Discovery at launch pad 39B following its rollout last month. Photo: NASA/KSC

NASA officials on Thursday announced another delay for space shuttle Discovery's long-awaiting mission to restore the Hubble Space Telescope to working order.

With work at launch pad 39B running behind schedule, the U.S. space agency opted to slip Discovery's liftoff by two days until December 11. The 38-minute launch window that day will open at 12:13 a.m. EST (0513 GMT).

"We are a full day behind and possibly more," NASA spokesman Bruce Buckingham said. "This date gives us time in reserve to work other problems."

NASA has held two senior-level readiness reviews over the past two weeks, the most recent on Wednesday, which failed to set a firm launch date because of ongoing inspections of electrical wiring running inside the $2 billion spaceship.

Buckingham said workers are still inspecting other wiring inside the engine compartment as the three-level area in the tail of Discovery is closed for launch.

Discovery's flight, the third and final shuttle mission of 1999, is running nearly two months behind schedule because of a series of technical gremlins that have plagued the shuttle fleet this year.

The troubles began when an electrical short-circuit knocked out two of Columbia's engine controllers during the last shuttle launch on July 23.

That prompted a fleet-wide inspection of wiring beneath the shuttles' payload bays.

Later problems on Discovery included a damaged range safety cable connecting the solid rocket boosters, an engine replacement after officials decided not to launch with a broken drill bit lodged inside a cooling chamber and a leaky valve in the shuttle's hydraulic system.

The $3 billion Hubble Space Telescope is orbiting above Earth but its mission to observe the universe is on hold. The reason: Hubble's precision pointing system failed in mid-November when the fourth of six devices called gyros malfunctioned. The telescope needs three working gyros in order to lock onto stars, planets and its other astronomical targets.

Two pairs of astronauts are scheduled to make four spacewalks during Discovery's flight to install six fresh gyros, a new main computer, replace other aging electronics and add new thermal shielding to the observatory.