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Assessment of Options for Extending the Life of the Hubble Space Telescope: Final Report 7 Benefit/Risk Assessment of Hubble Space Telescope Servicing Options INTRODUCTION A variety of risks and benefits are associated with on-orbit servicing of the Hubble Space Telescope (HST). There are risks to human safety, as well as several types of programmatic risks, including not meeting cost requirements, not meeting schedule, and not achieving mission objectives. As discussed in detail in Chapter 3, the benefits from HST servicing are in the continuation and enhancement of the science produced by HST, the enhancement of NASA’s image in the production of world-class science, and the educational value of inspiring the youth of the nation and the world in the pursuit of scientific careers. The assessment of a benefit/risk ratio for each alternative HST servicing option provides a measure of its efficacy. Ideally, benefit/risk ratios are quantitative. In the case of HST servicing, NASA has not as yet completed a quantitative assessment of risk, which is expected late in 2004, nor is there a quantitative measure of the benefits to be achieved. Nevertheless, the committee was able to assess the benefit/risk for alternative servicing options, based on qualitative risk assessments, and qualitative consideration of the specific scientific benefits expected as a result of the different servicing options. Benefit/risk comparisons were made for a human HST servicing mission and for a robotic servicing mission. Risk-related terms relevant to HST servicing are defined in Table 7.1. ASSESSMENT OF THE RISKS OF HUMAN AND ROBOTIC SERVICING The risk to crew safety is discussed in Chapter 6, where it is concluded that the risk of a single shuttle mission to HST is essentially the same as the risk of a single mission to ISS. Given that finding, it remains to assess the cost, schedule, and mission risks of the two types of Hubble servicing missions, human and robotic. The programmatic risks of meeting projected cost requirements and schedule are addressed throughout this report, and especially in Chapter 4 with respect to schedule. This chapter focuses on mission risk, which is the risk of not meeting the objectives of the servicing mission. The preference of the committee would have been to rely on the review of risk assessments currently
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Assessment of Options for Extending the Life of the Hubble Space Telescope: Final Report TABLE 7.1 Risk-related Terms Term Definition Risk An evaluation reflecting the combined answers to the questions (1) What can go wrong? (2) How likely is it? and (3) What are the consequences? Risk assessment The science of investigating the level of risk and the contributing factors associated with the risk of an event, process, or activity. Risk management The process of making decisions and taking actions to control risk based on a systematic process of risk assessment. Risk benefit analysis The evaluation of the risks and benefits of an activity, system, or program based on economic and performance considerations. Programmatic risk The potential for risk of an undesired impact on the cost, schedule, and success of a program, project, or activity. Mission risk The possibility of not meeting the objectives of a particular mission. Health and safety risk The potential for negative human health or safety consequences as a result of a particular event, process, or activity. Risk communication An interactive process of exchange of information and opinions regarding risk, among individuals, groups, and institutions, often involving multiple messages about the nature of risk and the expression of concerns, opinions, or reactions to risk messages. De minimis risk A level of risk considered below regulatory concern from the legal maxim “de minimis non curat lex” or “the law is not concerned with trifles.” Risk perception Sense of a hazard held by different groups of people and frequently dependent on factors other than the hazard itself, such as unfamiliarity, acuteness, and sensitization by catastrophic images. Risk characterization A synthesis and summary of information about a hazard that addresses the needs and interests of decision makers and of interested and affected parties. required by NASA’s procedures for probabilistic risk assessment.1 Unfortunately, primarily because NASA has only recently required full-scope risk assessments, the risk assessment of primary interest to the committee is currently in the process of being developed and therefore was unavailable to the committee. Furthermore, the risk assessment procedures for NASA programs and projects do not require risk assessments for non-human-related missions. As a result of not having a risk assessment for either the shuttle or the robotic HST servicing missions for its review and analysis, the committee performed its own qualitative assessment of the risks of the two HST servicing options based on briefings, meetings with NASA and contractor personnel, and selected references. 1 NASA, 2004, “Probabilistic Risk Assessment (PRA) Procedures for NASA Programs and Projects,” NASA Procedural Requirements: 8705.5, NASA Office of Safety and Mission Assurance, Washington, D.C.
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Assessment of Options for Extending the Life of the Hubble Space Telescope: Final Report The committee took a top-down qualitative approach to comparing risks. Its assessment focuses on those on-orbit activities and events most relevant to the actual servicing operations that are common to both the human and robotic options. Since this section is focused on mission risk (or the probability of success of a servicing mission), there is no assessment here of risk during reentry for either servicing option. The committee did develop for each of the two missions a representative success-oriented event sequence table (Tables 7.2 and 7.3) that shows its assessment of individual mission events at a high level in accordance with the “set of triplets” definition of risk (NASA;2 Kaplan and Garrick3)—What can go wrong?, How likely is it?, and, What are the consequences? Generally, the first question is answered in the form of a structured set of scenarios. The end states of the scenarios are the consequences; the likelihood of the scenarios occurring individually and collectively is assessed based on the supporting evidence. Although Tables 7.2 and 7.3 do not carry the process to completion, they do provide enough information to offer significant insight into the mission risks involved. Definitions Tables 7.2 and 7.3 were developed using the following terms and definitions. Mission Phase A servicing mission includes several major high-level phases that must be completed for the mission to be successful. For the pre-launch phase, the entries in Tables 7.2 and 7.3 relate more to cost and schedule risk than to mission success. The pre-launch phase is listed for completeness rather than as input used by the committee to assess the risk associated with mission success. The mission success risks are assessed for both mission options. What Can Go Wrong? Failures or undesired events are identified in Tables 7.2 and 7.3 for each mission phase and subphase. The list of phases and events is not complete, but the failures and events identified are considered by the committee to be the most important ones for each mission, and they are representative of the types of threats that can prevent a successful mission. In a quantitative risk assessment many of the undesired events and failures identified in Tables 7.2 and 7.3 would be analyzed in the context of scenarios as either initiating events or as downstream events leading to some end state or consequence. In a quantitative risk assessment the mitigation of failures or undesired events (automated or based on crew actions) is taken into account in the determination of likelihood and consequences. In the absence of detailed scenarios and failure data, judgments had to be made about the likelihood and consequences of failures and undesired events, including the effect of risk mitigation features for each type of HST servicing option. The ability to mitigate undesired events was considered by the committee on the basis of briefings and documents presented. 2 NASA, 2002, “Probabilistic Risk Assessment Procedures Guide for NASA Managers and Practitioners,” Version 1.1., NASA Office of Safety and Mission Assurance, Washington, D.C. 3 S. Kaplan and B.J. Garrick, 1981, “On the Quantitative Definition of Risk,” Risk Analysis 1(1): 11-27.
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Assessment of Options for Extending the Life of the Hubble Space Telescope: Final Report TABLE 7.2 Elements in Assessing the Risk Significance of a Shuttle Servicing Mission to the Hubble Space Telescope Mission Phase What Can Go Wrong? Likelihood/ Consequence Risk Significance Uncertainty Comments Pre-Launch Previous loss of vehicle or crew High/High High Medium Events occurring during pre-launch are more related to programmatic risk than to specific mission risk. Noted for completeness. Complete launch countdown Previous anomaly (major problem) Medium/Medium Medium Medium Launch and Ascent: Loss of vehicle or crew High/High High Medium Launch Abort/unable to effect rendezvous Medium/High Medium Medium Successful main engine cut-off Orbit: Orbit insertion Penetration of vehicle by orbital debris (before servicing) Low/High Low Medium Rendezvous Abort mission (systems, performance, etc.) High/High High Low Abort rendezvous Low/High Low Low Loss of radar High/Low Low Low Capture, grapple, mating RMS failure Low/High Low Low Mitigation alternatives available. RMS degradation Medium/Low Low Low Workarounds available for degraded performance. Tip-off rates generated Low/Low Low Low Workarounds available. HST tumbling/ loss of attitude control ~/High Low Low Very unlikely to occur. FSS latch failure ~/Low Low Low EVA required.
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Assessment of Options for Extending the Life of the Hubble Space Telescope: Final Report Mission Phase What Can Go Wrong? Likelihood/ Consequence Risk Significance Uncertainty Comments Capture, grapple, mating, continued Failure to make electrical power connection ~/Medium Low Low Workarounds available. Redundant connectors. Collision with Hubble Low/High Low Low Would significantly affect HST’s continuation. EVA/servicing (5 EVAs) Penetration of environmental mobility unit by orbital debris Low/High Low Low Loss of space-toground communication Low/Low Low Low Inadequate time for operations/ repair (failure to complete) Low/Low Low Medium Assumes partial mission completion. Failure of environmental mobility unit Medium/Low Low Low Early termination of EVA. Extra suits and contingency time. Latch failure High/Medium Low Low Latch replacement is routine. Assumes partial loss of mission. Effect access to HST instrument compartments Unexpected obstruction Low/Medium Low Low Easy to recover. Assumes partial loss of mission. Inability to remove hardware Low/Medium Low Low Assumes partial loss of mission. Removal of hardware from transport Carrier door does not open Low/Low Low Medium Assumes partial loss of mission. locations Incorrect cable length Low/Low Low Low Assumes partial loss of mission.
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Assessment of Options for Extending the Life of the Hubble Space Telescope: Final Report Mission Phase What Can Go Wrong? Likelihood/ Consequence Risk Significance Uncertainty Comments Removal and installation of hardware for each of 5 EVAs Bent pin Medium/Low Low Low Some repair capability exists. Assumes partial loss of mission. Inability to demate/mate connector Low/Medium Low Low Assumes partial loss of mission. Tool failure Low/Low Low Low Workarounds/backup tools available. RMS joint failure Low/High Low Low Some workarounds for limited tasks. RMS failure to release Low/High Low Low Jettison of arm would significantly affect HST’s continuation. RMS failure to grapple Low/High Low Low Failure of cable from excessive force Low/Medium Low Medium Possible EVA-induced loads. Crews train specifically for this. Loss of tool (includes tether failure) Low/Medium Low Low Recovery of tool or collision avoidance maneuver provides mitigation. Optics bay is highest risk. Misalignment/ binding of instrument during removal from HST Low/Medium Low Medium Misalignment/ binding of instrument during insertion into HST Medium/Medium Medium Medium Does not consider instrument getting stuck, preventing safe closeout. Exceeding of thermal limits/ attitude constraints Low/Low Low Low
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Assessment of Options for Extending the Life of the Hubble Space Telescope: Final Report Mission Phase What Can Go Wrong? Likelihood/ Consequence Risk Significance Uncertainty Comments Removal and installation of hardware for each of 5 EVAs, continued Contamination of Hubble arrays, control surfaces, etc. Low/High Low Low Close and secure all panels/doors Latch failure High/Medium Low Medium Assumes partial loss of mission. Panel deformation Low/High Low Medium Inability to close axial doors—severe consequence. Re-boost Hubblea Inadequate propellant for reboost Low/Medium Low Low Carefully planned for—requires orbiter propulsion system failure. Re-deploy Hubbleb RMS failure Low/Low Low Low Loss of contact with Hubble Low/High Low Low Would significantly affect HST’s continuation. aNot considered for robotic servicing mission due to assumption that re-boost will not be planned. bNot required for a robotic servicing mission.
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Assessment of Options for Extending the Life of the Hubble Space Telescope: Final Report TABLE 7.3 Elements in Assessing the Risk Significance of a Robotic Servicing Mission to the Hubble Space Telescope Mission Phase What Can Go Wrong? Likelihood/ Consequence Risk Significance Uncertainty Comments Pre-Launch Previous loss of vehicle (expendable launch vehicle (ELV) is less reliable than shuttle) High/High High High Return to flight can be months to years even for ELV. Previous anomaly (Orbital Express/ XSS-11) Medium/Medium Medium High Late in mission development process. Hardware development problems High/High High High Proximity operations sensor technology immature. Multivehicle systems engineering ground test failures High/Medium Medium Medium Ground test software not ready Medium/Medium Medium Medium Systems architecture currently immature. Flight software not ready High/High High High Systems architecture currently immature. Operator interfaces not developed Low/Medium Medium Medium Training and simulation not ready Low/Medium Medium Medium Resources not available when needed High/High High High Launch and Ascent: Loss of vehicle Low/High Low Medium Abort/unable to effect rendezvous Low/High Low Medium Rendezvous Failure to rendezvous Low/High Medium Medium
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Assessment of Options for Extending the Life of the Hubble Space Telescope: Final Report Mission Phase What Can Go Wrong? Likelihood/ Consequence Risk Significance Uncertainty Comments Rendezvous continued Proximity operations/rate matching failure High/High High High Capture, grapple, mating Robotic system checkout failure Low/High Medium Medium Failure to berth de-orbit module High/High High High Failure to ungrapple HST/deploy dexterous robot Medium/High Medium High Failure to de-mate/ mate power connectors Medium/High Medium Medium Failure to open access doors and keep them open Low/Medium Low Low Effect access to HST instrument compartment Failure to tether cables out of the way Low/Medium Medium Medium Connector cable failure (bent pin) Low/Medium Low Low Removal and installation of hardware Loss of control of connector Low/Low Low Low Dexterous robotic system arm failure Medium/High High High Vision system camera fails Medium/Medium Medium Low Failure of robotic system to grapple/ release tool Medium/Medium Medium Medium Failure of tool Medium/Medium Medium Low Two examples of tool failures on two of four HST servicing missions. Loss of servicing tool Medium/Low Low Low
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Assessment of Options for Extending the Life of the Hubble Space Telescope: Final Report Mission Phase What Can Go Wrong? Likelihood/ Consequence Risk Significance Uncertainty Comments Removal and installation of hardware, continued Misalignment/ binding of instrument during removal/ installation Low/High Medium Low Several occurrences on HST servicing missions. Failure of force feedback system in dextrous robotic system Medium/High Medium Medium Could result in instrument damage. Exceedance of attitude/thermal limitations during instrument change-out Low/Medium Low Medium Loss of control of instrument during change-out/ impact/ complete loss of instrument Low/Medium Low Medium Software incompatibility/ failure in integrated robotic system Low/Low Low Low Failure to stow removed instrument (no ability to jettison) Low/High Low Medium No jettison, no separation maneuver. Significant HST impact. Contamination of Hubble arrays, control surfaces, etc. Medium/High Medium Medium Close and Secure All Panels/Doors Latch failure High/Medium High Medium Assumes partial loss of mission. Panel deformation Low/High High Medium Inability to close axial doors—severe consequences.
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Assessment of Options for Extending the Life of the Hubble Space Telescope: Final Report Mission Phase What Can Go Wrong? Likelihood/ Consequence Risk Significance Uncertainty Comments Deorbit Preparationa Inability to separate de-orbit module from equipment module Low/Medium Low Medium Misalignment of de-orbit thrust vector due to incorrect attachment of de-orbit module Low/Medium Low Medium aNot considered for human servicing mission due to the assumption that SM-4 will not include installation of de-orbit module. Likelihood/Consequence The likelihood of an event is classified according to four broad categories, namely high, medium, low, and extremely low. Likelihood is defined as the frequency per launch of a failure or undesired event, taking into consideration any mitigating features. The category “High” was defined as indicating 1 undesired event in 100 missions or greater, “Medium” as between 1 in 100 to 1 in 300 missions, “Low” as less than 1 in 300 missions, and “Extremely Low” (designated by a tilde (~)) as much less frequent than 1 in 300 missions. A high-consequence event is defined as one that results in loss of the mission, a medium-consequence event implies loss of one mission element, and a low-consequence event signifies a recoverable loss of capability. Risk Significance “Risk significance” as a category reflects a qualitative attempt to take into consideration all three of the risk factors: what can go wrong (scenarios), likelihood (frequencies), and consequences (the end states of the scenarios). Risk significance integrates and interprets likelihood and consequences and accounts for the fact that catastrophic or even existential consequences do not always translate into a high risk. For example, a giant asteroid striking Earth would have catastrophic or possibly even existential consequences, but the extremely low frequency of its occurrence makes the risk of such an event low. Uncertainty In the absence of a quantitative expression of the risk, such as a frequency of occurrence parameter embedded in a probability distribution, a judgment of how uncertainty contributes to risk significance is based on an assessment of the quality of the supporting evidence. In particular, high uncertainty implies considerable weakness in the evidence supporting the assessment of risk significance. It is possible for uncertainty to dominate the assessed risk.
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Assessment of Options for Extending the Life of the Hubble Space Telescope: Final Report Risk Tables The entries in Tables 7.2 and 7.3 are based on input from several sources. They include briefings received by the committee members, committee expertise on Hubble servicing missions and risk assessment, meetings with NASA risk assessment experts, and studies performed by NASA and others on risk and safety, including the Aerospace Corporation’s independent analysis of alternatives to servicing HST.4,5 The entries in Tables 7.2 and 7.3 are based on current NASA plans and schedules. A change of plans would result in a different assessment of a particular entry or entries, or might eliminate one of the entries entirely in the case of a de-scoped mission. A number of factors could affect the assessed risk(s), possibly favorably in some cases. Examples of such factors are de-scoping the servicing mission, extending the time for servicing by 1 year, and performing on-orbit technology demonstrations (e.g., orbital express) and/or an aggressive program of robotic systems development and integration. Table 7.2 addresses some of the risks at issue with the human servicing option for Hubble. The previous highly successful servicing missions to HST have contributed significantly to a high overall confidence in mission success. For those cases where the likelihood/consequence is rated medium or high, experience indicates that there are a number of options for mitigating the associated risk significance. Examples of failures and undesired events during past human servicing, when a human presence was vital to risk mitigation, include remote manipulator system (RMS) failure or degradation, inability to remove hardware, bent pins on connectors, misalignment or binding of instruments during emplacement, and loss of tools. (See Chapter 6 for a more comprehensive discussion of previous successes.) Experience and analysis also indicate that several high-consequence failures or undesired events have a risk significance that is low because of the very low likelihood of their occurrence. Examples are penetration by orbital debris, loss of radar, loss of attitude control, loss of electrical power, and on-orbit catastrophic events. The on-orbit operation of highest mission risk for the human servicing mission is the possibility of having to abort the mission due to system problems. The experience base for human servicing of HST provides strong supporting evidence for the feasibility and reliability of the rendezvous and servicing operations. (See Chapter 6 for additional supporting evidence.) The preparation of HST for cooperative robotic capture to attach a de-orbit module is included in the scenario sequence of Table 7.2 for completeness of the discussion. The shuttle servicing mission is ideally suited to the preparation of HST for later robotic de-orbit (see “Benefit/Risk Assessment for Servicing Options” below and also Chapter 8 for further discussion of the de-orbit issue). Table 7.3 addresses some of the risk issues associated with a robotic servicing mission to Hubble. The overarching risk is the uncertainty in the reliability of the docking, servicing, and de-mating operations in the absence of any in-flight experience. Among the greatest threats to a successful robotic servicing mission are navigational problems close (within 5 to 50 meters) to HST, possible failure to grapple and dock with HST, failure of the dexterous robotic system during servicing, limitations of robotic alternatives in dealing with unanticipated events and failures, failure to berth the de-orbit module, and failure to de-mate. Among the most serious failures would be that of the dexterous robotic 4 Aerospace Corporation, “Hubble Space Telescope Servicing Analyses of Alternatives, Final Delivery Executive Summary,” August 3, 2004, Aerospace Corporation. 5 NASA, 2004, “Hubble Space Telescope Robotic Servicing Mission Project,” Hubble Space Telescope Review Team Report, July 26, NASA Independent Program Assessment Office, Washington, D.C.
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Assessment of Options for Extending the Life of the Hubble Space Telescope: Final Report 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. BENEFIT/RISK ASSESSMENT FOR SERVICING OPTIONS 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 6 Aerospace Corporation, “Hubble Space Telescope Servicing Analyses of Alternatives, Final Delivery Executive Summary,” August 3, 2004, Aerospace Corporation.
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Assessment of Options for Extending the Life of the Hubble Space Telescope: Final Report quality of science to be derived from the continuation of the HST mission and the enhancement of HST performance. A quantitative benefit/risk assessment cannot be made for either mission. However, the committee can conclude that the benefit/risk ratio for the human mission is high, and the benefit/risk ratio for the robotic mission is low. This conclusion takes into account the following: The enormous benefits to science of a serviced Hubble, including enhanced understanding of the physical universe, as articulated in Chapter 3. The conclusion in Chapter 6 that the safety risk for a single mission to the Hubble Space Telescope is comparable to the safety risk for a mission to the International Space Station. The analysis presented in this chapter on the mission risk for the two options, human and robotic servicing of HST.
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Assessment of Options for Extending the Life of the Hubble Space Telescope: Final Report 8 Conclusions and Recommendations The Hubble Space Telescope (HST) provides a host of unique and important capabilities for astronomical research, many of which will not be replaced by any existing or currently planned astronomy facility in space or on Earth. Hubble’s continuing and extraordinary impact on human understanding of the physical universe has been internationally recognized by scientists and the public alike. In recognition of the importance of this science facility to the development of human knowledge, a fifth shuttle servicing mission (SM-4) was in the planning stage prior to the Columbia accident in 2003. The SM-4 mission was being planned to install two new instruments and to perform a number of upgrades to HST’s avionics system. These upgrades are necessary because of the predictable decline in HST component performance over time. This decline in system reliability requires a timely and successful servicing mission in order to minimize further degradation and avoid a significant gap in the return of science data. The need for timely servicing of Hubble imposes difficult requirements on the development of a robotic servicing mission. The very aggressive schedule, the complexity of the mission design, the current low level of technology maturity, and the inability of a robotic mission to respond to unforeseen failures that may well occur on Hubble between now and the mission make it highly unlikely that the science life of HST will be extended through robotic servicing. A shuttle servicing mission is the best option for extending the life of Hubble and preparing the observatory for eventual robotic de-orbit; such a mission is highly likely to succeed. The committee believes that this servicing mission could occur as early as the seventh shuttle mission following return to flight, at which point critical shuttle missions required for maintaining the ISS will have been accomplished. The committee finds that the difference between the risk faced by the crew of a single shuttle mission to the ISS—already accepted by NASA and the nation—and the risk faced by the crew of a shuttle mission to HST is very small. Given the intrinsic value of a serviced Hubble, and the high likelihood of success for a shuttle servicing mission, the committee judges that such a mission is worth the risk.
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Assessment of Options for Extending the Life of the Hubble Space Telescope: Final Report RECOMMENDATIONS The committee reiterates the recommendation from its interim report that NASA should commit to a servicing mission to the Hubble Space Telescope that accomplishes the objectives of the originally planned SM-4 mission. The committee recommends that NASA pursue a shuttle servicing mission to HST that would accomplish the above stated goal. Strong consideration should be given to flying this mission as early as possible after return to flight. A robotic mission approach should be pursued solely to de-orbit Hubble after the period of extended science operations enabled by a shuttle astronaut servicing mission, thus allowing time for the appropriate development of the necessary robotic technology.
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Representative terms from entire chapter: