So far, two of the three space shuttles in NASA's dwindling fleet have resisted going quietly, as the shuttle program winds down.

The final launch of Discovery, which completed its last mission March 9, was delayed for months as technicians solved problems with the orbiter's external fuel tank.

Now Endeavour, originally scheduled for liftoff to the International Space Station on April 29, is dragging its 86-ton tail. Mission managers announced today that the orbiter's final mission will launch no earlier than May 16.

Endeavour's six-member crew is commanded by astronaut Mark Kelly, whose wife, US Rep. Gabrielle Giffords, was shot and seriously wounded in January during a meet-and-greet session with constituents near Tucson.

Her remarkable recovery prompted physicians to allow her to attend the scheduled April 29 launch. She reportedly will return for the launch when mission managers finally settle on a date.

Source of the delay: fuel-line heaters

The delay was triggered during the April 29 countdown, when a problem appeared with the heaters that keep the fuel lines to the orbiter's auxiliary power units from freezing in space. These units provide power to the ship's hydraulics systems, which operate flight-control surfaces on the wings and tail, the landing gear, and other key systems.

Technicians traced the problem to a 50-pound electrical switch box in the orbiter's aft section, which also houses the shuttle's three main engines.

Managers had hoped to launch May 8, but that date slipped as repair work progressed. Now, managers say they want to be sure to give technicians time to thoroughly test the repairs before signing off on a launch.

Endeavour's final mission

During its 14-day mission, the shuttle crew is scheduled to perform four spacewalks at the space station and deliver a $1.5 billion experiment that aims to help scientists answer riddles about the formation and evolution of the universe. Shuttle astronauts also will deliver spare parts and other supplies to the orbiting outpost.

The final mission in the shuttle program involves the orbiter Atlantis. It currently is scheduled to launch no earlier than June 28, although that could push back because of delays in launching Endeavour.