The third category contains all the other components, which the committee terms the “avionics system.” The failure model adopted for this last category is crucial, and some of its consequences are counter-intuitive, as explained below. The model assumes that components in this class exhibit random, unpredictable failures at a rate that is constant over time. Consequently, the avionics components do not wear out in the traditional sense; if a component lasts, say, 3 years, it is just as likely to keep working at the end of that time as it is today. Eventually, the avionics system will enter a wear-out stage, but the statistics for the failure of electronics parts combined with those for the performance of Hubble (and other spacecraft) indicate that that time frame is beyond the servicing window currently under consideration.
Above, the committee used the words “foreseen” and “unforeseen” to describe failures. Foreseen failures are the predictable failures that affect the wear-out components. Unforeseen failures are the random failures that affect the avionics system. The model for observatory lifetime computes the failure rate of the two categories separately to derive the projected lifetime of the system as a whole.
Previous shuttle servicing missions to HST have demonstrated that essentially all failures on HST are repairable. However, battery failure has unique consequences, since sufficient power must be available to prevent loss of temperature regulation in the optical system. If a battery is severely degraded or it fails, the temperature will drop below safe limits and the structural elements of the telescope will lose their proper shape. Recovery from this state is not possible.
The HST avionics system is currently fully operable and retains redundancy on all subsystems. (Redundancy is a vital element of spacecraft health; as soon as failures render a key system non-redundant, the projected lifetime becomes much shorter.) The observatory’s good condition since its launch in 1990 is the result of continuous extensive efforts by a dedicated and skilled team of scientists and engineers at the Space Telescope Science Institute (STScI) and the Goddard Space Flight Center (GSFC). The spacecraft is actively monitored on a daily basis and is conservatively operated with the objective of maximizing its performance and lifetime. The avionics system’s performance has also been extensively modeled and trended using flight telemetry data such that it is possible to credibly forecast system performance, failure trends, and replacement requirements.
FINDING: The HST avionics system is currently in a fully operable state and retains redundancy on all subsystems. Its performance is monitored regularly and is well understood by the operations team such that it is possible to credibly forecast system performance, failure trends, and replacement requirements.
Failure (both foreseen and unforeseen) rates are sufficiently high on HST that the spacecraft cannot function for an extended period without servicing. Servicing has been done in the past with crewed shuttle missions that have enabled three types of repairs. Some repairs replaced components that are subject to foreseen wear-out, such as the FGS units, gyros, and batteries. This class of repairs can be planned years in advance.
The second category, unforeseen failures (in the avionics system and science instruments), such as failure of the S-band single access transmitter (SSAT) or the reaction wheel assembly (RWA), are of a random or unpredictable type that cannot be planned for in advance. Repairs in response to failures in this category must be responded to at the time of occurrence and, historically, have been inserted as late as 3 months prior to a planned servicing mission. In the case of SM-3A, the mission itself (although other servicing work was also performed) was based on responding to an unexpected premature failure of gyros that resulted in an interruption of science operations.