Israel’s vaunted Iron Dome defense system is more like an iron sieve. It fails to destroy all but a few of the rockets that Hamas and other Palestinian militant groups fire at Israeli communities. But Israel’s early-warning civil-defense systems have proved highly effective. RELATED COLUMNS Uzi Rubin: Iron Dome is an ironclad success

EDITOR: Two views of Iron Dome's success The radar-guided Iron Dome missile, meant to intercept and smash incoming rockets in the seconds before they strike their targets, works just a small fraction of the time, according to a detailed analysis carried out by scientists at the Massachusetts Institute of Technology. The study was funded by the John D. and Catherine T. MacArthur Foundation and Ploughshares Fund.

Ted Postol, a physicist at the university and an expert in missiles and missile defenses, has found evidence that only about 5 percent of Iron Dome engagements result in the targeted rocket being destroyed or even sufficiently damaged to disable its explosive warhead. In the other 95 percent of cases, the interceptor either misses entirely or just lightly damages the enemy munition, allowing the rocket’s intact warhead to continue arcing toward the ground.

Postol based his conclusion on a careful analysis of amateur videos and photos of Iron Dome interceptions over the past three years. He admitted that most of his data is from a previous round of fighting in 2012. “The data we have collected so far [for 2014], however, indicate the performance of Iron Dome has not markedly improved,” Postol wrote on the website of the nonprofit Bulletin of the Atomic Scientists.

Richard Lloyd, another weapons expert, has also run studies that call into question Iron Dome’s high success rate. Other military analysts support his findings, though the Israeli government dismisses them, as it does the Postol study. An Israeli spokesman told the BBC, “The system saves lives.”

It should go without saying that guiding a missile to strike a particular spot on another missile is a very, very difficult achievement. The U.S. Missile Defense Agency likens rocket-on-rocket interceptions to “hitting a bullet with a bullet.”

The Israeli military, having spent billions of dollars on the system, appears to be exaggerating Iron Dome’s success rate. “Since the beginning of the operation, more than 1,260 rockets were launched from the Gaza Strip towards Israel,” the Israeli Defense Forces said on July 16, nine days into the latest spasm of violence pitting the Jewish state against Palestinian militias.

“Approximately 985 rockets hit Israeli territory and 225 rockets were intercepted by the Iron Dome missile-defense system,” the IDF stated, “with an overall success rate of 86 percent.”

Postol rejected that assessment. “The Israeli government is not telling the truth about Iron Dome,” the physicist asserted. Postol said if Iron Dome has such a high success rate, the Israeli government should release all the data it has.

When it comes to analyzing the effectiveness of missile defenses, Philip Coyle, who ran weapons testing at the Pentagon under the Clinton administration, told MIT’s Technology Review, Postol’s is “the best work that anybody has done outside the bowels of the Pentagon.”

Israel deployed the first Iron Dome missile battery in 2011. Each battery consists of a command post, a radar array and several launchers, each with 20 missiles. The United States has contributed more than $1 billion to Iron Dome’s development in exchange for access to the technology.

There is a lot of money – and credibility — invested in the system’s success.

Israel has so far purchased nine Iron Dome batteries from the manufacturer Rafael — and plans on buying several more. Each Iron Dome missile reportedly costs somewhere between $40,000 and $100,000, compared to less than a $1,000 apiece for the militants’ Qassam rockets.

The Israeli government has said the key point is not the cost but preventing Israeli deaths. “When we work, we do it to save lives,” Major Shay Kobninsky, a military Iron Dome commander, said in an official release.

“Every rocket intercepted would have hit populated areas,” the IDF added on its official blog. But Postol insisted that Iron Dome has not saved any lives. The fact that the Palestinian rockets kill so few Israelis — just two civilians have died in the recent attacks — is due to what Postol calls “civil-defense efforts.”

“Israel’s low casualty rate from Hamas rockets,” Postol wrote, “is largely attributable to the country’s well-developed early-warning and quick-sheltering system for citizens under imminent rocket attack.” Military radars and infrared sensors detect rocket launches the instant they happen. Air-raid sirens alert civilians to head for underground bunkers.

Iron Dome, Postol added, “appears to have had no measurable effect on improving the chances of Israelis escaping injury or death from Hamas artillery rocket attacks in Israel.”

By the time a rocket enters Iron Dome’s 40-mile engagement zone, it’s already arcing downward toward its target. One way or another, a part, or in some cases all, of the rocket is going to strike the ground. Iron Dome must strike an incoming rocket head-on to wreck its warhead and minimize the rocket’s destructive potential.

“If the Iron Dome interceptor instead hits the back end of the target rocket, it will merely damage the expended rocket-motor tube, basically an empty pipe, and have essentially no effect on the outcome of the engagement,” Postol asserted. “The pieces of the rocket will still fall in the defended area; the warhead will almost certainly go on to the ground and explode.”

Israel is not alone in pouring vast sums of money into ambitious missile-defense systems. The United States spends around $10 billion annually on a wide range of rocket interceptors that, like Iron Dome, have performed poorly in tests and combat.

Both countries want to be able to shoot down anything their enemies fire at them. But if Iron Dome is any indication, the technology just isn’t ready.

PHOTO (TOP): An Iron Dome launcher fires an interceptor rocket in the southern Israeli city of Ashdod, July 8, 2014. REUTERS/Baz Ratner

PHOTO (INSERT 1): An Iron Dome launcher fires an interceptor rocket in the southern Israeli city of Ashdod, July 9, 2014. REUTERS/Baz Ratner

PHOTO (INSERT 2): The entrance to an underground bomb shelter (L) is painted red near apartment blocks made from reinforced concrete in the Israeli town of Sderot, April 7, 2014. REUTERS/Finbarr O’Reilly

PHOTO (INSERT 3): Israeli police survey the scene after a rocket fired from Gaza landed in Ashdod, July 14, 2014. REUTERS/Amir Cohen

PHOTO (INSERT 4): People react as air raid siren sounds, warning of incoming rockets, and explosions are heard overhead as the Iron Dome anti-missile defence system intercepts rockets fired from the Gaza, in Tel Aviv, July 16, 2014. REUTERS/Finbarr O’Reilly