Shuttle Discovery's launch on a cargo resupply mission will be postponed from September to November 1, under a plan approved by NASA managers.
Sister ship Endeavour, carrying the $1.5 billion Alpha Magnetic Spectrometer particle detector, is now set to fly on February 26 on the program's 134th and final mission.
NASA initially planned to retire its three-ship fleet by the end of 2010. Congress is, however, expected to give the space agency a $600 million cushion to ease its deadline pressures.
NASA has also managed to trim the program's $200 million monthly costs to extend shuttle operations into March 2011.
The United States is retiring the shuttles primarily due to high operating costs. The Obama administration is pushing for Congress to approve a controversial plan to fly astronauts on commercial spacecraft, freeing NASA to focus on developing bigger rockets and new technologies needed for future missions to asteroids, Mars and other destinations in the solar system.
NASA also had to pick launch dates that did not conflict with Russian, European and Japanese missions to the station or previously scheduled rocket launches and other activities at Cape Canaveral Air Force Station, which provides critical support services for shuttle flights.
"There's so much traffic around the station it ultimately made the most sense to pick November 1," said NASA spokesman Kyle Herring. Discovery's delay, in turn, bumped Endeavour's flight from November to February.
A proposal for an additional station cargo run on shuttle Atlantis, which will be prepared as an emergency rescue vehicle for the Endeavour crew, is pending, with a decision expected in August, Herring said.