Hubble has six gyroscopes, operating in pairs, and the telescope needs at least three to orientate itself correctly. When a fourth gyroscope failed on 13 November Nasa had to close the telescope down and put it into a safety mode.
The next Shuttle mission to service the Hubble was planned for June 2000 but the problem with the gyroscopes forced Nasa to divide the mission into two flights, one which was due to be launched today and one in the middle of next year.
In addition to replacing all six gyroscopes, the seven-man crew, which includes British-born Michael Foale, will upgrade the telescope's computer. They will also install a new radio transmitter and battery pack, and replace and repair the instrument's outer insulation, which gets damaged by meteorite impacts.
The 10-day mission, which includes four space walks, will present serious technical difficulties, not least of which is capturing the 20-ton telescope as the Shuttle and the Hubble hurtle in tandem through space at a speed of 17,000 mph.
Once the Hubble has been secured and brought into the Shuttle repair bay, pairs of astronauts will go outside during an "extra-vehicular activity" (EVA) to replace or repair the telescope's hardware.
"One of the big issues for all the EVAs is preventing debris from getting inside the telescope," said Claude Nicollier, a French astronaut and mission specialist, who is part of the European Space Agency's contribution to the mission.
"At various points the guts of the telescope will be exposed to the outside. We are all aware of the potential damage and presence of debris and we don't want any of the debris to get inside."
The complex repair job will take place while orbiting around 370 miles above Earth - around twice the altitude of normal space walks. It will present an impressive backdrop to the complicated manoeuvres which the astronaunts are expected to make during each six-hour space walk.
The final walk will involve repairing the telescope's out protective layer, said Nicollier. "It will be like hanging wallpaper. We'll start at the top and gradually unroll the insulation."
2001 AND BEYOND
Second part of the third servicing mission to the Hubble Space Telescope will focus on installing the Advanced Camera for Surveys, a device for looking deep into space.
Two Mars missions are launched: Mars Surveyor Orbiter and Mars Surveyor Lander. The orbiter will begin the global mapping of Mars.
Mars Surveyor Lander sends out a rover vehicle, called the "Marie Curie" to test for signs of life and assess the feasibility of sending a manned mission of Mars.
Fourth servicing mission to Hubble. An advanced wide-field camera will enable the Hubble to view stars in an enormous range of wavelengths.
The Hubble comes to the end of its scheduled life. Nasa may send it into orbit permanently, or it may be brought back to Earth to become a museum exhibit.