Tim Furniss/LONDON

The 214th and final manned spaceflight this century will be launched from Kennedy Space Center, Florida, on 9 December.

Space Shuttle Discovery/STS103, flying the 126th US manned space mission and the 96th by the Shuttle, will conduct the third and most difficult service mission to the nine-year-old Hubble Space Telescope (HST).

The mission will involve a rendezvous and grapple of the spacecraft by a remote manipulator system (RMS) robot arm and four complicated spacewalks.

The grapple will be made more difficult now that the telescope is in "safe mode". The craft went into the mode when the fourth of six gyros on the HST failed on 13 November. At least three gyros on the HST must be operational. The STS103 mission will replace all six gyros. Since its launch in April 1990, the HST has made more than 260,000 astronomical observations and has transmitted pictures of the universe.

In addition to replacing the gyros, the seven-member Discovery crew will install a new computer, a replacement fine guidance sensor, a back-up S-band transmitter, back-up battery voltage and battery temperature improvement kits. The mission will also repair degraded multilayer insulation around the telescope.

Two previous servicing flights have been conducted, in December 1993, largely to correct myopic mirror optics, and in February 1997. The STS103 mission was originally scheduled in two parts - 3A and 3B - for next June.

Mission 3A is flying earlier to perform the more urgent work, while 3B will follow in 2001 to complete the servicing work, including the installation of new solar panels and camera and an instrument cooling unit. It is possible that the STS103 crew, including four spacewalkers, will also fly the 3B mission.

The first spacewalks, or extra vehicular activity (EVA), on 11 December, will replace the gyros and install battery enhancement kits. EVA 2 will install the new computer and fine guidance sensor, while the S-band transmitter will be installed on EVA 3, together with new electronics and new insulation layers.

EVA 4 will involve wrapping large blankets of Teflon insulation round parts of the spacecraft. Each EVA will require careful manoeuvring of the RMS. Virtual reality training has played a major role in the spacewalk preparations.

Source: Flight International