Lousy weather once again forced NASA to delay Discovery's flight to the Hubble Space Telescope today, and shuttle managers considered making one last shot at launch before giving up for the year.

It was the ninth postponement for the Hubble repair mission and the third in as many days.

NASA called off tonight's launch attempt before fueling had even begun. Dark rain clouds hovered over the Kennedy Space Center as the announcement boomed over radios.

Liftoff tentatively was rescheduled for 7:50 p.m. Sunday, with a final decision on whether to proceed expected in late morning. Forecasters put the chance of good weather at 60 percent.

Before committing to a Sunday launch, managers want to take another look at the weather and make sure the shuttle team can have Discovery back on Earth and all ground computers shut down before year's end.

Although NASA believes its computers are Y2K compliant, it does not want to have a shuttle up over New Year's Eve--just in case.

NASA had insisted for weeks that today would be the last chance to launch Discovery before bumping the mission into January. But after rain and thick, low clouds thwarted Friday night's effort, NASA's top space flight official said a Sunday run might be possible after all.

Sunday would really be the year's last opportunity, said the official, Joseph Rothenberg. That would put landing on Dec. 27. But if bad weather interfered, the shuttle might have to stay in orbit an extra day or two.

Rothenberg said the agency must project for two days beyond the scheduled touchdown as well as the possibility that bad weather at Kennedy Space Center could divert the shuttle to Edwards Air Force Base in California.

Although they insist they're putting no pressure on the shuttle program, astronomers want the $3 billion Hubble working again as soon as possible. Its eye to the universe closed in mid-November when its pointing system broke down.

Until Discovery's seven astronauts deliver new gyroscopes and other replacement parts, Hubble can perform no astronomical observations. It's an expensive hiatus; NASA spends close to $25 million on the telescope each month whether it's working or not.

Four space walks had been planned to install all of Hubble's new equipment, but only three will be carried out if Discovery lifts off Sunday. And the mission will be cut from 10 to eight days to ensure that Discovery is back with a few days to spare before New Year's Eve.

Discovery was supposed to fly to Hubble in October but was grounded by damaged wiring, a contaminated engine, a dented fuel line, and then last-minute concerns over welds in the main propulsion system.