arm. Such failures would result in total or partial failure of the mission (see Chapter 5 for more detailed discussions).

The analyses presented in Tables 7.2 and 7.3 indicate that the proposed HST robotic servicing mission involves a level of complexity that is inconsistent with the current schedule for the development robotics and would require an unprecedented reduction in the time required for system development, compared with that for space missions of similar complexity (see also Chapter 4). The likelihood of successfully developing the HST robotic servicing mission within the baseline 39-month schedule is deemed to be remote. The independent study performed by the Aerospace Corporation indicates that the most probable estimate is that 65 months are required for development of a combined servicing and deorbiting mission—26 months more than NASA’s 39-month schedule. The Aerospace Corporation report indicates a “high failure risk due to the unprecedented mission and unproven technologies (~50 percent probability of failure…).”6 Extending the robotics schedule to allow for a more reasonable development interval is not possible because the robotic mission does not “reset the avionics failure clock” the way a shuttle mission can, as explained in Chapter 4. As discussed in Chapter 4, this leads to a high probability that the spacecraft will fail due to some unforeseen failure in the avionics system before the end of the 3- to 5-year post-servicing science operations period.

Conclusions Regarding Risk for Servicing Options

Tables 7.2 and 7.3 show the risk significance of the various failure scenarios for the human and the robotic HST servicing options, respectively. A comparison of the analyses in these two tables indicates that there is strong evidence of a lower mission risk for the human servicing option. This assessment is supported by the experience base for the human servicing of HST, including the demonstrated capability of humans to diagnose unanticipated failures and take corrective action (see Chapter 6).

Great uncertainty remains about the range of corrective actions that can be performed robotically, as discussed in Chapter 5. In addition, there is strong supporting evidence that mission risk is high when successful system development and testing of the robotic servicing option must be done in the short time now available.

FINDING: Although a quantitative mission risk assessment does not exist for either a human or a robotic servicing mission to the Hubble Space Telescope, the committee’s qualitative evaluations lead it to conclude that the human servicing mission poses a low risk to mission success. Conversely, the risk posed by a robotic mission is high, considering the short time frame available for system development and testing, and the uncertainty concerning robotic performance.


Despite the absence of quantitative analyses of the risks and benefits from the two types of HST servicing missions, the committee has determined that a human mission poses a low mission risk, whereas a robotic mission poses a high mission risk. The benefits from either mission are comparably high (if the robotic mission performs all its intended activities), especially in terms of the quantity and


Aerospace Corporation, “Hubble Space Telescope Servicing Analyses of Alternatives, Final Delivery Executive Summary,” August 3, 2004, Aerospace Corporation.

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