Weather satellite imagery from mid-day shows the clouds over Florida. Photo: NASA TV/Spaceflight Now

After three frustrating launch delays - and a last-minute snag today - engineers finally began fueling the shuttle Discovery for blastoff tonight on the 100th shuttle mission, the most complex space station assembly flight yet attempted.

While Discovery appears ready to go, forecasters are predicting a 50 percent chance of low clouds and high crosswinds at the shuttle's emergency runway that could prompt yet another delay.

NASA flight rules prohibit shuttle launches if crosswinds at the 3-mile-long runway are predicted to exceed 15 knots a half-hour or so after liftoff.

That's when a shuttle crew would be attempting an emergency landing if one of the ship's three main engines failed during the first two minutes and 16 seconds of flight.

An engine failure after that would result in a landing in Europe or in an abort to a lower-than-planned orbit.

Forecasters at the Spaceflight Meteorology Group at NASA's Johnson Space Center in Houston are predicting crosswinds this evening from 40 degrees at 10 to 18 knots.

Winds from 60 degrees make up a direct crosswind at the shuttle runway, so the expected winds will be right at NASA's limit.

Hoping for the best, engineers began pumping a half-million gallons of rocket fuel into Discovery's external fuel tank at 12:02 p.m.

The procedure was held up an hour and 17 minutes because of last-minute troubleshooting to resolve questions about the status of a circuit breaker in the shuttle's mobile launch platform.

Discovery gets fueled for launch at pad 39A. Photo: NASA TV/Spaceflight Now

The circuit in question is part of a system that fires explosive charges to disconnect and pull away various data, electrical, fluid and gas lines attached to the tail of the orbiter at liftoff.

A team of engineers was dispatched to the launch pad to resolve the issue and they were able to reset the circuit breaker just before time ran out for fueling. A subsequent test from the firing room confirmed the system was ready for launch.

Fueling should be complete by 3 p.m. or so, 45 minutes before the countdown resumes after a built-in two-hour hold.

Discovery's crew - commander Brian Duffy, pilot Pamela Melroy, Japanese astronaut Koichi Wakata and spacewalkers Leroy Chiao, William McArthur, Peter "Jeff" Wisoff and Michael Lopez-Alegria - is scheduled to begin strapping in around 4:20 p.m. to await liftoff.

As with all space station assembly missions, Discovery's exact launch time will not be set until shortly before launch based on final radar tracking of the international space station.

The launch will be timed to coincide with the moment Earth's rotation carries pad 39A - and the space shuttle - into the plane of the space station's orbit. Launching into the orbital plane minimizes the propellant required to reach the target.

But it also reduces the duration of the launch window.

While the shuttle can, in theory, reach the station if it launches within five minutes of the moment the pad is "in plane" with the station's orbit, Discovery's window will last just two-and-a-half to five minutes.

The exact duration will be computed later this afternoon.

Assuming an on-time liftoff, Discovery will dock with the space station at 2:44 p.m. on Oct. 12.

Animation shows the Z1 truss after it is turned 180 degrees and mounted to the Unity node. Photo: NASA TV/Spaceflight Now

The goal of the mission is to install a new shuttle docking port on the orbital outpost and to attach an 18,300-pound truss to the U.S. Unity module that will serve as a temporary mounting point for a huge set of solar arrays scheduled for installation in December.

The Z1 truss also houses four stabilizing gyroscopes and the station's main KU-band and S-band communications antennas. Four back-to-back spacewalks will be required to hook up the new components and to position the antennas.

If all goes well, Discovery will undock from the station at 10:38 a.m. on Oct. 19 and land back at the Kennedy Space Center at 3:12 p.m. on Oct. 21.

But those times are pre-flight predictions that could be affected by a variety of factors, including the possibility the crew could be asked to enter the Russian Zvezda command module to troubleshoot problems with two of its eight batteries. vWhether that would require a mission extension is not yet known.