Engineers today readied the space shuttle Discovery for blast off this evening at 8:47 to kick off an oft-delayed nine-day mission to repair the crippled Hubble Space Telescope. In so doing, Discovery's seven astronauts hope to give NASA a year-end morale boost after the back-to-back failures of two high-profile Mars probes in September and early December.

But it will not be easy. Forecasters were predicting an 80 percent chance of low clouds, high winds and rain that would delay launch until Saturday. The outlook for Saturday -- NASA's last chance to launch Discovery before the end of the year -- calls for a 60 percent chance of unacceptable weather.

Hoping for the best, engineers at the Kennedy Space Center began pumping a half-million gallons of liquid oxygen and liquid hydrogen rocket fuel into Discovery's external tank at 11:49 a.m.

NASA managers gave the go-ahead for fueling after resolving last-minute concern about critical welds in main engine fuel lines that forced a 24-hour delay Thursday.

The goal of the 96th shuttle mission is to repair and service the 10-year-old Hubble Space Telescope, which went into scientific shutdown Nov. 13 when the fourth of six stabilizing gyroscopes failed.

Three such gyros are needed for astronomical observations and Discovery's crew plans to replace all six Monday during the first off our planned spacewalks. They also plan to install an upgraded computer, a new radio transmitter, a refurbished guidance sensor and other equipment intended to give Hubble a new lease on life.

If Discovery gets off the ground this evening, the astronauts will be able to carry out all four spacewalks. But if launch slips to Saturday, the lowest-priority spacewalk will be canceled to make sure the shuttle gets back to Earth before the end of the year and the Y2K computer rollover.

NASA originally planned to launch Discovery Oct. 14, but the flight has been repeated delayed because of work to repair frayed electrical wiring, to install a replacement main engine and to replace a dented hydrogen fuel line in the engine compartment.

Despite the rocky processing flow, NASA managers insist the veteran space shuttle is finally ready to go.

"There is no pressure to launch in December, none at all," said shuttle program manager Ron Dittemore. "We have found that our teams . . . are at their peak. And they are ready to go. And because the vehicle is ready, we think the right thing to do is launch this week."