The Hubble Space Telescope (HST) was launched from the space shuttle in 1990 and has operated continuously in orbit for the past 14 years. HST was designed to be serviced by astronauts, and a series of four shuttle servicing missions from 1993 to 2002 replaced nearly all the key components except the original telescope mirrors and support structure. Three of the four servicing missions added major new instrument observing capabilities. A fifth planned mission, designated SM-4 (servicing mission 4), was intended to replace aging spacecraft batteries, fine-guidance sensors, and gyroscopes and install two new science instruments on the telescope.
Following the loss of the space shuttle Columbia and its crew in February 2003, NASA suspended all shuttle flights until the cause of the accident could be determined and steps taken to reduce the risks of future shuttle flights. In mid-January 2004 NASA decided, on the basis of risk to the astronaut crew, not to pursue the HST SM-4 mission. This cancellation, together with the predicted resulting demise of Hubble in the 2007-2008 time frame, prompted strong objections from scientists and the public alike. NASA continued to investigate options other than a shuttle astronaut mission for extending Hubble’s science life and is currently in the early stages of developing an unmanned mission that would attempt to service Hubble robotically. NASA also plans to de-orbit HST by approximately 2013 by means of a robotic spacecraft.
This report assesses the options for extending the life of HST. In keeping with its statement of task (Appendix A), the Committee on the Assessment of Options for Extending the Life of the Hubble Space Telescope assessed the scientific value of continued HST operation, issues of safety in using the space shuttle for servicing HST with an astronaut crew, the feasibility of robotic servicing, the impacts of servicing options on HST’s science capability, and risk/benefit relationships between those servicing options deemed acceptable.
Approximately every decade the U.S. astronomical research community develops a decadal strategy for the field. A premise of the most recently developed strategy1 was that the HST SM-4 mission was an
integral part of NASA’s facility planning for the future of the field and that this servicing mission would occur as planned at the time necessary to prevent the demise of the telescope. The strategy’s advisory recommendations reflect this assumption, and the committee, which was neither asked nor constituted to address any possible changes in priorities for astronomical research or research facilities, assumed that NASA would follow the decadal survey advisory recommendations. If NASA concludes that it cannot move forward with portions of the decadal survey strategy, then NASA will have to carry out an in-depth examination of priorities for the research field. The committee does not endorse such a reexamination. The committee notes, however, that if a re-examination should occur it would have to be conducted in a very timely and very expeditious fashion in order to ensure the continued operation and integrity of Hubble.
ANTICIPATED HUBBLE FAILURES
The Hubble systems with the greatest likelihood of failing and thus ending or significantly degrading Hubble science operations are the gyroscopes, the batteries, and the fine-guidance sensor (FGS) units. In addition, the HST avionics system is vulnerable to the aging of the facility.
The telescope uses three gyroscopes to provide precision attitude control. There are currently four functional gyros on HST—three in operation plus one spare. It is likely that the HST system will be reduced to two operating gyros in the latter half of 2006. The HST engineering team is currently working on approaches to sustaining useful, though potentially degraded, astronomical operations with only two gyros, and NASA expects to have that capability by the time it becomes necessary. Eventually, without servicing, the telescope will be reduced to operation with a single gyro in mid to late 2007. The spacecraft can be held in a safe configuration with one or no operating gyros, but science operations will not be possible.
Battery failures are another likely cause of loss of science operations. HST now has six batteries, of which five are necessary for full operations. If battery levels fall too low, the temperature of the structural elements in the Optical Telescope Assembly will fall below permissible levels, causing permanent damage to the facility. Recovery of scientific operations from this state is not possible.
The FGS units (in combination with their electronics subsystems) are used for precision pointing of the observatory. Two operating FGS units are required to support the HST observing program, with a third to supply redundancy. Based on recent test and performance data, one of the three currently operating FGS units is projected to fail sometime between October 2007 and October 2009, and a second is expected to fail sometime between January 2010 and January 2012.
Based on its examination of data and numerous technical reports on Hubble component operations, as well as discussions held with Hubble project personnel, the committee developed the following findings predicated on an estimated SM-4 earliest launch date of July 2006 and a most likely robotic mission launch date of February 2010.
FINDING: The projected termination in mid to late 2007 of HST science operations due to gyroscope failure and the projected readiness in early 2010 to execute the planned NASA robotic mission result in a projected 29-month interruption of science operations. No interruption of science operations is projected for a realistically scheduled SM-4 shuttle mission.
FINDING: The planned NASA robotic mission is less capable than the previously planned SM-4 shuttle astronaut mission with respect to its responding to unexpected failures and its ability to perform
proactive upgrades. Combined with the projected schedule for the two options, the mission risk2 associated with achieving at least 3 years of successful post-servicing HST science operations is significantly higher for the robotic option, with the respective risk numbers at 3 years being approximately 30 percent for the SM-4 mission and 80 percent for the robotic mission.
BENEFITS OF SERVICING HUBBLE
Impact of Hubble
Over its lifetime, HST has been an enormous scientific success, having earned extraordinary scientific and public recognition for its contributions to all areas of astronomy. Hubble is the most powerful space astronomical facility ever built, and it provides wavelength coverage and capabilities that are unmatched by any other optical telescope currently operating or planned.
The four key advantages that Hubble provides over most other optical astronomical facilities are unprecedented angular resolution over a large field, spectral coverage from the visible and the near infrared to the far ultraviolet, access to an extremely dark sky, and highly stable images that enable precision photometry. Hubble’s imaging fields of view are also considerable, permitting mapping of extended objects and significant regions of sky. In contrast, ground-based telescopes have a view that is blurred by the atmosphere,3 and they are completely blind in the ultraviolet and large portions of the near infrared. Hubble can see sharply and clearly at all wavelengths from the far ultraviolet to the near infrared. Hubble images are 5 to 20 times sharper than those obtained with standard ground-based telescopes, in effect bringing the universe that much “closer.” Image sharpness and the absence of light pollution in orbit help Hubble to see objects 10 times fainter than even the largest ground-based telescopes. Moreover, Hubble’s images are extremely stable, in contrast to those obtained with ground telescopes, whose view is continually distorted by changing atmospheric clarity and turbulence.
Singly, each of these advantages would represent a significant advance for science. Combined, they have made Hubble the most powerful optical astronomical facility in history. Hubble is a general-purpose national observatory that enables unique contributions to and insights concerning most astronomical problems of greatest current interest. Among the most profound contributions of Hubble have been the following:
Direct observation of the universe as it existed 12 billion years ago,
Measurements that helped to establish the size and age of the universe,
Discovery of massive black holes at the center of many galaxies,
Key evidence that the expansion of the universe is accelerating, which can be explained only by the existence of a fundamentally new type of energy, and therefore new physics, and
Observation of proto-solar systems in the process of formation.
In addition to its impact on science, Hubble discoveries and images have generated intense public interest. Examples of Hubble data and images that have fascinated the public (and scientists) include the big “black eye” left by comet Shoemaker-Levy’s direct hit on Jupiter’s atmosphere, which alerted the
public to the dangers of asteroids impacting Earth; a panoply of jewel-like planetary nebulas that illustrate the ultimate death of our Sun; portraits of planets in the solar system, including auroras on Jupiter and Saturn; and such astronomical spectacles as the “pillars of dust” in the Eagle nebula that appeared on nearly every front page in America and became iconic for Hubble itself. The Hubble Space Telescope has clearly been one of NASA’s most noticed science projects, garnering sustained public attention over its entire lifetime.
Maintaining and Enhancing Hubble’s Capabilities
The four previous servicing missions to Hubble have added new observing modes and increased existing capabilities, typically by factors of between 10 and 100, since the telescope first flew in 1990. As a result, Hubble now produces more data per unit time than it did originally. The total rate of calibrated data has grown by a factor of 33 since launch.4 A further increase was expected with the installation of the two new science instruments, the Wide-field Camera 3 (WFC3) and the Cosmic Origins Spectrograph (COS), each of which would provide a greater than 10-fold improvement in scientific efficiency and sensitivity compared with previous instruments. Both of these instruments are already built.
With the installation of WFC3 and COS, and the continued operation enabled by a fifth servicing mission, a broad range of new discoveries would be expected from Hubble. In fact, the committee concluded that Hubble’s promise for future discoveries following a fifth servicing mission would be comparable to the telescope’s promise when first launched. For example, an important new technique that Hubble would offer for finding planets could enable detection of as many as 1000 new planets in the Milky Way Galaxy in the years after servicing. In addition, a large number of new supernovas could be found for the study of dark energy, reducing uncertainties in its properties by a factor of two. A wealth of data would also be collected to explore the nature of stars in the Milky Way Galaxy and in neighboring galaxies. Hubble is just now beginning to image objects being found by sister NASA missions such as Chandra (an x-ray observatory), Galaxy Evolution Explorer (GALEX; an ultraviolet imager), and Spitzer (an infrared imager and spectrograph), which are currently in orbit. These satellites are relatively wide-field survey telescopes whose goal in part is to detect objects for Hubble follow-up observations. These detailed follow-ups take time because of Hubble’s smaller field of view; a large fraction of the scientific benefit of these other satellites will be lost if Hubble’s mission is cut short prematurely. And finally, a servicing mission is needed to allow an orderly completion of large, homogeneous data sets such as spectral libraries and imaging surveys of large areas of the Milky Way Galaxy that Hubble is now gathering. These data sets will be archived to serve astronomers for decades to come, given that there are no foreseeable plans to replace Hubble with a telescope of comparable size, wavelength coverage, and high resolution.
The key findings of the committee related to the benefits of future servicing of Hubble are as follows:
FINDING: The Hubble Space Telescope is a uniquely powerful observing platform in terms of its high angular optical resolution, broad wavelength coverage from the ultraviolet to the near infrared, low sky background, stable images, exquisite precision in flux determination, and significant field of view.
FINDING: Astronomical discoveries with Hubble from the solar system to the edge of the universe are among the most significant intellectual achievements of the space science program.
FINDING: The scientific power of Hubble has grown enormously as a result of previous servicing missions.
FINDING: The growth in the scientific power of Hubble would continue with the installation of the two new instruments, WFC3 and COS, planned for the SM-4 shuttle astronaut mission.
THE RISKS OF ROBOTIC SERVICING
Because a robotic servicing mission does not involve risks to the safety of an astronaut crew, the principal concerns are the risk of failure to develop a robotic mission capability in time to service Hubble, and the risk of a mission failure that results in an inability to perform the needed servicing, or worse, critically damages Hubble during the mission. Both schedule risk and mission risk are composed of a large number of factors that were studied in considerable detail by the committee.
Some of the critical components of mission risk include lack of adequate development time to validate the hardware, level of software and system performance required to rendezvous with Hubble, failure to successfully grapple and dock with Hubble, failure to successfully execute the combination of complex autonomous and robotic activities required to actually accomplish HST revitalization and instrument replacement, and the risk of unforeseen Hubble failures prior to mission execution that the robotic mission will not have been designed to repair. One example of a mission risk that concerned the committee is the complicated docking maneuver required for a Hubble robotic servicing, which has never been performed autonomously or teleoperated with time delays. Specifically, the use of the grapple system to autonomously perform close-proximity maneuvers and the final capture of Hubble is a significant challenge and is one of the key technical aspects of a robotic servicing mission that has no precedent in the history of the space program.
The components of schedule risk examined by the committee included the readiness levels of such technologies as the sensors, software and control algorithms, and vision-based closed-loop support for autonomous docking operations, as well as NASA’s relevant programmatic and technical expertise, resources, and specific development plans for a robotic servicing mission. From the risk mitigation viewpoint, the committee judged that the planned use of the mature International Space Station robotic arm and robotic operational ground system helps reduce both the schedule risk and the development risk for the robotic mission. In addition, the committee assessed the development schedule for the robotic servicing mission based on its experience with programs of similar complexity and the historical spacecraft development schedule data provided by both NASA and the Aerospace Corporation. The committee’s key findings regarding the question of the risk of robotic servicing are as follows:
FINDING: The technology required for the proposed HST robotic servicing mission involves a level of complexity, sophistication, and maturity that requires significant development, integration, and demonstration to reach flight readiness and has inherent risks that are inconsistent with the need to service Hubble as soon as possible.
FINDING: The Goddard Space Flight Center HST project has a long history of HST shuttle servicing experience but has little experience with autonomous rendezvous and docking or robotic technology development, or with the operations required for the baseline HST robotic servicing mission.
FINDING: The proposed HST robotic servicing mission involves a level of complexity that is inconsistent with the current 39-month development schedule and would require an unprecedented improvement in development performance compared with that of space missions of similar complexity. The likelihood of successful development of the HST robotic servicing mission within the baseline 39-month schedule is remote.
Based on extensive analysis, the committee concluded that the very aggressive schedule for development of a viable robotic servicing mission, the commitment to development of individual elements with incomplete systems engineering, the complexity of the mission design, the current low level of technology maturity, the magnitude of the risk-reduction efforts required, and the inability of a robotic servicing mission to respond to unforeseen failures that may well occur on Hubble between now and the mission, together make it highly unlikely that NASA will be able to extend the science life of HST through robotic servicing.
THE RISKS OF SHUTTLE SERVICING
The risks that must be considered in making a decision to service Hubble with the shuttle are the risk to the safety of the crew and the shuttle, as well as the risk of failing to accomplish the servicing objectives. As part of its assessment of safety risk, the committee looked carefully at the findings and recommendations of the Columbia Accident Investigation Board (CAIB)5 and at NASA’s return-to-flight (RTF) requirements. Strong consideration was given to understanding differences in the safety risk factors between shuttle missions to the International Space Station (ISS)—to which NASA still plans to fly 25 to 30 missions—and a shuttle mission to Hubble. Technical considerations examined by the committee included comparisons of on-orbit inspection and repair capabilities at ISS and Hubble, various safe-haven and rescue options, and the likelihood of the shuttle being damaged by micrometeoroid orbital debris (MMOD). With regard to mission risk, the committee considered both the known on-orbit operations required for Hubble servicing and past experience with Hubble shuttle astronaut servicing, including such factors as unforeseen on-orbit contingencies.
The committee developed a large number of findings based on the various analyses cited above. Some of the key findings relevant to the question of the risk of shuttle servicing of HST are as follows:
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.
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.
FINDING: Previous human servicing missions to HST have successfully carried out unforeseen repairs as well as executing both planned and proactive equipment and science upgrades. HST’s current excellent operational status is a product of these past efforts.
FINDING: Space shuttle crews, in conjunction with their ground-based mission control teams, have
Columbia Accident Investigation Board, Report, Volume I, August, 2003. Available online at http://www.nasa.gov/columbia/home/CAIB_Vol1.html.
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.
COMPARISON OF THE RISKS AND THE BENEFITS OF SERVICING
As noted above, the Hubble Space Telescope provides unique capabilities for astronomical research. These capabilities 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.
Upgrading Hubble to address the predictable decline in HST component performance over time and thus ensure system reliability requires a timely and successful servicing mission in order to minimize further degradation and prevent a significant gap in science data return. Although it considered other options for extending the life of Hubble, the committee focused on two approaches: robotic servicing and shuttle astronaut servicing.
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 servicing mission to respond to unforeseen failures that may well occur on Hubble between now and a servicing mission make it unlikely that the science life of HST will be extended through robotic servicing.
A shuttle astronaut servicing mission is the best option for extending the life of Hubble and preparing the observatory for eventual robotic de-orbit by, for example, attaching targets to Hubble. The committee believes that a shuttle HST servicing mission could occur as early as the seventh shuttle mission following return to flight, at which point critical shuttle missions required for maintaining ISS will have been accomplished. All important systems needed to keep Hubble functioning well through 2011 were included in the original SM-4 shuttle servicing plan. Replacement of batteries and gyros and one FGS is deemed essential. Any spacecraft is subject to unanticipated failures, but if the repairs planned for the SM-4 mission are carried out promptly, there is every prospect that Hubble can operate effectively for another 4 to 5 years after servicing.
The committee finds that the difference between the risk faced by the crew of a single shuttle mission to ISS—already accepted by NASA and the nation—and the risk faced by the crew of a single shuttle servicing 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.
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.