Don't know the answer, but they were at the limits of their combat radius and every little bit helped in the drag department. Multiple sources agree that the tanks were jettisoned.
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Operational planning
The distance between Israeli military bases and the reactor site was significant—over 1600 km (1000 miles), which meant the military forces would be operating without easy resupply capability, and would have to arc across Jordanian and Saudi territory. Additionally, Israeli intelligence could not guarantee accurate intelligence on the state of Iraqi defences.

After much deliberation, the Israeli military finally concluded that a squadron of heavily fueled, and heavily armed F-16As, with a group of F-15As to provide air cover and fighter support, could perform a surgical strike to eliminate the reactor site, without having to refuel.

Israeli military also decided that it was essential to destroy the reactor before it was loaded with nuclear fuel, in order to minimize the effects of the reactor's destruction on the civilian population. Many European scientists were working on the reactor, and Israel decided to strike on Sunday, when most scientists would not be at work.

Having intelligence that the reactor's fuel rods were scheduled to be shipped to Iraq from France, the Israeli cabinet—then under the leadership of Menachem Begin—authorized the operation.

[edit] The attack

Israeli Air Force F-16A Netz 243, aircraft flown by Colonel Ilan Ramon in Operation Opera.
Nose of the F-16A flown by Ilan Ramon in Operation Opera, showing the triangular emblem of the attack.Operation Opera was carefully planned for a Sunday to minimize the loss of lives of any foreign workers and the attack was timed for the late afternoon to provide the Israeli Combat Search and Rescue Team (CSAR) all night to search for any downed Israeli pilots.

An Israeli Air Force flight of 8 F-16As—aircraft 107, 113, 118, 129, from 117 Squadron and 239, 240, 243 and 249 from 110 Squadron, each with two unguided Mark-84 2,000-pound delay-action bombs, and external fuel tanks—was set up. A squadron of 6 F-15As was also assigned to the operation to provide fighter support for the F-16As. Ilan Ramon, an F-16A pilot who would become Israel's first astronaut and died in the Space Shuttle Columbia disaster in 2003, during the time of the attack he was the youngest of the participants. He was two weeks away from his 27th birthday.

On 7 June 1981 at 15:55 local time (12:55 GMT) the plan was set in motion. The task force left Etzion Airbase, flying unchallenged at 800 feet in Jordanian and Saudi airspace.[8]

At 1,000 km into their flight, the operation was complicated by the F-16As' external fuel tanks. The planes were so heavily loaded that the external tanks (two underwing 1,400 l and one belly 1,100 l) were exhausted while the task force was still en route to the Osirak facility. These tanks were jettisoned over the Saudi desert before reaching the target.

Upon reaching Iraqi airspace the squadron split up, with two of the F-15s forming close escort to the F-16 squadron, and the remaining F-15s dispersing into Iraqi airspace as a diversion and ready back-up. The attack squadron descended to 30 m over the Iraqi desert, attempting to fly under the radar of the Iraqi defences.

At 18:35 local time (14:35 GMT), 20 km from the Osirak reactor complex, the F-16 formation climbed to 2,100 m and went into a 35-degree dive at 1,100 km/h, aimed at the reactor complex. At 1,100 m, the F-16s began releasing the Mark 84 bombs in pairs, at 5-second intervals. According to the Israeli reports, all sixteen weapons struck the reactor complex, although two reportedly did not detonate. As the anti-aircraft defenses opened fire the squadron climbed to an altitude of 12,200 m and started their return to Israel.

According to Israeli reports the Iraqi defenses were caught off guard and were slow to react. Whatever the reason, the anti-aircraft defenses of the facility did not damage any of the attacking aircraft. Despite the fears of encountering Iraqi interceptors, the squadron remained unchallenged and returned to Israeli airspace.
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