THE NATIONAL ACADEMIES
Advisers to the Nation on Science, Engineering, and Science
Aeronautics and Space Engineering Board
Space Studies Board
500 Fifth Street, NW
Washington, DC 20001
Phone: 202 334 3477
July 13, 2004
The Honorable Sean O’Keefe
National Aeronautics and Space Administration
Washington, DC 20546-0001
Dear Mr. O’Keefe:
At the request of the National Aeronautics and Space Administration, the National Research Council recently established the Committee on the Assessment of Options for Extending the Life of the Hubble Space Telescope.1 The committee’s statement of task charges it to assess the viability of a shuttle servicing mission, evaluate robotic and ground operations to extend the life of the telescope as a valuable scientific tool, assess telescope component failures and their impact, and provide an overall risk-benefit assessment of servicing options.2 The statement of task includes the possibility of transmitting an interim report to NASA prior to the submission of a final report.
The committee thanks you very much for your generous allocation of time in meeting with it on June 22, 2004. The information that you conveyed on the decision-making process that you and NASA followed when arriving at the Hubble-related decisions in January and in March 2004 was very important for us to hear directly from you. The additional information that you provided on NASA activities related to the shuttle return-to-flight program and robotic engineering in the broader context of long-term human space exploration was very useful, as was the extensive question-and-answer dialog that you enthusiastically engaged in with the committee.
Because you and your NASA colleagues have made clear to the committee that there is some urgency in issuing any recommendations related to Hubble, we are providing you with this interim report.3 It offers three principal findings and recommendations. These are based on the committee’s collective knowledge as well as input from other experts, both internal and external to NASA. This interim report does not address any one request in the statement of task in its entirety, but rather touches on aspects of task components 1, 2, and 4. Here the committee considers the degree of importance that a Hubble servicing mission would have for science, as well as some of the key factors involved in selecting a servicing mission option. Its aim is to provide useful guidance to NASA that can be utilized during the time that the committee (as well as NASA) continues to investigate the servicing options in greater detail. The work of the committee will continue during the coming weeks, and we expect to finish drafting a final report
See the statement of task in enclosure B.
Information about the independent review of the committee’s report under the supervision of the NRC’s Report Review Committee is provided in enclosure C.
by late summer or early fall. The final report will address in detail all four of the requests in the study’s statement of task.
Importance of a Hubble Servicing Mission
The Hubble Space Telescope (HST) is arguably the most important telescope in history. Much of Hubble’s extraordinary impact was foreseen when the telescope was being planned. It was predicted, for example, that the space telescope would reveal massive black holes at the centers of nearby galaxies, measure the size and age of the observable universe, probe far enough back in time to capture galaxies soon after their formation, and provide crucial keys to the evolution of chemical elements within stars.
All of these predicted advances have been realized, but the list of unforeseen Hubble accomplishments may prove even greater. Hubble did discover “adolescent” galaxies, but it also saw much farther back in time to capture galaxies on the very threshold of formation. Einstein’s theory of general relativity was bolstered by the detection of myriad gravitational lenses, each one probing the mysterious dark matter that pervades galaxies and clusters of galaxies. Gamma-ray bursts had puzzled astronomers for more than 20 years; in concert with ground and X-ray telescopes, Hubble placed them near the edge of the visible universe and established them as the universe’s brightest beacons, outshining whole galaxies for brief moments. Perhaps most spectacularly, Hubble confirmed and strengthened preliminary evidence from other telescopes for the existence of “dark energy,” a new constituent of the universe that generates a repulsive gravity whose effect is to drive galaxies apart faster over time. The resulting acceleration of universal expansion is a new development in physics, possibly as important as the landmark discoveries of quantum mechanics and general relativity near the beginning of the 20th century.
Closer to home, Hubble has zeroed in on our own cosmic past by uncovering virtual carbon copies of how the Sun and solar system formed. Dozens of protoplanetary disks have been found encircling young stars in nearby star-forming regions of the Milky Way. The sizes and densities of these disks show how surplus dust and gas collect near infant stars to form the raw material of planets. Dozens of large, Jupiter-like planets have been discovered, initially by other telescopes but recently by Hubble using a new and more precise method. Measuring the tiny drop in light as a planet transits the disk of its parent star, the new technique could lead to a method for discovering Earth-like planets—a discovery with tremendous long-term implications for the human race.
Riveting as they are, these scientific returns from Hubble are far from their natural end. With its present instruments the telescope could continue probing star formation and evolution, gathering more data on planetary systems, revealing planetary and cometary phenomena in our own solar system, and exploring the nature of the universe at much earlier times. However, two new instruments, already built for NASA’s next planned servicing mission (SM-4), would amplify the telescope’s capabilities by allowing qualitatively new observations in two underexploited spectral regions. Such rejuvenation via new instruments has occurred after every Hubble servicing mission, and the next one promises to be no different. Wide Field Camera-3 (WFC3) would increase Hubble’s discovery efficiency4 for ultraviolet and near-infrared imaging by factors of 10 to 30. The UV channel coupled with the camera’s wide field of view will image the final assembly of galaxies still taking place in the universe. The near-infrared channel of WFC3 favors discovery of the very youngest galaxies, whose light is maximally red-shifted. The available UV,
visible, and near-IR channels will combine to give a sweeping, panchromatic view of objects as diverse as star clusters, interstellar gas clouds, galaxies, and planets in our own solar system.
The second new instrument, the Cosmic Origins Spectrograph (COS), will increase Hubble’s observing speed for typical medium-resolution ultraviolet spectroscopy by at least a factor of 10 to 30, and in some cases by nearly two orders of magnitude. Ultraviolet spectra carry vital clues to the nature of both the oldest and the youngest stars, yet UV rays are totally invisible from Earth’s surface. COS will fill important gaps in our understanding of the birth and death of stars in nearby galaxies. Even more impressive, COS will use the light of distant quasars to spotlight hitherto undetectable clouds of dispersed gas between nearby galaxies, thereby mapping in unprecedented detail the properties of the so-called “cosmic web.”
FINDING. Compelling scientific returns will result from a servicing mission to the Hubble Space Telescope that accomplishes the scientific objectives of the originally planned NASA servicing mission SM-4.
RECOMMENDATION. The committee urges that NASA commit to a servicing mission to the Hubble Space Telescope that accomplishes the objectives of the originally planned SM-4 mission, including both the replacement of the present instruments with the two instruments already developed for flight—the Wide Field Camera-3 and the Cosmic Origins Spectrograph—and the engineering objectives, such as gyroscope and battery replacements. Such a servicing mission would extend the life of this unique telescope and maximize its productivity.
Other potential options to extend the useful life of Hubble—for example, by servicing components such as batteries and gyroscopes but without replacing instruments—will be studied by the committee as part of its charge. However, such a reduced level of servicing has not been featured in the repair strategies that the committee has heard about to date. The scientific impacts of reduced levels of servicing below that envisioned in SM-4 will be considered in the committee’s final report.
Servicing Mission Options
A wide range of factors must be considered when assessing the risk and effectiveness of HST servicing and deorbiting options. These options range from robotically attaching a deorbit module to Hubble to performing a mission (human or robotic) that replaces both scientific instruments and also services or repairs a number of engineering components. You discussed many of these options with us on June 22. One essential task is to enable the ultimate safe deorbiting of the spacecraft so that humans on Earth will not be at risk during its reentry. The present plan is to launch and robotically attach a deorbit module to the telescope around the year 2013.5 Consistent with this plan, NASA issued a Request for Proposals (RFP) on June 1, 2004, for a Hubble disposal vehicle.6
Another risk concerns robotic servicing and possible replacement of telescope instruments. You told the committee that a robotic mission “will be really tough.” NASA has proposed that a
This is the earliest date at which Hubble would be expected to reenter the atmosphere without intervention.
The RFP can be found at the following URL: http://www2.eps.gov/spg/NASA/GSFC/OPDC20220/HST%2DDM%2D0002%2DGDJ/listing.html.
deorbit module might be attached to the spacecraft at the time of robotic servicing, although the recently issued RFP does not specifically require either servicing or instrument replacement.7
The committee has been given detailed information on the plans for robotic servicing currently under consideration by NASA at its Goddard Space Flight Center. A subgroup of the committee visited Goddard and examined the current activities. The robotic servicing development effort at Goddard was officially initiated in 2004 and is a very recent undertaking. While considerable advances have been made in just a few months, there has been little time for NASA to evaluate and understand the technical and schedule limitations of robotic servicing.
The committee was gratified by your assurance that the robotic efforts will be adequately supported by the required resources in a timely manner. During the next year the robotic servicing mission project will have to achieve key milestones (including a critical design review in the summer of 2005) that will clarify the feasibility of a robotic servicing mission. Substantial resources will be required in Fiscal Year 2005 to accomplish this.
The committee finds the proposed robotic mission to be highly complex due to the inherent difficulties with supervised autonomy in the presence of time delays; the integration of vision and force feedback in six-degree-of-freedom assembly and disassembly tasks with high-degree-of-freedom, dexterous manipulators; and the coordinated control of the high-inertia HRV8 with a long-reach robotic arm grappling with a high-inertia payload. Robotic emplacement of a deorbit module and replacement of instruments and subsystems on Hubble will require a rendezvous with a non-cooperative vehicle9 together with a human in a telerobotic loop that has a substantial (on the order of 2-second) time delay.
The committee was informed about several current U.S. and foreign space programs that involve various concepts for robotic spacecraft rendezvous, capture, and servicing. Related U.S. experimental programs are currently scheduled for November 2004 (U.S. Air Force) and September 2006 (DARPA10). The committee has been informed that NASA is participating in some aspects of the DARPA program but this does not yet include a commitment to Hubble robotics servicing mission demonstrations. To the best of the committee’s current understanding, difficult challenges of the Hubble robotic scenario (such as the time delay and a non-cooperative target) are not currently covered explicitly in either the Air Force or the DARPA programs. Based on information provided to the committee and the knowledge of members who have deep experience with shuttle flights and spacecraft servicing, the committee believes that the proposed robotic mission to Hubble will essentially be an experimental test program that is expected to accomplish specific programmatic objectives at the same time.
FINDING. The proposed Hubble robotic servicing mission involves a level of complexity, sophistication, and technology maturity that requires significant development, integration, and demonstration to reach flight readiness.
RECOMMENDATION. As an early step, NASA should begin immediately to take an active partnership role that includes HST-related demonstrations in the robotics space experiments that are now under way in other agencies in order to ensure that the returns from these experiments can be beneficial to a potential robotic Hubble servicing mission.
The four HST shuttle servicing missions already completed have demonstrated that crew servicing and instrument replacement can be highly successful. Of course, there is risk to the astronaut crew in any human flight mission. As you informed the committee, some 25 to 30 additional shuttle missions are planned to complete the International Space Station (ISS). Based on its current assessment of the conclusions and recommendations contained in the Columbia Accident Investigation Board (CAIB) report11 and the Stafford-Covey reports (latest dated May 19, 2004),12 the committee concludes that a shuttle flight to the HST is not precluded by or inconsistent with the recommendations from these two NASA advisory groups.
The committee finds that the CAIB report makes clear distinctions between missions to the ISS and non-ISS missions. The CAIB report notes that the degree of difficulty is somewhat greater when conducting a non-ISS shuttle mission.13 This is partially due to the fact that a non-ISS mission such as one to Hubble would not have as long a “safe haven” opportunity as would a mission docking with the space station. The shuttle repair capabilities at a non-ISS location would also be less robust than at the ISS itself. Even so, the CAIB report does not prescribe operational constraints on how to conduct a non-ISS mission, but rather only general risk mitigation steps that should be followed. The CAIB consciously accepted lower risk mitigation efforts for non-ISS missions (such as a mission to Hubble).14
The committee was cognizant and most appreciative of your extensive discussions with us related to the ownership that you, and NASA, have for the shuttle return-to-flight and for astronaut safety in the nation’s civil space program. You stressed that total elimination of risk in crewed space flight is “impossible” and that you and NASA are “not risk averse.” From information it has received, including the risk information to date, the committee concludes that there would be little additional investment in time and resources required over the next year for NASA to keep open an option for a human servicing mission to Hubble.
According to briefings received by the committee, the risk assessments for viable Hubble servicing alternatives, both human and robotic, have not yet been completed or reported by NASA. The Hubble project office is currently investigating risks associated with robotic mission scenarios. Additionally, the committee was told that probabilistic risk assessment results for shuttle flights should be available in the fall or winter of this year. Such a study will be important in improving the comparisons between the risks of human flights to the ISS and to Hubble.
FINDING. Because of inherent uncertainties in the early stages of development of a robotic mission to the Hubble Space Telescope, as well as the uncertain current status of the shuttle
return-to-flight program, the key technical decision points for committing to a specific service scenario are at least a year in the future.
RECOMMENDATION. At the same time that NASA is vigorously pursuing development of robotic servicing capabilities, and until the agency has completed a more comprehensive examination of the engineering and technology issues, including risk assessments related to both robotic and human servicing options, NASA should take no actions that would preclude a space shuttle servicing mission to the Hubble Space Telescope.
We would be pleased to brief you and your staff regarding the views expressed in this letter. We remain committed to completing our final report in an expedited fashion.
Louis J. Lanzerotti,
Committee on the Assessment of Options for Extending the Life of the Hubble Space Telescope