The U.S. Missile Defense Agency (MDA) is extending the time needed to review proposals from contractors vying for a contract worth up to $10 billion for 10 years of work sustaining the Ground-Based Midcourse Defense (GMD) missile shield system.

Lockheed Martin/Raytheon and Boeing/Northrop Grumman are locked in a duel for the work. MDA had planned for a downselect in early summer; this is now slated for November.

“The source selection authority has determined that it is in the best interest of the government to ensure rigorous and comprehensive proposals by industry, and evaluation by the government, by extending the anticipated award date into November of this year,” according to a statement from Rick Lehner, MDA spokesman.

Officials from both teams say that they have been answering questions from MDA on their proposals. This is a normal process designed to ensure the agency understands each proposal.

Lehner notes that the contract amounts will be proposed by the bidders. The initial contract period of seven years has been estimated at about $600 million annually; total value could reach $10 billion if all options are exercised.

The contract would cover sustainment of the GMD system (including oversight of the interceptors and sensors), development of new capabilities, flight testing and disposal of outdated components. GMD includes a host of sensors across the globe and interceptors at Fort Greely, Alaska, and Vandenberg AFB, Calif. It is designed to protect much of United States from an ICBM attack from North Korea or Iran.

Boeing, which was the lead in designing the system, is the incumbent. However, MDA has at times been unhappy with the company’s management of the program. GMD production is currently on hold pending results from a failure review board. A Ground-Based Interceptor (GBI) failed in two attempts in 2010 to destroy a Lockheed Martin LV-2 target. Fixes to the GBI’s Exoatmospheric Kill Vehicle (EKV), made by Raytheon, are now under review.

MDA Director Army Lt. Gen. Patrick O’Reilly says that a non-intercept characterization test could take place early next year to run the EKV fixes through their paces. An intercept attempt, which would be the third for this head-on hit-to-kill scenario, could follow late in 2012. The EKV used is an upgraded version, though officials decline to outline its specific capabilities.

Stakes are high in the GMD sustainment contract competition. The loser will likely be edged out of a significant, and potentially lucrative, piece of the missile defense market. The winner, by contrast, could have a foothold for 10 years.