Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 74
Assessment of Options for Extending the Life of the Hubble Space Telescope: Final Report 6 Space Shuttle Servicing of Hubble The Hubble Space Telescope (HST) was specifically designed to be serviced by space shuttle astronauts on an approximate 3- to 6-year cycle (also see Chapter 2). As noted in Chapter 2 and more thoroughly addressed in the section “Relevant Space Shuttle Mission Successes” below, HST has been serviced four times and each servicing mission has fully met its objectives. Each mission improved the observatory’s capabilities and enhanced its reliability while also satisfying the overriding servicing maxim “do no harm.” This chapter examines certain mission viability factors, other operational considerations, relevant prior servicing missions successes, and mission and crew safety risk considerations in servicing HST using the space shuttle during the flight operations that will follow the return to flight following the Columbia accident. REQUIREMENTS AFFECTING THE VIABILITY OF A SHUTTLE MISSION TO HST WHILE MEETING THE CAIB AND NASA RETURN-TO-FLIGHT REQUIREMENTS The committee takes as its starting point that NASA will meet the Columbia Accident Investigation Board (CAIB) and NASA requirements for the International Space Station (ISS) shuttle missions, and that the ISS shuttle missions are viable. The committee makes no determination or judgment as to whether the ISS missions are worth the human risk, but accepts the implied assessment from NASA that they are. Based on this assumption, this chapter assesses the differences between a shuttle mission to the ISS versus a shuttle HST servicing mission. NASA is currently planning 25 to 30 missions to the ISS to complete its assembly following return to flight (RTF). CAIB Requirements The orbiter Columbia was lost on February 1, 2003, during the re-entry of flight STS-107. After its loss, the CAIB was formed, chaired by retired Navy Admiral Harold Gehman. The CAIB formally reported its findings in an August 2003 report that contained 29 recommendations. Fifteen of these
OCR for page 75
Assessment of Options for Extending the Life of the Hubble Space Telescope: Final Report recommendations were identified as those that should be met prior to RTF. They can be found in their entirety in Chapter 11 of the Columbia Accident Investigation Board’s Report, Volume 1. The CAIB recommendations were stated as a desired end result; the report did not specifically identify how each was to be achieved. Only two CAIB requirements are directly applicable to the viability of a space shuttle servicing mission to HST. They are the following:1 CAIB Requirement 6.4-1 For missions to the International Space Station, develop a practicable capability to inspect and effect emergency repairs to the widest possible range of damage to the Thermal Protection System, including both tile and reinforced carbon-carbon, taking advantage of the additional capabilities available when near to or docked to the International Space Station. For non-Station missions, develop a comprehensive autonomous (independent of Station) inspection and repair capability to cover the widest possible range of damage scenarios. Accomplish an on-orbit Thermal Protection System inspection, using appropriate assets and capabilities, early in all missions. The ultimate objective should be a fully autonomous capability for all missions to address the possibility that an International Space Station mission fails to achieve the correct orbit, fails to dock successfully, or is damaged during or after undocking.2 CAIB Requirement 4.2-4 Require the Space Shuttle to be operated with the same degree of safety for micrometeoroid and orbital debris as the degree of safety calculated for the International Space Station. Change the micrometeoroid and orbital debris safety criteria from guidelines to requirements. NASA is focusing the RTF effort on the ISS mission and has chartered the Return to Flight Task Group to review and evaluate the agency’s compliance with all RTF recommendations. Additional NASA Requirements NASA has determined that it is insufficient for RTF to simply meet the CAIB recommendations and has concluded that it should go beyond CAIB requirements to increase crew safety. Additional applicable NASA RTF activities that affect the viability of an HST mission follow: Space Shuttle Program Action SSP-3—Contingency Shuttle Crew Support [Safe Haven]:3 NASA will evaluate the feasibility of providing contingency life support on board the International Space Station (ISS) to stranded Shuttle crewmembers until repair or rescue can be effected. 1 Columbia Accident Investigation Board, Report, Volume I, August, 2003, p. 174 (Requirement 6.4-1), p. 95 (Requirement 4.2-4). Available online at http://www.nasa.gov/columbia/home/CAIB_Vol1.html. 2 The committee interprets this statement from the CAIB as a requirement for an autonomous capability (if the mission fails to rendezvous with the ISS) on an ISS mission that could also be used on a mission other than ISS. 3 NASA, NASA’s Implementation Plan for Space Shuttle Return to Flight and Beyond, December 3, 2004, p. 2-5. Available online at www.nasa.gov/news/highlights/returntoflight.html.
OCR for page 76
Assessment of Options for Extending the Life of the Hubble Space Telescope: Final Report Space Shuttle Program Action SSP-2—Public Risk of Over-flight:4 The Space Shuttle Program will evaluate relative risk to the public underlying the entry flight path. This study will encompass all landing opportunities from each inclination to each of the three primary landing sites. NASA Administrator’s Considerations In the committee’s discussions with the NASA administrator, it was clear that he considers that three key elements differentiate a shuttle mission to ISS from a servicing mission to Hubble; these elements determined his overarching rationale for cancellation of the shuttle HST servicing mission: Crew safety—Additional crew risk incurred on an HST mission versus an ISS mission; Time and resource considerations—The additional time and resources required to provide the desired inspection and repair on a non-ISS mission; and Disciplined implementation of requirements—The discipline of fully implementing the CAIB and additional NASA shuttle program requirements. Additional Considerations for a Space Shuttle Mission to HST In his letter to Senator Barbara Mikulski dated March 5, 2004, the CAIB Chair, Admiral Gehman, amplified the intent of the CAIB by stating, “We called for a less technically challenging inspection capability for non-ISS missions. Do the best you can.” Due to the capability required to detect and repair thermal protection system (TPS) damage on an HST mission, the CAIB clearly recognized this additional difficulty but did not state any requirement that precluded a non-ISS mission. The committee believes that Admiral Gehman’s phrase “Do the best you can” means NASA’s best effort to meet the CAIB requirements while maintaining a balanced consideration of the risk mitigation provided by the effort. NASA’s Response to Recommendations NASA has publicly stated that the agency intends to comply with all of the CAIB recommendations, and it has initiated a comprehensive program to address CAIB recommendations and NASA RTF requirements. Design changes are being implemented to reduce ascent debris to acceptable limits, and improved ground-based and airborne systems are being implemented to image the ascent phase of the launch. Cameras installed on the external tank (ET), the solid rocket boosters, and the orbiter will provide additional imagery of the TPS during ascent. Following separation of the ET, and once the orbiter is on orbit, the shuttle remote manipulator system (SRMS) with an attached orbiter boom sensor system (OBSS) will inspect the TPS for damage.5 On ISS missions, inspections will also be accomplished by the ISS crew during orbiter approach. Following docking, inspections will be by ISS equipment and/or extravehicular activity (EVA). TPS 4 NASA, NASA’s Implementation Plan for Space Shuttle Return to Flight and Beyond, December 3, 2004, p. 2-3. Available online at www.nasa.gov/news/highlights/returntoflight.html. 5 The OBSS is an integrated system being produced to attach to and augment the existing shuttle.
OCR for page 77
Assessment of Options for Extending the Life of the Hubble Space Telescope: Final Report repair techniques are being developed to permit repair to both tile and reinforced carbon-carbon (RCC) components. Initially, TPS repairs are planned while the orbiter is attached to the ISS using the SRMS to position the orbiter relative to the ISS to provide an astronaut repair work station. After the ISS node 2 is deployed (currently scheduled on the eighth flight following return to flight), the SRMS will no longer be able to reach the ISS grapple fixture, and so different procedures will have to be developed. The SRMS/OBSS is an integrated system that consists of the normal SRMS and an attached boom (the OBSS). The system consists of the following: The normal SRMS with its options for television cameras on the end effecter and the elbow. The SRMS can be used alone or with the OBSS attached. A 50-foot extension (the OBSS) with its supporting electro-mechanical infrastructure in the shuttle payload bay and crew cabin. Two sensor packages attached to the end of the OBSS that can be used to image the orbiter TPS. Sensor package 1 (primary) consists of an integrated television camera (ITVC) (black and white, high resolution, low light capability) and a laser dynamic range indicator (LDRI) (three-dimensional laser mapper to detect and measure the extent of damage to tile and RCC surfaces). These two imagers are mounted on a standard orbiter pan and tilt unit to enhance and expedite the total acreage survey and/or detailed damage inspection of the orbiter surface. Sensor package 2 (back up) consists of a single laser camera system (LCS) with a fixed field of view perpendicular to the long access of the boom and mounted with vibration isolation apparatus. Rated in qualitative terms of level of resolution, the ITVC is good, the LDRI is higher, and the LCS is highest. THE VIABILITY OF A SHUTTLE MISSION TO HST WHILE MEETING THE CAIB AND NASA RTF REQUIREMENTS Based on NASA briefings and materials supplied by the Space Shuttle program, the following represent the committee’s considerations for the viability of a space shuttle mission to HST that will satisfy the CAIB as well as the additional NASA requirements. On-Orbit Inspection Planning and Flexibility An ISS mission incorporates a series of inspections that take advantage of the observations of astronauts on board the ISS as well as the ISS imaging resources to minimize the time required for inspection. NASA is planning the following inspections for an ISS mission: The shuttle SRMS/OBSS will be used in inspecting the wing leading edge RCC early in flight. Prior to docking at the ISS, the shuttle will execute a rotational pitch maneuver to permit visual observation and photography of the tile areas by ISS astronauts using digital cameras. The data will be transmitted to the Mission Control Center for evaluation. After docking, the SSRMS and window views will be used for visual observations and digital photography, if required. Detailed inspection of areas of concern found during the ISS observations will be performed, if warranted, using the SRMS/OBSS. If required, a spacewalk can also be carried out for a close-up inspection. NASA is currently developing EVA inspection techniques. On an HST mission, SRMS (standalone) and the SRMS/OBSS, without augmentation from any other system, could be used to do a complete inspection.
OCR for page 78
Assessment of Options for Extending the Life of the Hubble Space Telescope: Final Report NASA reported that the inspection on an HST servicing mission would require more time than one on an ISS mission. This increased time results from not having the advantage of an inspection from astronauts and from the imaging resources on board the ISS. NASA has not done a complete time line to determine the exact amount of time required for an ISS inspection. The committee believes that it is possible to develop additional sensors that would reduce the time required to perform an inspection on a shuttle HST mission. The options range from new techniques to scaled versions of the current sensors to fill the SRMS coverage gap. The committee concludes that there are at least two approaches that could satisfy the inspection requirements during a shuttle HST servicing mission: The resources and procedures developed for the ISS missions could be used to accomplish the inspection and to add the required time to the HST time line as follows: Use the SRMS/OBSS to inspect the wing leading edge (WLE). Use the SRMS (standalone) to inspect areas where it provides adequate resolution for a large portion of the acreage tile. The remaining area cannot be imaged at the required resolution with the SRMS due to limitations on its physical reach and the field of view of its wrist camera. Complete the remaining inspection of the acreage tile with the SRMS/OBSS. A detailed inspection could be accomplished via a spacewalk as a backup if deemed necessary. Develop additional sensors to reduce the inspection time. Use the SRMS/OBSS to inspect the WLE RCC. Use the SRMS to inspect areas where it provides adequate resolution. Use the additional sensors to inspect the remaining areas. Use the OBSS to inspect areas of concern if required and available. A detailed inspection could be accomplished via a spacewalk as a backup if deemed necessary. The committee notes that implementation of either approach would satisfy the CAIB and NASA requirements for shuttle inspection. The ultimate objective would be a fully autonomous capability for all shuttle missions in order to address the possibility that an ISS mission fails to achieve the correct orbit, fails to dock successfully, or is damaged during or after undocking. FINDING: A complete inspection of the orbiter thermal protection system can be accomplished on a shuttle servicing mission to HST using the SRMS and the SRMS/OBSS. On-Orbit Repair Capability and Limitations To repair damage to the outer surface of the shuttle while it is on orbit, one or more astronauts must make the repair during a spacewalk. The astronaut must be able to access the area to be repaired and have a stable work site at the area. For return to flight to the ISS, the shuttle can be positioned using the SRMS to an attitude that allows access to the work site from the ISS, either directly or from the SRMS. However, after the installation of the node 2 on the ISS (currently scheduled for the eighth flight after RTF), due to the inability to reach the ISS grapple fixture, the SRMS will no longer position the shuttle for inspection and repair. A different work site plan will be required after the eighth flight after RTF. NASA is currently developing a technique using the SRMS/OBSS to position the crew at the work
OCR for page 79
Assessment of Options for Extending the Life of the Hubble Space Telescope: Final Report site. This technique could also be used on an HST mission. Since this technique provides access to the repair work site independent of ISS, it can be used on all flights or on an ISS flight that fails to achieve the correct orbit or to dock successfully. Therefore, it will satisfy the repair site access portion of the “fully autonomous” capability requirement recommended by the CAIB. FINDING: The orbiter thermal protection system repairs can be accomplished on a shuttle servicing mission to HST following the development of work site and repair techniques for ISS to meet the CAIB and NASA requirements. Safe Haven and Crew Rescue The CAIB did not make a recommendation for a safe haven capability for future space shuttle missions. Nevertheless, NASA has recognized that ISS missions provide some capability for a safe haven, and has baselined a Contingency Shuttle Crew Support (CSCS/Safe Haven) requirement for the first two flights following RTF. In the future, the program will consider extending this requirement. ISS Safe Haven The ISS can be used as a safe haven to provide additional time to deal with emergency problems. If the shuttle is docked to the ISS, NASA analysis indicates that the astronauts could be housed in the ISS for 30 to 90 days beyond the shuttle mission time frame. This conclusion assumes (1) that the zero-faulttolerant6 ISS life support system (i.e., ability to support 10 people) is available, and (2) that sufficient supplies (food, water, and so on) have been pre-positioned aboard ISS. The additional time provided by the ISS safe haven capability, assuming it is available, provides the following attributes: Additional time to repair the damaged shuttle and prepare the shuttle for re-entry, Additional time to make modifications to the rescue vehicle and its cargo if required and to launch the rescue shuttle, and Schedule relief for the shuttle launch team. Although these attributes provide desirable operational flexibility, use of the ISS safe haven also results in a strategy that has significant risks. First, the ISS’s ability to support 10 people for 30 to 90 days depends on a zero-fault-tolerant life support system that may fail at any time. This capability also requires the pre-staging of adequate resources on the ISS to support 10 people for the desired time. Important ISS areas that are zero-fault-tolerant or that have negative margins are oxygen generation, carbon dioxide removal, waste removal, water supply, and condensate processing. FINDING: The ISS safe haven offers operational flexibility and time to adapt to real-time problems in the case of a critical ascent impact event that is both detected and repairable, or that affords the option of a shuttle rescue mission. However, the availability of the ISS safe haven is zero-fault-tolerant, requires significant pre-positioning of supplies, and, therefore, poses significant risks due to its limited redundancy and margins. 6 Zero-fault tolerance means that any single failure renders the system inoperative and results in loss of the system’s function.
OCR for page 80
Assessment of Options for Extending the Life of the Hubble Space Telescope: Final Report HST Shuttle Servicing Mission Safe Haven On an HST shuttle mission, a safe haven could be provided by an extreme power-down of the shuttle. The duration is limited, due to critical consumables, to between 17 and 30 days depending on when the contingency power-down is done. This would require the launch of a rescue vehicle within days after launch of the servicing shuttle that encountered the problem. However, during the committee deliberations, other options for increasing the safe haven time at HST surfaced.7 Following the Challenger accident, NASA developed an Extended Duration Orbiter (EDO) capability using additional cryogenic tanks. This permitted the shuttle to fly up to 15 additional days in an extreme powered-down condition. NASA reported that orbiter vehicle 104 (OV104) and OV105 are equipped to utilize the EDO system, whereas OV103 is not. Outfitting OV103 for EDO would provide highly desirable scheduling flexibility if a mission required the EDO’s capability. However, it would take 6 months of work to install the EDO system in the orbiter processing facility. The oxygen and nitrogen tanks are long-lead items that would take 24 months to design, build, and certify. The completed hardware is required at Kennedy Space Center 8 months before flight. Therefore, the earliest the EDO could fly is 2.8 years from project initiation. Space Shuttle Rescue Mission As Integral to All Safe Haven Concepts A shuttle rescue mission is part of the NASA requirements in planning for the ISS mission. It entails being prepared to launch a rescue shuttle to retrieve the stranded crew of a damaged shuttle. On an ISS mission, the ISS safe haven is expected to provide the additional time required to mount the rescue. This would allow the orbiter to be in a normal flow in the orbiter processing facility at any call-up for rescue. A shuttle rescue mission of an HST crew would require launch and rescue within 17 to 30 days, depending on the timing of an emergency power-down of the shuttle, while the ISS mission nominally provides 30 to 90 days. If the capability for rescue is deemed mandatory for a single shuttle mission to service HST, the rescue vehicle would have to be pre-positioned on the launch pad to allow launch as soon as possible. The workload for launch preparation in such a case would be more intense. However, with careful planning and preparation, the committee believes it is well within the capability of the shuttle team. The shuttle processing team is regularly processing up to three orbiters at any time. Planning, scheduling, and prioritizing the total work at KSC should allow the processing of an early rescue launch without an “unprecedented workload.” The shuttle program has experienced several periods where balancing workload and the short-term manifest has overcome workload challenges. For example, in July of 1995, STS-71 was launched 14 days after STS-70. To minimize the impact to downstream flight, this strategy would also require careful manifesting of rescue hardware and ISS hardware in order to allow proceeding with the next ISS flight in an orderly fashion. NASA has indicated that, if the required cargo is changed out on the launch pad, an ISS mission could be launched 30 days after a rescue mission is called off. The prompt initiation of limited mission planning for a shuttle servicing mission to HST, including the requirement for parallel processing for a 7 In addition to the safe haven consideration discussed in this section, it came to the committee’s attention that commercial companies have suggested options to launch a “safe haven” vehicle into the HST orbit in order to provide a longer-term capability. The committee understands that NASA has been provided these proposals, which will naturally require a balancing of crew safety, risk reduction, cost and schedule, and so on, if any are pursued.
OCR for page 81
Assessment of Options for Extending the Life of the Hubble Space Telescope: Final Report rescue orbiter on the second launch pad, would ensure maximum flexibility and a minimum impact on resources. FINDING: An HST shuttle rescue mission can be ready on the second launch pad to respond to any need for rescue. Such an approach would involve some costs and ISS schedule delays, principally because of the impact of parallel orbiter processing. Limited time would be available to execute a rescue. Micro-Meteoroid Orbital Debris Risks The risks to the shuttle from micro-meteoroid orbital debris (MMOD) are different for an ISS and an HST flight. The debris density at the HST altitude (~570 km) is higher than the density at the ISS altitude (~355 km), resulting in a higher risk of collision at the HST orbit. On the other hand, the orbiter attitude during an HST mission affords better protection from the debris. When altitude and attitude considerations are combined, an HST flight has a smaller chance of experiencing a catastrophic debris collision than does an ISS flight. However, NASA is currently working to modify the ISS attitude to reduce the risk of collision on an ISS mission and to validate the collision and damage models to better understand this problem. When this work is completed, NASA expects that the MMOD risk will be smaller on an ISS mission. The committee expects that the ultimate differences will be small and will not be a significant contributor to the risk factors. NASA plans considerable effort on the MMOD issue prior to resuming shuttle missions in order to develop a better understanding of the MMOD risks and to develop the flight rules to control the risks. Public Risk of Overflight The shuttle de-orbit burn and subsequent re-entry and landing can be accomplished from either ascending or descending orbital tracks. The tracks have been designed to optimize reception of telemetry data, structure the crew work/rest cycle, provide daylight landings, and deal with the weather at the landing site. The combination of landing site location and the choice of ascending (vehicle traveling northeast) or descending (vehicle traveling southeast) re-entry tracks determines the amount of populated landmass that is overflown. The populated landmass overflown during entry is the driver for public risk. Since the Columbia accident, this has become a heightened concern. NASA is currently developing mission rules to manage the entry flight path in order to deal with the public risk of overflight. The committee is confident that flexibility of ascending orbits versus descending orbits and landing site selection will allow the development of flight rules that will result in comparable public risk of overflight for both the ISS and HST missions. Summary of Viability for Meeting Both the CAIB and the NASA Requirements Considerations regarding the viability of meeting the CAIB and the NASA requirements are discussed above. Inspection techniques developed for ISS missions can be used to accomplish the requirements for TPS inspection. The committee believes that additional work can be done to reduce the time required for implementation of the shuttle HST servicing mission. Repair requirements can be accomplished on an HST mission once work site positioning techniques are developed. Techniques currently being developed for use after node 2 installation on the ISS could also be used on a shuttle HST servicing mission. The committee believes that an emergency power-down as soon as a credible indica-
OCR for page 82
Assessment of Options for Extending the Life of the Hubble Space Telescope: Final Report tion of a catastrophic problem is detected will afford additional HST mission safe haven time. Shuttle rescue would involve pre-positioning of a rescue shuttle on the launch pad ready for rescue and performing the necessary mission preparation. The committee believes that once the scheduled work of developing mission rules and procedures is completed, the MMOD risk and the public risk from overflight will not be a consideration. FINDING: Meeting the CAIB and NASA requirements (relative to inspection and repair, safe haven, shuttle rescue, MMOD, and risk to the public) for a shuttle servicing mission to HST is viable. ADDITIONAL OPERATIONAL CONSIDERATIONS In addition to meeting the CAIB and NASA RTF requirements, the following considerations affect the ability to execute a shuttle HST servicing mission. Shuttle Rescue Operations Complexity Crew rescue on an HST mission would require planning and training for a complex set of EVA operations to effect the transfer of the crew. In an example scenario provided by the Shuttle Program Office, a rescue shuttle would launch within days after an endangered HST servicing mission and would rendezvous with the damaged shuttle. After rendezvous, the damaged shuttle would grapple the rescue shuttle with the robotic arm. Three spacewalks would be conducted to transfer the rescued crew and launch and entry suits (LES). Two of the spacewalks would be conducted while grappled, while the third would be conducted while flying in formation (a crewperson is required to ungrapple the rescue vehicle from the damaged vehicle). The rescue shuttle’s SRMS would be used to transport the crew members from the damaged shuttle to the rescue shuttle. The first two EVAs would be long spacewalks involving two depressurizations and repressurizations of the shuttle airlock. The third spacewalk would be conducted while flying in formation since the SRMS of the damaged shuttle must be un-grappled before the last crewperson leaves the vehicle.8 The shuttle has rendezvoused with and grappled numerous satellites, including the HST four times, without any major problem. The rendezvous and grappling of the damaged shuttle are well within the experience base and the capabilities of the shuttle program. The spacewalks that transfer the flight crew are complex and result in a higher risk than the transfer on an ISS mission. However, the shuttle program has considerable experience in complex spacewalks as described below in “Relevant Space Shuttle Mission Successes.” FINDING: The extravehicular activities (spacewalks) for transferring the crew from a damaged vehicle on a shuttle HST flight, although complex, are well within the experience base of the shuttle program. 8 Randall Adams and Wayne Hale, NASA Johnson Space Center, “Shuttle and Mission Operations: Requirements for Human Servicing Mission,” presentation to the Committee on the Assessment of Options for Extending the Life of the Hubble Space Telescope, June 2, 2004.
OCR for page 83
Assessment of Options for Extending the Life of the Hubble Space Telescope: Final Report HST Manifesting Options The flight position selected for an HST servicing mission in the space shuttle manifest is crucially important, since a balance must be struck between shuttle RTF, construction and logistics requirements of the ISS, and the necessity to preserve and upgrade the HST science mission before failures aboard HST make that impossible. NASA reported to the committee that, if flown, the HST mission would be positioned in the shuttle manifest after the completion of the twelfth ISS flight currently scheduled in July 2007. This flight positioning gives priority to the deployments of international partner elements to the ISS. If the RTF launch date of March 2005 slips significantly, or if subsequent ISS mission delays are incurred, this position in the shuttle schedule flight would put the HST at risk (see Chapter 4). After the shuttle returns to flight, the first two missions (FL1 and UFL1.1, both to the ISS) are planned to be devoted to RTF activities and to initial ISS logistics and utilization. In discussing with NASA the ISS planning for subsequent flights, the committee was informed that the critical flights for the ISS are those that will ensure its power and thermal configuration (flights 12A, 12A.1, 13A, and 13.A.1). Inserting an HST servicing mission before this sequence is complete would not be advisable. Therefore, the earliest opportunity to fly the HST mission is the seventh flight after RTF (currently scheduled for July 2006). This flight position would provide the best opportunity for HST mission success but would delay the completion of the ISS assembly and deployment of the international partner elements by about 4 to 6 months. The exact time delay will depend on the approach to the HST mission, the resources expended to prepare for the HST mission and the next mission, and the planning for processing at KSC. Implementation of an HST flight on the seventh mission will require careful ISS logistics planning and associated manifesting. FINDING: To avoid putting the Hubble at risk and to maintain continuous science operations, the HST servicing mission could be flown as early as the seventh flight after return to flight without a critical operational impact on the ISS. RTF Workload NASA informed the committee that the agency is concerned about the time and effort needed to attain the level of additional safety that is required to successfully complete the RTF, and is further concerned that adding the burden of a non-ISS flight to the shuttle flight manifest could seriously threaten its capacity to return to flight in a timely fashion. Although much of the mission planning for a shuttle HST mission was well along prior to the announcement of cancellation of the HST SM-4 mission, additional planning and training for the crew and ground team still remain to be accomplished in order to prepare for the additional activities to meet the CAIB and NASA requirements on the HST mission. As examples, the on-orbit TPS inspection plan would be different from that on an ISS mission. Additional repair site stabilization techniques and/or hardware may be required, and contingency planning for the transfer of a stranded crew to a rescue mission vehicle will require the initiation of new planning and training. While recognizing that additional work must be done to position the program to perform an HST servicing mission, the committee believes that the earliest HST servicing mission could occur on the seventh mission following RTF and that the major HST workload can come after the workload required for RTF.
OCR for page 84
Assessment of Options for Extending the Life of the Hubble Space Telescope: Final Report FINDING: Major HST mission preparation work for a shuttle servicing mission to HST can be deferred until after return to flight. This approach would avoid a significant expenditure of human resources until the shuttle is flying again. HST De-orbit Module on a Shuttle Servicing Mission The committee did not give consideration to the option of flying an HST de-orbit module on a shuttle servicing mission because of the possible mission complications, the additional time required for mission preparation, potentially excessive cargo weight/volume, and possible problems with de-orbit module reliability given the required long stay on orbit before de-orbit. However, during NASA planning for a shuttle HST servicing mission, an in-depth assessment should be conducted to determine if there is any merit to flying a de-orbit module on the shuttle servicing mission rather than conducting an end-of-mission robotic de-orbit. TIME AND RESOURCES NEEDED TO OVERCOME UNIQUE TECHNICAL OR SAFETY ISSUES ASSOCIATED WITH HST SERVICING After the cancellation of the shuttle HST servicing mission, NASA stopped all work on a non-ISS mission and is concentrating on RTF and the ISS missions. As a result, the NASA data available to the committee to allow it to assess the time and resources required to overcome any unique technical or safety issues associated with HST servicing required to meet the CAIB and NASA requirements was limited to qualitative statements provided by the Shuttle Program Office and other NASA personnel. The actual amount of unique time and resources required to fly an HST versus an ISS mission depends on the approach selected to implement this single HST mission. The committee believes that the range of possible options available to NASA is broad and includes the following: Adapt and implement the ISS-developed inspection and repair capability on the HST flight. Do not develop additional major inspection and repair procedures to support the single HST mission. Focus on inspection and repair capability and forego the rescue capability. Current plans indicate that such an approach would require either adding time to the inspection task with the shuttle OBSS or augmenting the OBSS ITVC with new and/or additional sensors to provide an overview inspection similar to the ISS capability. This option would also require providing techniques and/or hardware to ensure that the OBSS could be certified to serve as a work site for TPS repair. Focus on the rescue capability while augmenting the inspection capability to the minimal extent deemed acceptable. This approach would require parallel orbiter processing at KSC to provide a second launch-ready shuttle as well as simultaneous, equal-priority crew training at JSC for both the HST and rescue mission crews. Focus on both a full repair capability and a rescue capability. Ultimately the decision on the approach to an HST mission is the responsibility of NASA. Although all options may not meet the full intent of the CAIB and NASA requirements, the committee believes that after consideration of an appropriate crew safety risk analysis for a single HST mission, any of the above options would be acceptable. Regardless of the NASA approach taken on an HST flight, the committee also believes, based on its accumulated experience, that the increases in required resource and time impacts would be small compared with the total cost of servicing HST. Any of the approaches are within the framework of the shuttle program capacity and experience base.
OCR for page 85
Assessment of Options for Extending the Life of the Hubble Space Telescope: Final Report FINDING: Compared with the total cost of a shuttle flight, the resources required to overcome unique technical or safety issues involved in flying a shuttle mission to HST are small and are well within the experience base of work done in the past to enable unique shuttle missions. ADDITIONAL COSTS TO REINSTATE A SHUTTLE SERVICING MISSION Detailed information on the cost of performing a servicing mission of HST using the shuttle was not available to the committee, but the committee did receive a portion of NASA’s input to the Government Accountability Office (GAO),9 which estimated costs as follows: Hubble Project Costs $614 million Shuttle and ISS Program Costs $1.1 billion to $1.8 billion Total $1.7 billion to $2.4 billion The NASA letter report to the GAO notes that $400 million to $1.0 billion of the shuttle and ISS program’s cost will be incurred in the last year of shuttle program life, currently targeted for 2010. The variations are due to marginal versus proportional annual cost accounting methods. The HST project costs of $614 million will be expended through 2012 for HST to sustain engineering, mission operations and analysis, and for delay of the de-orbit module to 2012. NASA’s letter states that the estimated costs of standalone TPS inspection and repair capability ($260 million to $300 million) and development of standalone rescue ($290 million to $340 million) are due to the fact that “no design solution is currently available.” The committee believes that careful planning for, and implementation of, the additional HST-unique activities to meet the CAIB and NASA requirements will result in substantially lower actual costs to service HST using the shuttle than those projected above. For example, the inspection techniques can be a direct derivative of the ISS techniques. The repair techniques could be, as discussed above, the same as those for the ISS after node 2 installation. The ongoing GAO assessment of shuttle servicing costs may provide greater insight into these questions when it is released at the end of 2004. HST VERSUS ISS CREW SAFETY RISK NASA reports that the agency is currently in the process of updating the shuttle probabilistic risk assessment (PRA) model planned to be available in late 2004. The agency was therefore unable to provide specific, quantitative information on difference in risk for HST and ISS crew safety for examination by the committee. The data provided during the committee’s discussions with NASA were based on engineering judgment and were qualitative, and in most cases specific elements of the HST mission risk were described as “higher risk” or “lower risk” (in comparing an ISS mission to an HST mission), but otherwise were not quantified. Since the Columbia accident, NASA has been developing many safety improvements to be implemented prior to RTF and beyond. The committee reviewed progress on RTF issues, qualitatively assessed the risk reduction expected from the safety improvements for missions to HST and ISS, and qualitatively compared the risks of the two types of missions. The committee agrees that post-RTF missions to the ISS will have some safety advantage over an HST mission, such as total time required to 9 NASA, “Review of Hubble Space Telescope Servicing Mission Cost,” letter report dated August 13, 2004, NASA, Washington, D.C.
OCR for page 86
Assessment of Options for Extending the Life of the Hubble Space Telescope: Final Report perform ascent damage detection and the availability of a crew safe haven and rescue (see previous sections of this chapter). However, the committee concludes that this post-RTF advantage will be small—because the need for such repairs and crew rescue will have been sharply reduced by elimination of critical ascent debris. That is, the NASA shuttle program’s rationale for return to flight from the STS-107 mission failure is based on the identification and elimination of critical ascent debris. Sources of critical ascent debris are defined as those from which there is an unacceptably high probability of liberation of debris during launch, and have an aerodynamic transport mechanism that would permit the debris to impact a vulnerable location with enough energy to cause catastrophic damage to the TPS. Following flight certification for the improved external tank, NASA will consider the ascent debris risk to the shuttle TPS as acceptable. All other corrective actions are considered additional risk-reduction measures. These include the on-orbit TPS inspections, repair capability, and safe haven for both the ISS and HST missions. In a meeting with a subgroup of the committee that reviewed risk questions associated with a shuttle HST servicing mission, NASA personnel stated that the risks associated with the launch/ascent and entry/landing phases of any mission comprise the vast majority of the safety risks of a mission, and that these phases are comparable for the ISS and HST missions. Therefore, in terms of risk to vehicle and crew, the committee concludes that the difference in risk of loss of the vehicle and crew between a single servicing mission to Hubble and a single mission to the ISS is extremely small. The committee further believes that adding a shuttle flight for an HST servicing mission adds approximately a percent to the total risk, which is a small percentage of the risk of losing astronauts in the course of completing the already planned ISS program. FINDING: The shuttle crew safety risks of a single mission to ISS and a single HST mission are similar and the relative risks are extremely small. RELEVANT SPACE SHUTTLE MISSION SUCCESSES Human Response to Unforeseen On-Orbit Contingencies As noted in “Avionics Reliability Model” in Chapter 4, the flexibility provided by astronauts is highly valuable in repairing unforeseen anomalies in HST’s avionics system (see findings in “Avionics Reliability Model”). Between 1984 and 1992, prior to the first HST servicing mission, there were five space shuttle missions in which astronauts were called upon to respond to unexpected scenarios in the conduct of spacewalks (or EVAs) or leading to EVAs. The five incidents are summarized below. STS-41-C, April 1984. The mission was planned to retrieve, repair, and redeploy the Solar Maximum Mission (SMM) satellite, which was de-spun to enable the on-orbit work. During the attempt to capture the SMM using the manned maneuvering unit with an attached trunnion pin attachment device, the grapple mechanism failed to operate multiple times and an attempt by the extravehicular crewperson to stabilize the satellite by hand resulted in increased instability of the SMM. Following a night and day of re-planning on the part of mission control and flight crews, the shuttle was flown so as to bring the SMM into close proximity with the shuttle payload bay where it was grappled using the SRMS. The SMM was subsequently berthed in the shuttle, repaired (a faulty attitude control system and one science instrument were replaced) and redeployed using the SRMS.10 10 NASA, STS-41-C National Space Transportation Systems Program Mission Report (JSC-19642), NASA Johnson Space Center, Houston, Tex., May 1984.
OCR for page 87
Assessment of Options for Extending the Life of the Hubble Space Telescope: Final Report STS-51-A, November 1984. The mission was planned to rendezvous with and retrieve the Palapa B2 and WESTAR VI communications satellites that had failed to reach their operational orbits following failure of their upper-stage rocket motors on a previous shuttle mission. When the EVA crewperson flew to the first satellite using the manned maneuvering unit and attempted to grapple it using a specially designed capture bar, the bar failed to fit properly and the satellite capture failed. Following re-planning by the mission control and flight crews, the shuttle was flown sequentially to re-rendezvous with each satellite and the satellites were literally flown into the cargo bay where EVA crewpersons, tethered in the payload and on the end of the RMS, manually grappled each satellite and docked them in the bay for the flight back to Earth. Each satellite was subsequently refurbished and successfully launched into service.11 STS-51-D, April 1985. The mission was planned to deploy the SYNCOM IV-3 communications satellite. Following deployment, it was determined that the satellite failed to activate as planned. The shuttle rendezvoused with the malfunctioning satellite and, while flying in close formation, two EVA crewpersons attempted to manually activate the power switch on the satellite by utilizing a device that was fabricated on board. The activation attempts failed, but the shuttle program had again demonstrated the ability of human crews to make a real-time response to an on-orbit contingency. There had been no EVA planned for this flight.12 STS-51-I, August/September 1985. The flight was planned to rendezvous with and retrieve the errant SYNCOM IV-3 communications satellite left on orbit by STS-51-D earlier in the year. After the rendezvous was accomplished, two EVA crewpersons manually grappled the satellite, brought it into the payload bay for the installation of a new battery/starter mechanism, and subsequently manually redeployed the satellite for successful on-orbit operation.13 STS-49, May 1992. This mission was planned to rendezvous with and retrieve an INTELSAT-VI communications satellite that had been left in a useless orbit from its earlier launch. In a never-before-done three-person EVA, the crew manually grappled the satellite as it was flown into the payload bay. The crew installed a replacement upper-stage rocket motor while the satellite was in the shuttle payload bay and redeployed it for subsequent successful on-orbit operation.14 Space Shuttle Servicing Missions to the Hubble Space Telescope To date, there have been four completely successful space shuttle servicing missions flown to Hubble. These missions have continuously enhanced the performance of HST, resulting in a huge increase in the data-gathering capability of this observatory. The following are summaries of the accomplishments as well as some contingency responses that were necessary during the conduct of these four HST servicing missions. STS-61 (SM-1), December 1993 Servicing mission (SM)-1 was the first of the HST servicing missions, and its primary goals included the installation of the Corrective Optics Space Telescope Axial Replacement (COSTAR) to 11 NASA, First 100 Manned Space Missions, NASA Kennedy Space Center Public Affairs Office, June 14, 1995. Available online at http://science.ksc.nasa.gov/shuttle/missions/51-a/mission-51-a.html. 12 Ibid. Available online at http://science.ksc.nasa.gov/shuttle/missions/51-d/mission-51-d.html. 13 Ibid. Available online at http://science.ksc.nasa.gov/shuttle/missions/51-i/mission-51-i.html. 14 NASA, Flight 047 STS-49 Mission Highlights. Available online at http://spacelink.nasa.gov/NASA.Projects/Human.Exploration.and.Development.of.Space/Human.Space.Flight/Shuttle/Shuttle.Missions/Flight.047.STS-49/.index.html.
OCR for page 88
Assessment of Options for Extending the Life of the Hubble Space Telescope: Final Report correct for the spherical aberration that was discovered in the telescope’s primary mirror weeks after its initial deployment in 1990. In the process of conducting five EVAs, the crew encountered six documented anomalous situations.15 Retraction of the positive axis solar array (+V2 SA) was halted when the crew visually detected slack in the blanket. The problem was caused by bowing and a kink in the outer bi-stem of the solar array. The decision was made to manually remove and jettison the damaged array to avoid the risk of having an improperly stowed component in the shuttle payload bay during reentry and landing. During initial attempts to close the –V3 aft shroud door on HST, the EVA crew encountered alignment problems that prevented closure. The misalignment was subsequently corrected through the impromptu use of a payload retention device and the door was closed and locked. In the process of examining the integrity of HST, the EVA crew discovered two loosened sides of protective covering on the magnetic sensor system 2. A thermal blanket available elsewhere on HST was wrapped on the magnetometers to protect them from further degradation caused by exposure to atomic oxygen and ultraviolet light. On the second and fourth EVAs, two-way communications between an EVA crewperson and the orbiter crew was lost. Communications between this crewperson and the other two EVA crew members, however, remained good and both EVAs were continued using relay of communications. When the primary deployment mechanisms of the new solar arrays were commanded to deploy, neither responded. An EVA crewperson manually deployed both of them. During the original SADE-1 removal, two connector screws and mounting clips became disengaged and were captured by the EVA crewperson. A mounting screw also came loose and was retained. When the EVAs were completed, the following operations had been successful completed: Installed COSTAR, Installed Wide Field Planetary Camera-2 (WFPC-2) to replace the original instrument, Replaced both solar arrays, Replaced the solar array drive electronics (SADE), Replaced the original magnetometers, Replaced the co-processor for the flight computer, Installed two replacement rate sensor units (RSU), Installed two replacement gyroscope electronic control units, and Installed a Goddard High Resolution Spectrograph (GHRS) redundancy kit. STS-82 (SM-2), February 1997 The objective of the second servicing mission to HST was to significantly upgrade the scientific capabilities of the observatory. All of the HST primary and secondary objectives for this mission were fully accomplished. Although four EVAs were originally scheduled, it was decided to conduct a fifth EVA for the purpose of repairing a damaged (torn) thermal blanket. The crew also fabricated patches that were installed to cover thermal blanket tears on HST bays 8 and 10. Both bays contained components requiring thermal protection. There were two documented anomalies during EVAs:16 15 NASA, Space Shuttle Mission Report NSTS-08288, NASA Johnson Space Center, Houston, Tex., February 1994. 16 NASA, Space Shuttle Mission Report NSTS-37413, NASA Johnson Space Center, Houston, Tex., April 1997.
OCR for page 89
Assessment of Options for Extending the Life of the Hubble Space Telescope: Final Report During opening of the +V2 aft shroud doors on EVA-1, the bottom latch bolt backed out only 3½ turns (as opposed to the expected 6 to 8 turns) when the door was initially opened. Furthermore, during door closure another latch would not drive closed with the nominal tool setting. When the lowest latch was attempted, it also did not drive. The EVA crewperson increased the torque setting on the pistol grip tool to start both fasteners and then reset it to the planned torque. When attempting to mate an electrical harness to the HST connector, the EVA crewperson noticed a bent pin in the corner on the short side. A spare harness was obtained from an onboard storage locker and installed successfully. At the completion of the five EVAs on SM-2, the following tasks had been successfully accomplished: Installed the Space Telescope Imaging Spectrograph (STIS), Installed the Near Infrared Camera and Multi-Object Spectrometer (NICMOS), Installed a refurbished fine-guidance sensor (FGS), Installed an Optical Control Electronics Enhancement Kit (OCE-EK), Installed a solid-state recorder (SSR), Replaced a reaction wheel assembly (RWA), Replaced a data interface unit (DIU), and Replaced the solar array drive electronics (SADE). STS-103 (SM-3A), December 1999 The objective of the third HST servicing mission was to further upgrade the scientific capabilities of the observatory. Because HST had gone into safe mode with the failure of a fourth gyroscope, the decision was made to divide the scheduled third mission into two missions in order to launch an earlier “emergency mission” to replace the failed gyros that had led to the HST “going to sleep.” From the time of the decision to fly the “emergency mission,” it was planned, launched, and successfully accomplished in 7 months. All the EVA tasks for this flight were fully accomplished over the course of the three EVAs. Only one EVA anomaly occurred, and that was a failure of the power ratchet tool (PRT) during EVA-1. After an unsuccessful attempt to correct the problem with a change-out of the batteries, the PRT was replaced with the pistol grip tool for the remainder of the flight.17 At the completion of SM-3A, the following had been successfully accomplished: Installed three new rate sensor units (six replacement gyroscopes), Installed batter voltage/temperature improvement kits, Installed a faster (486) main computer, Installed a next-generation solid-state data recorder, Installed a new S-band single-axis transmitter-2 (SSAT2), Installed a replacement enhanced fine-guidance sensor (FGS), Installed new outer blanket layers (NOBL) on bays 9 and 10, and 17 NASA, STS-103 Space Shuttle Mission Report NSTS-37426, NASA Johnson Space Center, Houston, Tex., February 2000.
OCR for page 90
Assessment of Options for Extending the Life of the Hubble Space Telescope: Final Report Performed the NICMOS valve-opening procedure in preparation for reservicing on the next servicing mission. FINDING: The shuttle mission planning process provides flexibility in the final manifest and in mission execution that can be used to respond to known or unforeseen HST anomalies. STS-109 (SM-3B), March 2002 Five EVAs were successfully conducted on the fourth HST servicing mission with no documented EVA activity anomalies.18 At the completion of the mission, the following had been accomplished: Replaced the original Faint Object Camera (FOC) with the Advanced Camera for Surveys (ACS), Installed solar array 3 +V2, resulting in a 30 percent power increase, Installed a new power control unit (PCU) requiring the power-down of HST, Installed the Electronic Support Module (ESM) for NICMOS, Installed the NICMOS cryocooler (NCC), Installed the NICMOS cooling system radiator, Replaced the new outer layer blanket on bay 6, Replaced a reaction wheel assembly, and Completed several minor get-ahead tasks on the HST structure. FINDING: In the case of every documented anomaly encountered during the conduct of extravehicular activities on all four HST missions, the onboard crew, in conjunction with its ground-based mission control team, worked around each anomaly and successfully completed every task planned for these missions. HST SERVICING MISSION RISK HST servicing mission risk depends on the availability of the shuttle and, once launched, the likelihood that the mission will be successfully accomplished (see also Chapter 7). Based on discussions with the probabilistic risk assessment experts in NASA and drawing on the expertise of committee members, the committee concludes that there is an 80 to 90 percent probability that the shuttle will be available for an HST servicing mission by the time such a mission is scheduled to fly. Reasons that the shuttle would not be available include loss of a vehicle on a previous flight, or a major anomaly that would ground the shuttle fleet for 6 months or more. The mission risk assessment by this committee is based on the accomplishments of previous shuttle missions involving satellite rescues utilizing EVAs, including the four successful shuttle missions to service HST as discussed in the sections “Human Response to Unforeseen On-Orbit Contingencies” and “Space Shuttle Servicing Missions to the Hubble Space Telescope.” In the nine flights considered, the EVA activities resulted in complete mission success in all but one instance, and that case was an unplanned EVA for the unexplained failure of a SYNCOM IV-3 communications satellite to activate on deployment.19 18 NASA, STS-109 Space Shuttle Mission Report NSTS-37437, NASA Johnson Space Center, Houston, Tex., May 2002. 19 NASA, First 100 Manned Space Missions, NASA Kennedy Space Center Public Affairs Office, June 14, 1995, available online at http://science.ksc.nasa.gov/shuttle/missions/51-d/mission-51-d.html.
OCR for page 91
Assessment of Options for Extending the Life of the Hubble Space Telescope: Final Report A more detailed discussion of the comparative mission risk between a human servicing mission and a robotic mission to HST is presented in Chapter 7. FINDING: Space shuttle crews, in conjunction with their ground-based mission control teams, have consistently developed innovative procedures and techniques to bring about desired mission success when encountering unplanned for or unexpected contingencies on-orbit. FINDING: The risk in the mission phase of a shuttle HST servicing mission is low.
Representative terms from entire chapter: