4
Short Reports

During 2004, the Space Studies Board and its committees issued two short reports. The main text of each is reprinted in this section.

4.1 Assessment of Options for Extending the Life of the Hubble Space Telescope: Letter Report

On July 13, 2004, SSB/ASEB Committee on the Assessment of Options for Extending the Life of the Hubble Space Telescope Chair Louis J. Lanzerotti sent the following letter to Sean O’Keefe, NASA administrator.

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

1  

The committee roster, enclosure A, is not reprinted in this annual report. Additional background material on the motivation for the study, contained in enclosure B, is not reprinted in this annual report.

2  

See the statement of task, enclosure B, which is not reprinted in this annual report.

3  

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, which is not reprinted in this annual report.



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Space Studies Board Annual Report 2004 4 Short Reports During 2004, the Space Studies Board and its committees issued two short reports. The main text of each is reprinted in this section. 4.1 Assessment of Options for Extending the Life of the Hubble Space Telescope: Letter Report On July 13, 2004, SSB/ASEB Committee on the Assessment of Options for Extending the Life of the Hubble Space Telescope Chair Louis J. Lanzerotti sent the following letter to Sean O’Keefe, NASA administrator. 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 1   The committee roster, enclosure A, is not reprinted in this annual report. Additional background material on the motivation for the study, contained in enclosure B, is not reprinted in this annual report. 2   See the statement of task, enclosure B, which is not reprinted in this annual report. 3   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, which is not reprinted in this annual report.

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Space Studies Board Annual Report 2004 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 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 4   Throughput multiplied by the area of the field of view.

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Space Studies Board Annual Report 2004 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 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 5   This is the earliest date at which Hubble would be expected to reenter the atmosphere without intervention. 6   The RFP can be found at the following URL: http://www2.eps.gov/spg/NASA/GSFC/OPDC20220/HST%2DDM%2D0002%2DGDJ/listing.html. 7   The RFP requires only submissions for a vehicle to provide end-of-life controlled reentry or other safe disposal of the HST; the RFP invites but does not require that submissions include life extension or servicing capabilities. 8   Hubble Robotic Vehicle.

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Space Studies Board Annual Report 2004 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 9   A non-cooperative vehicle is a vehicle that is not equipped with transponders or active sensors, meaning that it cannot respond to electronic interrogation from other spacecraft or emit signals enabling its identification or localization. 10   Defense Advanced Research Projects Agency. 11   Columbia Accident Investigation Board Report, Volume 1, August 2003, NASA and the Government Printing Office, Washington, D.C. 12   Return to Flight Task Group Interim Report, January 20, 2004, and Return to Flight Task Group Second Interim Report, May 19, 2004. 13   CAIB Report Recommendation R6.4-1, p. 174. 14   Ibid.

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Space Studies Board Annual Report 2004 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. Signed by Louis J. Lanzerotti Chair, Committee on the Assessment of Options for Extending the Life of the Hubble Space Telescope

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Space Studies Board Annual Report 2004 4.2 Review of Science Requirements for the Terrestrial Planet Finder: Letter Report On September 23, 2004, SSB/BPA Panel to Review the Science Requirements for the Terrestrial Planet Finder Chair Wendy L. Freedman sent the following letter to Anne L. Kinney, director of the Universe Division, Science Mission Directorate, NASA Headquarters. This letter report reviews the science goals of the current Terrestrial Planet Finder (TPF) project as well as NASA’s plan for acquiring the necessary precursor knowledge to successfully meet those goals. This review by the Panel to Review the Science Requirements for the Terrestrial Planet Finder complements recommendations made in the National Research Council report Astronomy and Astrophysics in the New Millennium (referred to here as the 2000 decadal survey)1 and was conducted in response to your request of January 29, 2004, asking for a science assessment of the TPF project. Your original letter of request was followed by one dated April 15, 2004, announcing NASA’s intention to proceed with both coronagraphic and interferometric planet finder missions on an accelerated schedule. Both are included in this letter report’s attachment. The Space Studies Board and the Board on Physics and Astronomy, in response to your requests, developed the following charge: This panel will review NASA’s current scientific objectives for the Terrestrial Planet Finder (TPF) mission and prepare a brief letter report conducting an independent scientific assessment as to whether these objectives remain consistent with the priority given to the mission by the Astronomy and Astrophysics Survey Committee. In carrying out this charge, the panel will consider (1) the scientific goals of the mission as developed by the NASA TPF-Science Working Group; (2) plans for acquiring the necessary precursor scientific knowledge; and (3) the rationale for the mission that formed the basis of the priority assigned by the NRC’s decadal survey report Astronomy and Astrophysics in the New Millennium. This charge, to which NASA raised no objection, governed the scope of the current letter report; the panel emphasizes here that it was not constituted to carry out a technical assessment of the current TPF project plans and did not attempt to do so. The panel met at the Keck Center of the National Academies in Washington, D.C., on May 18, 2004, to conduct the review (see the attachment for a panel roster and the meeting agenda). Drawing extensively on the current membership of the NRC’s Committee on Astronomy and Astrophysics, the panel’s membership covered a broad range of astronomical expertise. The panel also included members with specific expertise in coronagraphy and extrasolar planets. The panel received presentations from Zlatan Tsvetanov (NASA) on the programmatic plans for TPF and from Charles Beichman (JPL) on the scientific and engineering plans for the project. Also participating in the discussion were Marc Kuchner (Princeton University), Alan Boss (Carnegie Institution of Washington), Dan Coulter (NASA), and Garth Illingworth (University of California, Santa Cruz). The 2000 decadal survey report ranked the Terrestrial Planet Finder third in its list of major NASA missions behind the James Webb Space Telescope (then called the Next Generation Space Telescope) and the Constellation-X Observatory and sixth overall: The main goal of TPF is nothing less than to search for evidence of life on terrestrial planets around nearby stars. The present concept calls for a space-based infrared interferometer of enormous sensitivity, capable of nulling out the light from the host star. TPF’s angular resolution will also enable it to peer into the innermost regions of protoplanetary disks, galactic nuclei, starburst galaxies, and galaxies at high redshift. By a large margin, TPF is the most costly and the most technically challenging mission discussed in this report. Both SIM and NGST involve key technologies that must be demonstrated if TPF as currently envisioned is to go forward. The committee’s recommendation of this mission is predicated on the assumptions that TPF will revolutionize major areas of both planetary and nonplanetary science, and 1   National Research Council, 2001, Astronomy and Astrophysics in the New Millennium, National Academy Press, Washington, D.C. Attachments are not reprinted in this annual report.  

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Space Studies Board Annual Report 2004 that, prior to the start of TPF, ground- and space-based searches will confirm the expectation that terrestrial planets are common around solar-type stars. NASA should pursue a vigorous program of technology development to enable the construction of TPF to begin in this decade. (p. 39) The original mission that was considered by the 2000 decadal survey ranked highly based on its potential science impact on terrestrial planet finding2 and on the astrophysics reach afforded by the high angular resolution at infrared wavelengths. However, the widely recognized technical challenges of the interferometer prohibited the decadal survey committee from prioritizing it as a flight mission. Rather, that committee gave TPF its high ranking as a technology development activity with the aim of pushing the technology forward in this decade, and enabling the mission to be flown in the following decade. Specifically, “The committee attributes $200 million [in FY2000 dollars] of the $1,700 million total estimated cost of TPF to the current decade….” (p. 37). At the time of NASA’s initial request in January 2004 for the current vision, the TPF project was considering both a free-flying infrared interferometer and an optical coronagraph, with the goal of downselecting to a single architecture in the near future. The course of the TPF project has since changed in order to take advantage of the new opportunities presented by NASA’s new space exploration goals3 and to maximize the scientific potential for terrestrial planet finding. Specifically, the TPF project team is now proposing to fly TPF-C (an optical telescope with a coronagraph) followed by TPF-I (a free-flying infrared interferometer) within its planet-finding portfolio. The level-1 requirement for TPF-C’s wavelength coverage is proposed to be 0.5 to 0.8 µm, with “stretch” goals of 0.5 to 1.05 µm. The level-1 requirement for TPF-I’s wavelength coverage is proposed to be 6.5 to 13 µm, with “stretch” goals of 6.5 to 17 µm. The primary scientific goal of the TPF mission (direct detection and spectroscopic analysis of Earth-like planets in orbit about some of the nearest main-sequence stars) arguably requires both TPF-C and TPF-I. This requirement was not well understood at the time the TPF mission was presented to the decadal survey committee, because understanding was imperfect then concerning the spectrum that our own Earth would present to a nearby solar system. Furthermore, the identification of biomarkers (i.e., spectroscopic features indicative of chemical balances attributable to biogenic activity) requires observations in spectra that span not only the optical but also the mid-infrared (IR) bands. Assuming there are planets to be found within the range of these telescopes, the combination of the two could provide evidence suggesting the presence of living organisms outside our solar system. The TPF-C coronagraph is being designed to be able to identify planets that are Earth-sized or slightly smaller within the habitable zones of about 35 single, solar-type (F, G, K) stars within about 10 parsecs of the Sun. Liquid water is required for Earth-like life, and O2 is under most circumstances a good indicator of photosynthetic life. Equipped with a modest-resolution spectrometer, the coronagraph should be able to identify several near-IR absorption bands of H2O, along with the 0.76-µm “A” band of O2 in light passing through the atmosphere of such planets. Hence, this mission has the capability by itself of at least suggesting whether life is present on planets around other stars. Flying an IR interferometer, TPF-I, several years after the coronagraph is launched (potentially in a joint mission with the European Space Agency, as is under current discussion with NASA) could help to advance the science goals of the field in several ways. The currently envisioned free-flier concept for the interferometer would make long baselines possible and could enable this mission to extend the search for planets to more than 150 single, solar-type stars—a fourfold increase over the TPF-C sample. This larger search space would, of course, be critical if the frequency of Earth-like planets is low. Even if Earth-like planets are abundant, and the coronagraph is able to see many of them, the additional information obtained by the interferometer will likely prove crucial in characterizing these planets and determining whether any of them could harbor life. Thus the spectroscopic information provided by the interferometer is complementary to that provided by the coronagraph. For example, with sufficient cooling, the interferometer is expected to be able to observe the strong 15-µm band of CO2. The presence of CO2 is perhaps the best indicator that a planet being observed is terrestrial (i.e., rocky) and that it has an atmosphere, as opposed to being an airless body similar to Mercury or the Moon. CO2 is also required for photosynthesis, both aerobic and anaerobic, and hence it is a requirement for many Earth-like forms of life. Even more importantly, the IR interferometer will have 2   Terrestrial planets are planets similar in size and composition to Earth. In our solar system Mercury, Venus, and Mars (as well as Earth) are considered to be terrestrial planets. 3   National Aeronautics and Space Administration, The Vision for Space Exploration, NP-2004-01-334-HQ, NASA, Washington, D.C., 2004.

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Space Studies Board Annual Report 2004 the capability to detect the 9.6-µm ozone band. Ozone, formed photochemically from oxygen, may be a more sensitive indicator of photosynthetic life than is oxygen itself. The TPF-C and TPF-I data together will provide simultaneous information for the same molecules in different states (e.g., the presence of oxygen from the A-band, as well as ozone at 9.6 µm or CO2 at both the near and the thermal infrared), thereby removing some of the degeneracies in the interpretation of TPF-C or TPF-I data alone. The simultaneous detection of ozone (TPF-I) and methane (TPF-C)—oxidizing and reducing gases—implies that life may be present. What can be learned from the combination of TPF-C and TPF-I data is therefore far greater than what either mission alone would yield. Since the 2000 decadal survey the TPF project has made progress in technology development and scientific definition of the mission. NASA reported that shortly after the survey, a number of detailed studies of TPF system architectures showed that an extremely precise coronagraphic imaging telescope could achieve some—though not all—of the science goals outlined originally for the interferometer approach. The TPF team believes that it will be ready to move TPF-C into Phase-A development by 2006, pursuant to an ambitious schedule to launch the coronagraphic imaging telescope by 2014. Because of its greater complexity, TPF-I is currently planned to follow about 6 years later. The technology development plans for both TPF-C and TPF-I are aggressive. However, a promising development is that the High Contrast Imaging Test Bed at the Jet Propulsion Laboratory has successfully imaged a region next to a simulated star within which an average contrast of 1.5 × 10−9 has been achieved. If this result proves applicable to the broader TPF-C mission, the mission’s development may meet the 2006 goal for entering Phase A. Nevertheless, TPF-C would satisfy only part of the science requirements previously ascribed to the interferometric version of TPF that was ranked in the 2000 decadal survey. TPF had two goals of equal importance: planet finding and astrophysics. As emphasized in the 2000 survey: “To ensure a broad science return from TPF, the committee recommends that, in planning the mission, comparable weight be given to the two broad science goals: studying planetary systems and studying the structure of astronomical sources at infrared wavelengths” (p. 12). The 2000 decadal survey’s companion volume of panel reports specified that TPF cover a range from 3 to 30 µm for general imaging, and 7 to 20 µm for planet finding, with an angular resolution of 7.5 × 10−4 arcsec at 3 µm.4 TPF-I is necessary not only to enhance the project’s planet-finding ability but also to complete the astrophysical goals laid out in the 2000 decadal survey. A conceptual ancillary science case for TPF-C has been developed with the addition of a 5-arcminute wide-field camera. The science achieved with this camera would be synergistic with that made possible by the James Webb Space Telescope and a 30-m ground-based telescope. An example involves extremely deep observations, significantly more sensitive than the Hubble Space Telescope ultradeep field, of the annular region around the stars targeted for the planet search. TPF-C might have additional astrophysics reach, but the TPF project has not allowed broader astrophysics goals to drive the design or the cost of the TPF-C optical telescope assembly. Ancillary science for the TPF-I mission is not as clearly developed at this point, although ideas include extended spectral coverage for exoplanetary science or fine-resolution studies of high-red-shift galaxies and protostellar disks. The 2000 decadal survey report was also very explicit about the importance of studies to be carried out prior to designing TPF. NASA’s plans for acquiring the necessary precursor science include (1) an assessment of the extent of exozodiacal dust in other planetary systems and the effects of this dust on the detectability of terrestrial planets, (2) a determination of the biomarkers that would be optimal indicators that life exists on such planets, and (3) an estimation of the minimum number of stars in the sample necessary to detect terrestrial planets with confidence. Both the Spitzer Space Telescope, which was launched in 2003, and new ground-based IR interferometry (using the Keck interferometer or Very Large Telescope Interferometer telescopes) should address the extent of exozodiacal dust. The NASA TPF Working Group presented a reasonable case that the combination of visible (obtainable with a coronagraphic mission) and infrared (obtainable with an interferometric mission) biomarkers would be a far stronger discriminant of life than either set of wavelength-dependent biomarkers separately. Launching both TPF-C and TPF-I would provide this combination of evidence. The greatest unknown remains the number of stars TPF-C needs to be able to observe, in order to assure that it will detect terrestrial planets.5 Achieving the primary scientific goal for the TPF mission is still hampered, perhaps 4   See National Research Council, Astronomy and Astrophysics in the New Millennium: Panel Reports, Chapter 5, National Academy Press, Washington, D.C., 2001.

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Space Studies Board Annual Report 2004 crucially, by a lack of information about how common Earth-mass, let alone Earth-like, planets are around the sorts of F-, G-, and K-type stars identified for the TPF sample. Estimates of the probability that Earth-like planets exist vary widely. The fraction of these planets that will have even remotely Earth-like atmospheres is also unknown, but must strictly be less than 1 (i.e., over most of its lifespan Earth’s atmosphere has been chemically quite different from its composition today). The fraction of stars with planets bearing Earth-like atmospheres is a matter of conjecture at this point. This is precisely why the 2000 decadal survey report recommended that strong constraints on these fractions be established before the TPF conceptual design is finalized. As envisioned by the survey, the mission could be designed to accommodate whatever fraction nature provides, thus maximizing the chances of success. Thus, the precursor science is of paramount importance to the success of TPF. The target stars must be surveyed with any and all available resources before a detailed preliminary design is finalized. These searches can be undertaken with both ground- and space-based precursor missions, such as SIM and other lower-sensitivity projects (e.g., the planned “extreme” adaptive optics coronagraph for the Gemini Observatory). The Kepler mission may place strong constraints on the frequency of Earth-mass planets around Sun-like stars. Unfortunately, on NASA’s proposed schedule these missions may not produce results before the design of TPF-C is completed. The panel concludes that accelerating the schedule for TPF-C development carries considerable risk of settling on a design that the results obtained with SIM, Kepler, and microlensing and other observations will subsequently reveal to be incapable of seeing terrestrial planets. Therefore, the panel urges NASA to plan the development of TPF-C at a pace that allows the design to take into account the results of SIM, Kepler, and other observations as outlined above. A TPF flight mission could then be well positioned for a high ranking, possibly including both TPF-C and TPF-I, in the next decadal survey. The 2000 decadal survey took into account, among other things, the broad programmatic implications of TPF. The proposed addition of TPF-C represents a major new mission of the Great Observatory class and is proposed for launch in 2014, 3 years after the James Webb Space Telescope. The panel agrees that TPF as envisioned in the 2000 decadal survey remains an exciting mission scientifically. The combination of TPF-C and TPF-I will cover at a minimum the planet-finding goals as laid out in that report. Although NASA gave the panel no cost estimates, presenters did suggest a few bounds that lead toward a conclusion that the mission cost of TPF-C will be at least the cost of the current James Webb Space Telescope. According to NASA, the decision to fly both TPF-C and TPF-I was triggered by NASA’s new space exploration goals, in which planet finding received a very high priority. Neither the 2000 decadal survey nor any prior NRC reports had considered the added value for terrestrial planet finding of having an optical mission such as TPF-C as a complement to TPF-I. The panel finds that the current scientific goals of the TPF project are consistent with those envisioned in the 2000 decadal survey, Astronomy and Astrophysics in the New Millennium. But the panel does not consider that this finding justifies advancing at this time the priority that can be accorded TPF as combined TPF-C and TPF-I missions. A decision after the fact to initiate a major project such as TPF-C implicitly reorders without due process the prioritized list developed by the 2000 decadal survey. Any such decision about prioritization should be made with the input of a broadly constituted committee that has sufficient time to weigh all of the scientific and technical issues. In summary, the panel reaffirms that TPF, as envisioned in the 2000 decadal survey, remains an exciting mission scientifically. The panel concludes that, with the addition of TPF-C, there is considerable potential for interesting ancilliary science in addition to the science connected with the search for life-bearing planets. The panel also concurs with the 2000 decadal survey on the importance of precursor missions (e.g., SIM and Kepler) toward enhancing TPF’s overall scientific productivity. It is critical that their results continue to drive the development of the project. The panel also concurs with the 2000 decadal survey’s recommendation that the astrophysics goals of TPF be weighted comparably to the planet-finding goals. Yet although the proposed new camera for TPF-C possesses interesting capabilities, the associated science case has been neither carefully developed nor critically reviewed. The panel recommends that NASA solicit input from the astronomical community in order to develop the strongest possible science case. A strong science case would enhance TPF’s competitiveness in any priority-setting process, whether conducted in the context of the next decadal survey of astronomy and astrophysics or in an exercise of smaller scope conducted before the next survey. 5   Precursor studies will constrain the fraction of Sun-like stars that are orbited by Earth-like planets. This information determines the minimum volume of space that needs to be searchable by TPF-C in order to reasonably ensure that it will directly detect at least one Earth-like planet, thus driving the physical design of the telescope.

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Space Studies Board Annual Report 2004 Finally, the panel is concerned about the process by which NASA’s decision to propose two TPF missions and to start one of them this decade was reached. The statement of task asked for input on the likely impacts on the 2000 decadal survey priorities, and these are large. The plan for TPF-C is clearly not consistent with the 2000 decadal survey’s recommendations regarding TPF. Even though NASA has rearranged the order of missions occasionally in the past when funding or technology concerns warranted such changes, TPF-C is so expensive and challenging that the panel believes that, from the perspective of astronomy and astrophysics,6 it must be placed in the broader context of the other highly ranked space missions identified in the 2000 decadal survey. The panel is very concerned about breaking with a process for developing a strategy that has served astronomy and astrophysics very well—the broadly debated, carefully balanced, and widely endorsed portfolio that the 2000 decadal survey presented. If implementation of TPF-C were to delay, or even preclude, other highly ranked astronomy and astrophysics missions, such an outcome would represent a substantial tipping of the portfolio’s scientific balance. The panel urges NASA to consider the addition of TPF-C within the broader context of the entire astronomy and astrophysics program. Signed by Wendy L. Freedman Chair, Panel to Review the Science Requirements for the Terrestrial Planet Finder 6   The panel acknowledges that the TPF mission is of interest to disciplines throughout the space sciences, and that the mission could conceivably be of higher priority to other disciplines.