The GBU-57A/B Massive Ordnance Penetrator (MOP) is a precision-guided, "bunker buster" bomb used by the United States Air Force. This is substantially larger than the deepest-penetrating bunker busters previously available, the GBU-28 and GBU-37. It is designed to accomplish a difficult, complicated mission of reaching and destroying an adversary's weapons of mass destruction located in well-protected facilities. Development In 2002, Northrop Grumman and Lockheed Martin were working on the development of a earth-penetrating weapon, but funding and technical difficulties resulted in the development work being abandoned. Following the 2003 invasion of Iraq, analysis of sites that had been attacked with bunker buster bombs revealed poor penetration and inadequate levels of destruction. This renewed interest in the development of a larger bunker buster, and the MOP project was initiated by the Defense Threat Reduction Agency to fulfill a long-standing Air Force requirement. The U.S. Air Force has not officially communicated a specific military requirement for an ultra-large bomb, but it does have a concept for a collection of very large penetrator and blast weapons: the so-called "Big BLU" collection, which includes the MOAB (Massive Ordnance Air Blast) bomb. Development of the MOP was performed at the Air Force Research Laboratory, Munitions Directorate, Eglin Air Force Base, Florida with design and testing work performed by Boeing. It is intended that the bomb will be deployed on the B-2 bomber, and will be guided using GPS. It is also planned to be deployed on the B-21 bomber. Northrop Grumman announced a $2.5-million stealth-bomber refit contract on 19 July 2007. Each of the U.S. Air Force's B-2s is to be able to carry two 14-ton MOPs. The initial explosive test of MOP took place on 14 March 2007 in a tunnel belonging to the Defense Threat Reduction Agency (DTRA) at the White Sands Missile Range, New Mexico. On 6 October 2009, ABC News reported that the Pentagon had requested and obtained permission from the U.S. Congress to shift funding in order to accelerate the project. It was later announced by the U.S. military that funding delays and enhancements to the planned test schedule meant the bomb would not be deployable until December 2010, six months later than the original availability date. The project has had at least one successful Flight Test MOP launch. On 7 April 2011, the USAF ordered eight MOPs plus supporting equipment, for $28 million. The Air Force took delivery of