The National Aeronautics and Space Administration said the modification "aligns and extends all activities associated with the production contract to include final assembly of one tank, partial manufacture of a tank and the acquisition of the component parts for one additional tank to serve as spares."

NASA said the modification supports its priorities of safely flying the space shuttle, completing construction of the International Space Station and the long-term plan to return astronauts to the moon.

The cost-plus-award, fee-incentive contract ends Sept. 30, 2010, and takes the total value of the contract -- initially awarded in October 2000 -- to $2.93 billion.

The contract calls for the delivery of 18 external tanks to NASA.

The 154-foot tall external rocket fuel tanks hold the liquid hydrogen fuel and liquid oxygen for the shuttle's three main engines. They are the largest single component of the space shuttle and the only part of the shuttle that isn't reused.