JERUSALEM—Israel said Wednesday it has begun testing a massive inflatable missile detection system designed to hover at high altitudes and detect long-range threats. Israel already boasts an array of sophisticated missile defenses, which were used successfully during the 11-day Gaza war this year. The High Availability Aerostat System resembles a giant blimp or zeppelin. The Defense Ministry says it’s one of the world’s largest systems of its kind. It was developed in cooperation with a subsidiary of state-owned Israel Aerospace Industries and TCOM, a U.S. aerostat manufacturer. Israel has moved aggressively in recent years to counter potential threats from Iran, the Hezbollah terrorist group in Lebanon and Gaza’s terrorist Hamas rulers, all of which boast large arsenals of rockets capable of hitting major cities. During the Gaza war, Hamas fired barrages of rockets at Tel Aviv in an attempt to overwhelm Israel’s Iron Dome defense system, but Israeli officials say …