(11/04/1999) -- Orbiter Discovery begins its rollover to the Vehicle Assembly Building (in the background) after leaving the Orbiter Processing Facility bay 1. Launch date for Discovery on mission STS-103, the third Hubble Space Telescope servicing mission, is under review for early December. The mission is a 'call-up' due to the need to replace portions of the pointing system, the gyros, which have begun to fail on the Hubble Space Telescope. Although Hubble is operating normally and conducting its scientific observations, only three of its six gyroscopes are working properly. The gyroscopes allow the telescope to point at stars, galaxies and planets. The STS-103 crew will also be replacing a Fine Guidance Sensor and an older computer with a new enhanced model, an older data tape recorder with a solid-state digital recorder, a failed spare transmitter with a new one, and degraded insulation on the telescope with new thermal insulation. The crew will also install a Battery Voltage/Temperature Improvement Kit to protect the spacecraft batteries from overcharging and overheating when the telescope goes into a safe mode