Space shuttle Discovery's flight next month to the Hubble Space Telescope has been pushed back a few more days, this time because of engine trouble.

One of Discovery's three main engines must be replaced because a broken drill bit was left in a coolant chamber, NASA said today.

Discovery had been scheduled to blast off Dec. 2 on a 10-day mission to carry crucial replacement parts for the Hubble telescope. The original October launch date was scrapped because of wiring repairs.

"We're not going to make Dec. 2, that's for sure," NASA spokesman Rob Navias said. But he said the delay could be just a few days. It will take 10 days to replace the engine, but the work will set back other launch preparations.

The drill bit ended up in the engine chamber during routine maintenance last spring, Navias said. Shuttle managers decided today to replace the entire engine as a precaution.

The last thing NASA wants is debris inside a shuttle main engine, even a drill bit only a half-inch long that weighs less than a gram.

During Columbia's launch in July, hydrogen fuel seeped from three ruptured tubes in the right engine nozzle after a one-inch pin came loose inside the engine and hurtled down through the combustion chamber, damaging the tubes.

An exposed wire caused a short circuit during the same Columbia flight. NASA ordered a fleetwide inspection that found exposed wire in 57 places in Discovery, leading to last month's flight delay. The exposed wires have been repaired.

NASA also double-checked thermal tiles on Discovery after a batch of tile material was found unsuitable for flight. All of the shuttle's tiles checked out fine.

If NASA does not launch Discovery by mid-December, the mission will be bumped into the new year. At the end of December, the computers in Mission Control must be shut down and loaded with new software that is Y2K-compliant.