How Missile Defense Works (and Why It Fails)

Once a ballistic missile is fired into the air, a defender has only minutes to identify its precise trajectory and try to shoot it down.

The target, an enemy warhead, is inside an object about the size of a car that typically flies through the edge of space at many times the speed of sound.

In April and October, Iran launched two of the largest ballistic missile attacks in history at Israel. The world’s most advanced missile defense systems, deployed by Israel and the United States, stopped some of the missiles — but not all of them.

Neither attack caused major damage, but some Israeli and American officials said it may have been a matter of luck that it was not worse. After Israel struck back at Iran last week, Iranian officials threatened another round of retaliation, although their plans remain unclear.

Here’s why stopping a ballistic missile attack is hard.

Ballistic missiles like the ones Iran fired last month escape the atmosphere and accelerate to great speeds as they fall back down to Earth. It took only about 12 minutes for Iran’s missiles to reach Israel, analysts estimate. But there was far less time to make critical decisions about how to stop them.

Within seconds, satellites must detect the heat signature given off by a missile launch. Radars must find the missile and try to calculate its exact course.

A defensive missile called an interceptor must be fired soon after that to reach the incoming missile in time.

That’s all hard enough with one missile. But Iran fired a large volley of them last month — nearly 200 ballistic missiles in less than an hour. The goal appeared to be to overwhelm Israeli defenses.

Radars can only track so many targets at once, and launchers, once emptied, may need a half hour or more to reload.

Beyond that, if they are concerned about future attacks, targeted countries may also need to make a critical split-second decision to reserve valuable interceptors only for the incoming missiles that appear likely to do the most damage.

After Iran’s second barrage, in October, Israel said its defense systems had shot down many of the missiles, and those that struck appear to have caused limited damage.

But satellite imagery does show that the barrage, which analysts said used more advanced missiles, left dozens of craters at one air base, Nevatim. Had those missiles landed in a populated area, the death and destruction could have been extensive.

Israel’s best-known defense system, the Iron Dome, was built to stop short-range rockets, and is too slow and limited when it comes to ballistic missiles. For that, Israel relies on several more advanced layers of defense designed to counter ballistic missiles at different stages of flight.

The most advanced systems, long-range interceptors like the Arrow 3, operate in space, where ballistic missiles like those Iran fired spend most of their time. They are the first chance to stop a missile, but high above the atmosphere, there is no room for error.

Both an interceptor and an enemy missile shed the boosters that power them into space. Just two smaller vehicles remain, hurtling toward each other.

The interceptor seeks a direct hit to destroy the warhead. To home in, the interceptor carries sensors to track the enemy missile and thrusters to move toward it. But by the time an interceptor senses its target a mile away, it has only a split second to adjust.

That’s because missiles like Iran’s latest are only about three feet wide at the base by the time they are in space, and they are traveling about two miles every second.

If that weren’t hard enough, some ballistic missiles carry decoys to trick the interceptor. Debris leftover from the boosters can also confuse it.

It’s unclear how often interceptions above the atmosphere actually work. Governments tend to avoid disclosing specific interception rates, and they have every reason to present a positive picture, even when interceptions fail. So do the companies that manufacture the pricey systems.

When combined with U.S. antimissile systems in the region, Israel currently has the most layers of missile defense in the world. If outer-layer defenses fail to stop a missile, shorter-range systems that intercept missiles closer to the ground may have another chance.

But time runs out quickly. The closer a ballistic missile gets, the more dangerous it becomes. And even if a lower-altitude intercept is successful, the resulting debris can still be deadly.

If a missile succeeds in re-entering the atmosphere, often less than a minute remains before it strikes.

Defenses that work in the upper atmosphere — like Israel’s Arrow 2 or the THAAD system that the United States recently sent to Israel — must fire their interceptors within seconds.

As the missile nears the ground, close-range defenses like the Patriot system from the U.S. provide a final chance to stop it. But these systems have a range of about 12 miles and can only protect limited areas.

An attacker can draw on a variety of tactics. To distract the enemy, it can fire a volley of cheaper weapons timed to arrive at the same time as the ballistic missiles. This is what Iran tried in its April attack, but Israel and its allies appear to have been able to triage between the faster and slower weapons, using other defenses like fighter jets to counter them.

Israel’s retaliatory strike at Iran last week targeted missile production sites and degraded Iran’s ability to produce the kind of missiles it fired in October. The attack damaged at least one rocket production facility, as well as fuel mixers that make propellant for Iran’s missile fleet.

It is unclear how Iran will respond or whether it will fire another round of ballistic missiles, but if it does, the fundamental imbalance of missile warfare will remain: Firing a ballistic missile is a lot easier than stopping one. And making a ballistic missile is cheaper and faster than making an interceptor to shoot one down.

Over the course of a protracted conflict, it could become a question of which side runs out of missiles first.