The Massive Ordnance Penetrator Bomb Israel Wants to Destroy Iran’s Fordo Nuclear Facility

Why This Is the Only Bomb That Could Destroy Iran’s Nuclear Bunker—Under 300 Feet of Rock

American military engineers designed the GBU-57/B bomb to devastate deeply buried bunkers without radioactive fallout. It’s the only nonnuclear weapon that can reach Iran’s hardest target

By Deni Ellis Béchard edited by Dean Visser

The challenge sounds unreal: How do you blast a hole through a mountain of concrete and granite and then blow up what hides beneath it―all without crossing the nuclear line? The solution is a bomb that weighs 30,000 pounds, about as much as a city bus, but that is compressed into a cylinder roughly 20 feet long and 2.5 feet thick. Since the Israel-Iran conflict broke out last week, much speculation has centered on this weapon: the GBU-57/B, the most powerful nonnuclear bomb that is capable of destroying targets deep below the earth. The question many are asking is whether the U.S.—the only country possessing the bomb—will supply it to Israel.

To understand what the GBU-57/B is and why Israel might want the weapon, it is important to understand the presumed target: Fordo, Iran’s most advanced nuclear enrichment facility, which lies 18 miles northeast of the central city of Qom. According to an Institute for Science and International Security report, the site has the ability to produce enough highly enriched uranium to make a nuclear warhead in the span of mere days. It is also ensconced deep beneath a mountain, under 260 to 300 feet of rock that is reinforced with concrete, and surrounded by a ring of air-defense batteries. After Israel bombed Iraq’s nuclear reactor in 1981 and Syria’s in 2007, Iran decided to spread its nuclear program out over multiple sites, with its crucial elements hidden so deep beneath the earth that not even Israel’s 5,000-pound bunker busters could reach them. Intelligence reports suggest Iran began construction in 2002, while Iran has said that work began in 2007. Tehran acknowledged the site’s existence only in September 2009.

The first three letters in GBU-57/B stand for “guided bomb unit” (a precision bomb that can home in on its target), and it’s the 57th design in the series of such bombs. The second B refers to the bomb’s iteration (designations such as A/B, B/B, C/B, and so on are used for each adjustment made by military engineers). After the U.S.’s 2003 invasion of Iraq, engineers studied bunker strikes with smaller and older GBU models and found that they hadn’t penetrated deeply enough and had done limited damage. The military required a more powerful weapon that would respect the “nuclear taboo,” a widely accepted international consensus that the use of nuclear weapons is morally abhorrent and dangerous because it creates radioactive fallout, invites escalation, and risks driving allies and neutral states into diplomatic revolt. GBU-57/B—also known as a Massive Ordnance Penetrator (MOP)—was a solution. Its Air Force fact sheet sums it up as “a weapon system designed to accomplish a difficult, complicated mission of reaching and destroying our adversaries’ weapons of mass destruction located in well protected facilities.”

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When dropped from 50,000 feet, the cruising ceiling for the B-2 Spirit bomber, the GBU-57/B uses sophisticated fins to guide it to its target. While the exact impact velocity is classified, it is estimated to exceed Mach 1—the speed of sound (767 mph). This strike delivers 800 to 900 megajoules (about 758,000 to 853,000 British thermal units) of kinetic energy—comparable to a 285-ton Boeing 747-400 touching down at 170 mph or a 565-ton Amtrak Acela train moving at 120 mph. With the bomb, however, all that energy is concentrated into a tiny area. According to a 2012 Congressional Research Service briefing, the GBU-57/B has been reported to burrow through 200 feet of concrete or bedrock with a density of 5,000 pounds per square inch (comparable to the strength of bridge decks or parking-garage slabs). Then its 5,300-pound explosive charge detonates.

The GBU-57/B is designed to enhance this ability to pierce deep below the surface. Its nose has an ogive shape, like that of a Gothic arch. Just as such arches are known for distributing weight effectively, the bomb’s nose has no sharp corners that cause air drag. Then, upon impact, the rounded shape spreads the initial crushing load gradually through the bomb’s steel casing instead of concentrating it at one brittle point. This allows the casing to remain intact as it bores into the earth. The casing also has high sectional density. In this context, sectional density refers to the ratio of an object’s mass to the size of the face that first meets whatever surface the object is moving through. A hammer striking a board has high sectional density because it concentrates its mass in a small area; a pillow does not. In the case of the bomb, a great deal of mass and momentum is packed behind a small point. Whereas the ogive nose minimizes drag and structural shock, the high sectional density concentrates velocity and force, allowing the bomb to punch deep into stone.

About one fifth of the warhead’s 5,342-pound total weight is made up of two explosives: 4,590 pounds of AFX-757 plus 752 pounds of PBXN-114. Both create a larger blast than earlier bombs while being sufficiently insensitive to survive the shock of the initial strike. The detonation itself is timed by a specialized fuse that can be programmed from within the bomber cockpit. The fuse “counts” layers of rock or concrete, “hears” the hollow of a tunnel or chamber and then fires a detonator into the smaller PBXN-114 charge so that the main charge explodes only after the bomb has bored well inside the target. The released energy is roughly equivalent to about three to four tons of TNT. But because so much of the bomb’s 30,000 pounds is hardened steel, much of its destructive effect comes from its kinetic punch, delivered at sonic speed.

Though suspected of having nuclear weapons, Israel has so far respected the nuclear taboo. To destroy Fordo without them, it would need both the GBU-57/B and a B-2 Spirit bomber, the only aircraft designed to drop the bomb (it’s capable of releasing two, one from each bay). Otherwise Israel could, in theory, only chip at Fordo’s periphery—by destroying power sources, collapsing entrances or sending saboteurs—while uranium continues to be enriched below. Fordo is so well protected that a Royal United Services Institute op-ed states that “even the GBU-57/B would likely require multiple impacts at the same aiming point to have a good chance of penetrating the facility.” Despite media claims that GBU-57/B bombs were used to strike Houthi targets in Yemen, the U.S. Air Force states that these weapons have never been used in combat and that only a few dozen are stockpiled. So far, the U.S. has refused to hand Israel any of them—or the B-2 Spirit bomber required to drop them and collapse a mountain.