The Iron Dome missile defense system succeeded on Tuesday where it had been entirely ineffective during the 2014 Gaza war: in shooting down incoming mortar shells that are in the air for just 15 seconds.

“The Iron Dome has been quite effective,” IDF spokesperson Lt. Col. Jonathan Conricus told reporters. “It’s probably something [Gazan terrorist groups] didn’t take into account.”

This was not the first time that the Iron Dome has intercepted mortar fire, but Tuesday represented a significant test for the system, which the military believes it passed.

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Over the course of Tuesday, scores of mortar shells, and some rockets, were fired at southern Israel by the Hamas and Palestinian Islamic Jihad terrorist groups in the Gaza Strip.

Conricus said the shells, fired by the Iran-backed Islamic Jihad, appeared to be an Iranian-made variety that had been smuggled into the coastal enclave.

According to the Israel Defense Forces, Iron Dome batteries succeeded in shooting down at least 25 incoming projectiles over the course of the day, most of them mortar rounds, though the army said it had yet to calculate a final tally.

Mortar fire by terrorist groups in the Gaza Strip killed several Israelis during the 2014 Gaza war, including four-year-old Daniel Tragerman from Kibbutz Nahal Oz.

In the four years that have passed, Israel has worked continuously to improve the system to counter this threat. This included technological improvements and efforts to streamline the work of the soldiers operating the system.

The Walla news site reported that the army also changed the way in which the batteries were deployed along the Gaza border on Wednesday in order to give them a better chance of intercepting the dozens of mortar shells fired at southern Israel.

The Iron Dome system was initially designed to shoot down incoming rockets and missiles, which it did effectively during the 2014 conflict, known in Israel as Operation Protective Edge.

Over the years, the system has been upgraded to also be able to intercept drones and mortar rounds.

The Iron Dome’s first reported interception of mortar fire occurred in September 2016, when a battery in northern Israel shot down two incoming errant shells fired from Syria that were heading toward the Golan Heights, in an apparent case of spillover fire from the civil war there.

Mortar shells present a far greater challenge for the Iron Dome than rockets, owing to the shells’ shorter range — normally no more than four kilometers (2.5 miles).

In order for the Iron Dome to intercept an incoming projectile, the system must first spot it, determine based on its trajectory if it is heading toward a populated area and, if so, launch an interceptor missile to shoot it down.

Rockets and missiles remain in the air for far longer as they travel toward their target, which gives the Iron Dome’s automated systems and the soldiers that operate it more time to carry out these steps.

Mortar shells, on the other hand, are in the air for far less time. Residents of the Israeli communities closest to the Gaza border have 15 seconds to reach a bomb shelter once a mortar shell is fired.

Tuesday showed that the Iron Dome is capable of shooting down mortar shells in this amount of time, though military officials repeatedly stressed that the system was not perfect.

“Nothing is hermetic, nothing is 100 percent,” IDF Spokesperson Ronen Manelis told Israel Radio on Tuesday afternoon.

For instance, one mortar shell from the first barrage of the day exploded in the yard of a kindergarten in southern Israel’s Eshkol region, just before children were due to arrive.

A caretaker at the school was lightly wounded by shrapnel from the shell.

Another mortar shell struck a military position east of the southern Gaza Strip on Tuesday, injuring three soldiers.

Two of the servicemen were lightly wounded and released from the hospital a few hours later. The third was moderately injured and required surgery, a spokesperson for Beersheba’s Soroka Medical Center said.