What Happened
- The US is reportedly shifting towards the use of gravity bombs, including bunker-busting variants, in its military operations against Iranian nuclear facilities
- The B61 nuclear gravity bomb family and the GBU-57 Massive Ordnance Penetrator (MOP) are the primary weapons under discussion
- Iran's deeply buried nuclear facilities, particularly Fordow (built inside a mountain), require earth-penetrating weapons capable of reaching hardened underground targets
- The US Air Force has been buying additional bunker-buster bombs following strikes on Iranian nuclear sites in early 2026
- The shift highlights the limitations of precision-guided munitions against deeply fortified targets
Static Topic Bridges
Gravity Bombs vs Precision-Guided Munitions (PGMs)
A gravity bomb (also called an unguided or "dumb" bomb) is a conventional or nuclear weapon that follows a ballistic trajectory after release, relying on gravity alone for its path. In contrast, precision-guided munitions (PGMs or "smart bombs") use guidance systems — laser, GPS, infrared, or inertial navigation — to steer towards a designated target. Modern gravity bombs like the B61-12 blur this distinction by incorporating tail-kit guidance while retaining the "gravity bomb" designation due to their free-fall delivery method.
- Gravity bombs date back to World War I; PGMs were first widely used in the Vietnam War (Paveway laser-guided bombs, 1960s)
- B61 nuclear gravity bomb: primary US thermonuclear weapon since 1968; 14 variants (Mod 0 through Mod 13); variable yield ("dial-a-yield") from 0.3 to 340 kilotons
- B61-12: newest variant with GPS/INS tail-kit guidance; four yield options (0.3, 1.5, 10, and 50 kt); weighs approximately 825 lb
- B61-11: earth-penetrating variant deployed in 1997 specifically for bunker-busting; assigned to B-2 Spirit stealth bomber
- B61-13: higher-yield strategic bunker buster; first unit produced at Pantex Plant in May 2025; designed for the B-21 Raider
Connection to this news: The US shift towards gravity bombs for Iran reflects the need for earth-penetrating capability against deeply buried facilities, where the sheer kinetic energy and nuclear yield of gravity bombs may be more effective than conventional PGMs.
GBU-57 Massive Ordnance Penetrator (MOP)
The GBU-57A/B Massive Ordnance Penetrator is the largest non-nuclear bunker-busting bomb in the US arsenal, developed by Boeing specifically to destroy hardened and deeply buried targets (HDBTs). It uses a GPS/INS dual-mode guidance system and a Large Penetrator Smart Fuze (LPSF) that adjusts detonation timing based on penetration depth.
- Weight: approximately 30,000 lb (13,600 kg); length: 20.5 ft (6.2 m); diameter: 31.5 inches (0.8 m)
- Penetration capability: up to 60 m (200 ft) through earth or 18 m (59 ft) through reinforced concrete (5,000 psi)
- Casing: Eglin steel (ES-1) alloy designed to withstand extreme penetration pressures
- Warhead: BLU-127 with enhanced-blast explosives (AFX-757 and PBXN-114)
- Delivery platforms: Only the B-2 Spirit and B-21 Raider strategic bombers can carry it due to its size
- Developed in response to Iran's Fordow facility and North Korea's underground installations
Connection to this news: The GBU-57 MOP is the primary conventional alternative to nuclear gravity bombs for striking Iran's Fordow enrichment facility, which is built 80 metres underground inside a mountain near Qom.
Nuclear Non-Proliferation and Iran's Nuclear Programme
Iran's nuclear programme has been a central issue in international non-proliferation efforts since 2002, when covert enrichment facilities at Natanz and Arak were revealed. The Joint Comprehensive Plan of Action (JCPOA) of 2015 was designed to limit Iran's uranium enrichment in exchange for sanctions relief, but the US withdrew in 2018 under President Trump.
- Nuclear Non-Proliferation Treaty (NPT): signed 1968, entered into force 1970; Iran is a signatory but not a nuclear-weapon state under the treaty
- JCPOA (2015): limited Iran to 3.67% enrichment, 300 kg stockpile, and 5,060 centrifuges at Natanz; Fordow converted to research-only; Arak heavy water reactor redesigned
- US withdrawal from JCPOA: May 2018; reimposed sanctions under "maximum pressure" campaign
- IAEA: International Atomic Energy Agency monitors compliance; has reported Iran enriching up to 60% (weapons-grade threshold is approximately 90%)
- Fordow Fuel Enrichment Plant (FFEP): Built inside a mountain near Qom; discovered by Western intelligence in 2009; depth makes it resistant to conventional strikes
- India's position: India has consistently advocated for diplomatic resolution and opposed nuclear proliferation while maintaining strategic autonomy
Connection to this news: The military shift towards earth-penetrating weapons reflects the assessed inability of conventional strikes to neutralise Fordow, raising the stakes of the confrontation and the global non-proliferation framework.
Key Facts & Data
- B61 gravity bomb: 14 variants; variable yield 0.3-340 kt; approximately 505 in US stockpile (2026)
- GBU-57 MOP: 30,000 lb; penetrates up to 200 ft of earth; carried only by B-2 and B-21 bombers
- Fordow enrichment facility: Built 80 m underground inside a mountain near Qom, Iran; discovered 2009
- JCPOA signed: July 2015; US withdrew: May 2018
- NPT: 191 state parties; five recognised nuclear-weapon states (US, Russia, UK, France, China)
- Iran enrichment level: Up to 60% (IAEA reports); weapons-grade threshold: approximately 90%
- B61-13: First unit produced May 2025 at Pantex Plant, Texas