks this year, Israel and its partners employed a “layered defense,” which means it used several different systems to engage various kinds of targets at separate points in their flight, such as ballistic missiles, cruise missiles, and drones. Layered defenses are essential because each of these fly at different speeds and altitudes and vary in their ability to maneuver in flight and deploy countermeasures.

Iran’s salvos included a combination of all these weapons. Cruise missiles are powered by an engine that burns fuel, which limits their range. They’re cheaper to build and easier to launch than ballistic missiles.

Ballistic missiles are fired into orbit by a booster and travel at altitudes up to several hundred kilometers, depending on the missile. They don’t require an engine because they use the earth’s gravitational pull to travel through space, and they reenter the atmosphere above their target. Ballistic missiles can go farther and faster than cruise missiles because most of their travel is unencumbered by atmospheric drag. Their high speeds after reentry—traveling several times the speed of sound—make them very difficult to shoot down in the so-called terminal phase.

For a defender like Israel, one way to solve this is by shooting down ballistic missiles in their mid-course phase, while they’re still in space, immediately prior to re-entering the atmosphere. This, too, is hard because detonating explosives in space doesn’t produce a shock wave. Defenders can’t simply explode an interceptor in the missile’s proximity to destroy it; they must hit it directly.

Israel’s layered defense systems include David’s Sling, Iron Dome, Arrow 2, and Arrow 3. Allies and partners have contributed shipboard ballistic missile defense, which are on some U.S. Navy destroyers, as well as Patriot anti-air batteries, and fighter aircraft that can engage cruise missiles and drones, which are slower-moving targets.

Why is multinational cooperation so important?

The responses to Iran’s two missile attacks were a multinational effort of remarkable sophistication. Only a few militaries in the world possess the necessary technology and can accomplish the degree of organizational integration that the United States, France, the United Kingdom, and Israel demonstrated. Each ally brings unique defensive capabilities designed for the different kinds of targets.

During the April attack, for instance: French and British aircraft shot down drones; U.S. Navy destroyers intercepted ballistic missiles; and Israel used its Arrow, David’s Sling, and Iron Dome systems against missiles and drones at varying altitudes. In the days following Iran’s October strikes, the Pentagon announced it would further bolster this regional missile defense network by temporarily deploying a Terminal High-Altitude Area Defense (THAAD) battery, along with dozens of U.S. troops to operate it. The deployment is in part a recognition that nearly any country, including Israel, needs help in effectively defending against such a large barrage of missiles (in April, the success rate was 99 percent, the Israeli military said).

Since radars, launch platforms, and interceptor missiles are optimized for different threats, allies need to assign responsibilities among themselves ahead of time. Once an attack begins, warnings from advanced infrared satellites and other intelligence sources have to be rapidly gathered and communicated to defenders. Every second counts. The ability to share tipping and cueing among allies to help refine the fire control for the defensive systems is essential. While responsibilities can be designated before time, the shooting of interceptors must be quarterbacked in real time.

With especially large inbound salvoes of missiles, the defenders need to prioritize targets among them, as it could be impossible to intercept them all. This is possibly what happened during the October attack, when several Iranian missiles reached their targets and damaged Israeli air bases. Israel may have de-prioritized intercepting these missiles, judging that missiles headed toward populated areas were more important to destroy.

Does Iran have any advantages to exploit?

Faced with a single salvo attack, a defender such as Israel may not be particularly challenged if they have an adequate stockpile of interceptors. But if an aggressor such as Iran makes repeated attacks over time it can become a strategic problem: as the tit-for-tat exchange plays out over weeks or months, the defender spends down its missiles faster than the attacker. Israel faced one attack in April, another in October, and it could yet face another.

The challenge is heightened by the fact that sometimes multiple interceptors need to be fired to ensure a single missile is destroyed. Even the most sophisticated defensive systems can’t hit their targets with 100 percent accuracy, partly for reasons described above (e.g., the need to hit inbound missiles directly). Instead, missile defenses hit their targets a portion of the time. Even a 90 percent “probability of kill” may not good enough if the inbound ballistic missile is headed toward a populated area or critical infrastructure, for example. Therefore, to achieve a higher certainty of intercept, the defender would fire two or more interceptors at a single missile, putting the exchange ratio in favor of the attacker.

The problem is compounded by limited magazines (the firing capacity of a ship or land-based interceptor). For instance, during both of Iran’s attacks, U.S. Navy ships helped intercept ballistic missiles. Out of a U.S. destroyer’s 90 vertical launch cells, however, not all are dedicated to interceptors. Some of those cells are reserved for the ship’s self-defense capabilities, such as anti-submarine rockets. The more salvos a ship is forced to engage, the sooner it has to return to port and reload, which takes time. Not all of the navy’s destroyers and cruisers are equipped for ballistic missile defense, either. Land-based missile batteries, too, must rearm. Therefore, missile defense has as much to do with the defender’s in-theater magazine depth and industrial base capacity to produce interceptors, as it does with its general technical capabilities.

Are there broader security implications for the United States?

Iran is not the only threat the United States faces. Over time, the U.S. military can’t necessarily afford to have its ships constantly expending interceptors. This is why missile defense can be a strategic challenge—one that has to do with questions including defense production, as previously noted, but also alliance relationships, and political questions such as threat prioritization. The United States should also spread its limited missile defense capabilities against other looming threats from China, North Korea, and Russia. The constant expenditure of resources, and the potential that allies’ tolerance for repeated joint defensive efforts may wane with time, will undoubtedly influence how U.S. and Israeli leaders think about what to do next.

Will Merrow created the graphic for this Expert Brief.