(12/17/1999) --- A cloud-streaked sky provides backdrop for Space Shuttle Discovery as it waits for liftoff on mission STS-103 from Launch Pad 39B. The tower at its left is the Fixed Service Structure, topped by the 80-foot-tall fiberglass mast that helps provide protection from lightning strikes. Below it, extending outward, is the external tank gaseous oxygen vent arm system with the vent hood (commonly called the 'beanie cap') poised above the external tank. The retractable arm and the beanie cap are designed to vent gaseous oxygen vapors away from the Space Shuttle. The arm truss section is 65 feet long and the diameter of the vent hood is 13 feet. Extending toward the cabin of the orbiter below is the orbiter access arm, with the environmental chamber (called the White Room) at the end. Through this chamber the crew enters the orbiter. The STS-103 mission, to service the Hubble Space Telescope, is scheduled for launch Dec. 17 at 8:47 p.m. EST. Mission objectives include replacing gyroscopes and an old computer, installing another solid state recorder, and replacing damaged insulation in the telescope. The mission is expected to last about 8 days and 21 hours. Discovery is expected to land at KSC Sunday, Dec. 26, at about 6:25 p.m. EST