gram. Hubble’s structure and general avionics system are based on those of satellite systems of similar size and complexity that were developed by Lockheed and associated optical contractors in the 1970s and early 1980s. An exploded view of the telescope system is shown in Figure 2.1.
Hubble was designed with an anticipated 15-year lifetime based on the expected integrity of the main mirror. It was believed that over HST’s 15-year life the space environment in low Earth orbit would cause sufficient degradation of the mirror that the telescope’s light-gathering capabilities would be severely damaged by cosmic rays and orbital debris. To date, since the first shuttle servicing mission’s correction for a significant aberration in the mirror, there has been no measurable degradation. The operations of the telescope over the 14 years since launch have provided an extensive database on HST’s performance and failure mode and effects that can be used for engineering purposes to attempt to anticipate the spacecraft’s future performance.
An important feature of Hubble is that it was the first spacecraft to be designed specifically for on-orbit servicing by astronauts. At the same time, however, the telescope’s avionics subsystems, largely included in the Support Systems Module Equipment Section (see Figure 2.1), were not specifically designed to be accessible for servicing. These included such subsystems as the Data Management Unit, the Data Interface Unit, the Power Control Unit, and transponders. Even so, astronauts could change out some of these subsystems during servicing missions.