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Launching Science: Science Opportunities Provided by NASA’s Constellation System B Summary Analysis of Mission Concepts That Would Not Benefit from the Constellation System In its interim report, the Committee on Science Opportunities Enabled by NASA’s Constellation System determined that 4 of the 11 Vision Mission studies that it evaluated would not directly benefit from the Constellation System.1 (As described in Chapter 2 of the present report, Vision Missions are mission concepts for future space science missions studied at the initiation of NASA between 2004 and 2006.) Those four missions are the Advanced Compton Telescope (ACT), the Kilometer-Baseline Far-Infrared/Submillimeter Interferometer, the Single Aperture Far Infrared (SAFIR) Telescope, and the Solar System Exploration/Astrobiology Vision Mission (Palmer Quest) Mars lander. The committee did not receive any comments from the mission teams that developed these concepts challenging the committee’s conclusions. In its interim report, the committee also issued a request for information, in response to which it received six mission concepts (plus other responses that were not actual mission concepts). The committee determined that one of those mission concepts, Super-EUSO (Extreme Universe Space Observatory), would not benefit from Constellation.2 That mission concept, plus the four listed above, are summarized in this appendix. (See Chapter 2 for the committee’s evaluations of the 12 mission concepts that it deemed worthy of further study as Constellation missions.) ADVANCED COMPTON TELESCOPE3 Scientific Objectives of the Mission Concept The Advanced Compton Telescope is a concept for a powerful new survey instrument for studying supernovae, galactic nucleosynthesis, gamma-ray bursts (GRBs), compact objects, and the laws of physics (see Figure B.1).4 1 National Research Council, Science Opportunities Enabled by NASA’s Constellation System: Interim Report, The National Academies Press, Washington, D.C., 2008. 2 Information describing the scientific objectives and current development status of the characteristics of each mission concept evaluated here is derived from the materials provided to the committee by the teams that developed the respective proposed mission concepts. 3 The first two subsections in this section are based on S.E. Boggs, University of California, Berkeley, for the ACT Study Team, “ACT, Advanced Compton Telescope: Witness to the Fires of Creation,” NASA Vision Mission Concept Study Report, arXiv:astro-ph/0608532v1, December 2005; and J. Kurfess, U.S. Naval Research Laboratory and Praxis, Inc., “The Advanced Compton Telescope Mission,” presentation to the Committee on Science Opportunities Enabled by NASA’s Constellation System, February 21, 2008. See footnote 2 above. 4 The original Compton Gamma Ray Observatory (CGRO) was the second of NASA’s “Great Observatory” telescopes (after the Hubble Space Telescope) and operated in low Earth orbit from 1991 to 2000. At the time of its launch, it was the heaviest scientific instrument placed in orbit.
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Launching Science: Science Opportunities Provided by NASA’s Constellation System FIGURE B.1 Artist’s illustration of the Advanced Compton Telescope both in mission configuration and stowed within a Delta IV Heavy Launch Vehicle payload shroud. NOTE: GLAST, Gamma-ray Large Area Space Telescope. SOURCE: Courtesy of S. Boggs, NASA. Since the gravitational collapse of matter into stars and galaxies a few hundred thousand years after the big bang, much of the visible matter in the universe has been processed through the slow but spectacular life cycle of matter—stellar formation and evolution ending in novae or supernovae, with the ejection of heavy nuclei back into the galaxy to seed a new generation of stars. Nuclear gamma-ray astrophysics is the study of emission from radioactive nuclei as tracers of this cycle of creation. In particular, ACT would map our galaxy in a broad range of nuclear line emission from radioactive decays, nuclear de-excitations, and matter-antimatter annihilations. It would measure the radioactive gamma-ray and positron emitters among the particles propagating from supernovae, novae, and stellar winds populating our galaxy. Additionally, gamma rays from accretion of matter onto galactic compact objects and massive black holes in active galactic nuclei (AGN) are used to test accretion disk and jet models and to probe relativistic plasmas. Gamma-ray polarization can be used to study the emission processes in GRBs, pulsars, AGN, and solar flares. The origins of the diffuse cosmic MeV background can also be identified. The ACT telescope would increase detection efficiency by up to two orders of magnitude over the COMPTEL instrument (the Compton Telescope on the Compton Gamma Ray Observatory [CGRO]). The ACT instrument design is driven by its primary science goal—spectroscopy of the 56Co (0.847 MeV) line from type Ia supernovae (SNe Ia), which is expected to be Doppler-broadened to ~3 percent. ACT would allow hundreds of SNe Ia detections over its primary 5-year survey lifetime. In the process, ACT becomes an all-sky observatory for all classes of gamma-ray observations, Table B.1 provides estimates of potential ACT observations compared with those of the COMPTEL. The ACT science would be complementary to the Gamma-ray Large Area Space Telescope (GLAST) science.5 GLAST addresses the high-energy gamma rays from ~20 MeV to 300 GeV, and ACT would cover the region from 0.2 MeV to 10 MeV. GLAST also includes a 10-keV to 30-MeV GRB detector. 5 GLAST (now called the Fermi Gamma-ray Space Telescope) was successfully launched in June 2008.
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Launching Science: Science Opportunities Provided by NASA’s Constellation System TABLE B.1 Estimates of Potential Observations and/or Detections of Various High-Energy Events by the Advanced Compton Telescope (ACT) over a 5-Year Survey Period, Compared with Those by the COMPTEL Sources Number of Observations and/or Detections Actual by COMPTEL Potential by ACT Supernovae 1 100-200 Active galactic nuclei and blazars 15 200-500 Galactic 23 300-500 Gamma-ray bursts 31 1,000-1,500 Novae 0 25-50 NOTE: COMPTEL, Compton Telescope on the Compton Gamma Ray Observatory. SOURCE: Adapted from ACT, Advanced Compton Telescope: Witness to the Fires of Creation, NASA Vision Mission Concept Study Report, December 2005, available at http://arxiv.org/ftp/astro-ph/papers/0608/0608532.pdf, p. 3. The ACT mission involves a single instrument composed of a large array of multichannel gamma-ray detectors, surrounded by anticoincidence (ACD) shields on all sides, mounted on a zenith-pointing spacecraft. ACT can be launched from the Kennedy Space Center on a Delta IV vehicle into a 550-km circular orbit with 8 degrees inclination for a 5-year minimum (10-year desired) lifetime. The ACT total mass is ~4,000 kg, power is 3,340 W, and the data rate is 69 megabits per second (Mbps). The ACT concept fits within the Delta IV’s 4-m fairing. The ACT mission is not specifically identified in any decadal survey of the National Research Council (NRC). However, advanced gamma-ray mission concepts have been under consideration by NASA over the past decade, and such missions were considered as part of NASA’s roadmap activities, including what is called the Universe Strategic Roadmap.6 NASA’s Gamma Ray Astrophysics Program Working Group named ACT as its highest-priority major mission in reports published in 1997 and 1999. The committee believes that this mission offers a significant advance in its scientific field. However, this mission would require priority endorsement in the next NRC astronomy and astrophysics decadal survey before initiation of development. Characteristics of the Mission Concept as Developed to Date The ACT concept has matured to the point that flight detector technology development and selection appear to constitute the major open item. Several detector alternatives are available, all entailing considerable and challenging development efforts: (1) Si-Ge baseline D2 germanium detectors, (2) thick Si detectors, (3) liquid Xe detectors, or (4) silicon controlled-drift detectors. These detector alternatives, combined with the significant improvement in collecting area over prior instruments, appear to provide reasonable risk-mitigation paths and alternatives to satisfy the ACT requirements. There are also major challenges to be met for these technologies related to power and cryogenics system needs. Consequently, the highest priority is to focus on detector development and final selection. Relative Technical Feasibility of the Mission Concept New technology development required for ACT is relatively modest, with the most important technology development needed for the detectors. There are no significant mission design issues. The ACT mission is comparable in size, mass, and complexity to GLAST and has similar spacecraft and mission operations requirements (GLAST’s mass is 4,627 kg). The mission mass (~4,000 kg) and orbit (550 km low Earth Orbit, <10°) are within the capabilities of the 6 NASA, Universe Exploration: From the Big Bang to Life: A Strategic Roadmap of Universe Exploration to Understand Its Origin, Structure, Evolution and Destiny, Washington, D.C., May 20, 2005.
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Launching Science: Science Opportunities Provided by NASA’s Constellation System Delta IV and Ares I launch vehicles. The European Space Agency’s (ESA’s) Ariane V could provide a low-altitude, near-equatorial-orbit minimizing background, therefore making this a potentially attractive mission for international cooperation. General Cost Category in Which This Mission Concept Is Likely to Fall The ACT study team estimates an ACT mission cost of ~$760 million in fiscal year (FY) 2004 dollars. Comparing ACT with the comparable mission, GLAST, and considering the later time frame for development, ACT is likely a ~$1-billion-class mission. Benefits of Using the Constellation System’s Unique Capabilities Relative to Alternative Implementation Approaches The Ares V launch vehicle’s capabilities are not required for this mission concept, since ACT can easily be packaged to fit on a Delta IV. Although the Ares V would allow for significantly greater collecting area of detectors, the science provided by the Delta IV ACT concept is a significant order-of-magnitude improvement in gamma-ray astrophysics. A gamma-ray mission of the Ares V class would logically follow only after an ACT Delta IV-class mission had been implemented. Should This Concept Be Studied Further as a Constellation-Enabled Science Mission? The committee determined that the ACT mission concept does not deserve further study as a Constellation-enabled science mission. The primary reason is that the spacecraft size does not exceed current Evolved Expendable Launch Vehicle (EELV) capabilities. The committee determined that it is worthy of further study as a Delta IV-class mission. ACT provides significant science that deserves to be addressed by the next astronomy and astrophysics decadal survey. KILOMETER-BASELINE FAR-INFRARED/SUBMILLIMETER INTERFEROMETER7 Scientific Objectives of the Mission Concept The long-term goal of far-infrared astronomy has been a combination of large collecting area with high angular resolution. This combination is provided by the Submillimeter Probe of the Evolution of Cosmic Structure (SPECS), a Michelson interferometer consisting of two 4-m telescopes separated by up to 1 km (Figure B.2) to provide angular resolutions of a few tens of milliseconds of arc, similar to the Hubble Space Telescope (HST), James Webb Space Telescope (JWST), and the Atacama Large Millimeter Array (ALMA) at far-infrared and submillimeter wavelengths. The spacecraft would be located at the Sun-Earth L2 point. Far-infrared (>40 μm) observations of distant galaxies, the intergalactic and interstellar medium, and circumstellar winds and disks are considered necessary to an understanding of the formation of stars and galaxies and the energy balance in the universe. Two big problems have prevented far infrared astronomy from becoming as important as optical astronomy is as an information channel for studying the universe—relatively low sensitivity and poor angular resolution owing to the small diameters of those telescopes, and the strong, variable far infrared (FIR) background from the zodiacal cloud, galactic cirrus, and extragalactic light from unresolved galaxies. The poor angular resolution is especially 7 The first two subsections in this section are based on M. Harwit, Cornell University, and the Interferometer Vision Mission Team, “Kilometer-Baseline Far-Infrared/Submillimeter Interferometer,” Vision Mission Final Report, May 2005; and M. Harwit, Cornell University, and D. Leisawitz, NASA Goddard Space Flight Center, “A Kilometer-Baseline Far-Infrared/Submillimeter Interferometer in Space: Submillimeter Probe of the Evolution of Cosmic Structure, SPECS,” presentation to the Committee on Science Opportunities Enabled by NASA’s Constellation System, February 20, 2008. See footnote 2 above.
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Launching Science: Science Opportunities Provided by NASA’s Constellation System FIGURE B.2 Artist’s illustration of the Kilometer-Baseline Far-Infrared/Submillimeter Interferometer. SOURCE: Courtesy of the SPECS Consortium, prepared as part of a NASA Vision Missions study. problematic compared with the typical size of distant galaxies, ~0.1 arcsec or less at high redshifts, and it will limit the sensitivity of even large infrared telescopes, such as the SAFIR Telescope. If it works as planned, the SPECS will advance many areas of astrophysics by substantial factors. For example, typical galaxies at redshifts beyond 1 are on the order of 0.1 arcsec or less in size. Resolving these galaxies in the far infrared will be essential to study ongoing star formation in the same manner as is done now for local galaxies by comparing optical, infrared, and radio images. Most regions of active star formation have significant obscuration from dust penetrated only in the far infrared. Very high resolution is also required to see placental stars under assembly. Circumstellar disks in the act of creating new planets will produce gaps a few tens of milliseconds of arc in size around the nearest young stars, and the temperatures of the disk material will be cool enough, ~50 K, that most of the energy will be emitted at FIR wavelengths. Line radiation at these wavelengths will provide even more diagnostic information about galaxies, disks, and the interstellar medium than the continuum light. These are only a few of the important observations that a SPECS could do. Sensitive FIR observations at the resolution provided by a kilometer-baseline interferometer will be both essential to and unique for solving a wide suite of astrophysical problems. It is for this reason that the 2000 astronomy and astrophysics decadal survey8 ranked far-infrared space telescopes very highly, and the scientific case for these capabilities will remain strong and uncompromised for the foreseeable future. There is little doubt that if the goals of the currently proposed SPECS can be achieved, it will represent a major advance in observational astronomy that is unlikely to be obtained any other way. The committee believes that this mission does offer a significant advance in its scientific field. An ESA study of a similar telescope is currently examining both a free-flying option and a connected spacecraft. This mission could be attractive for future international cooperation. 8 National Research Council, Astronomy and Astrophysics in the New Millennium, National Academy Press, Washington, D.C., 2001.
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Launching Science: Science Opportunities Provided by NASA’s Constellation System Characteristics of the Mission Concept as Developed to Date This mission pushes far-infrared technology to new limits. No large FIR space telescope has been operated in space, and no interferometer of the scale proposed by this mission concept has ever been tested. The challenges of beam combination and aperture synthesis, particularly with the current generation of FIR detectors, are daunting. Several levels of technology development would be needed to bring this mission to maturity, including the following: Large, cooled telescopes operating in space (these might derive from JWST heritage); Controlled tethered telescopes or formation flying to maintain baseline control; Advanced far-infrared array detectors and heterodyne detectors working efficiently at the quantum limit; and Large, rapid, cooled delay lines operating in space. Additionally, aperture synthesis routines for a rapidly rotating two-telescope interferometer with a constantly changing baseline would need to accommodate complex engineering data sets (metrology of the baselines) with complex data streams from the detectors to assemble useful astronomical data. By most traditional measures, this mission is immature and would require several technical developments, probably as heritage from precursor missions, to reach flight readiness. Relative Technical Feasibility of the Mission Concept Every approach to far-infrared space astronomy considered by the mission team is challenging. The approach adopted in this Vision Mission concept is one reasonable choice among several. It appears technically feasible only after considerable investment to develop space hardware as outlined above. General Cost Category in Which This Mission Concept Is Likely to Fall At present, this mission appears to be considerably more expensive than the SAFIR Telescope owing to the amount of technical development yet to be done, meaning a cost of more than $5 billion. Future developments in tethered flight, FIR detectors, cooled telescopes, and fast delay lines might decrease these costs substantially, but the upfront investment at this time appears large. Benefits of Using the Constellation System’s Unique Capabilities Relative to Alternative Implementation Approaches The current project as proposed does not require the Ares V capabilities because it meets the mass and volume limits of the EELV. However, the technical goals are ambitious, and it appears likely that many of the packaging problems of fitting an interferometer into a small fairing and deploying it in space might be alleviated by using the Ares V. Complete analysis of the fuel budget needed for moving the array may show the advantages of using an Ares V to increase the lifetime of the mission. But the committee determined that the fundamental technological challenges of the mission are substantial and would not benefit from the capabilities of the Ares V or other aspects of the Constellation System. Should This Concept Be Studied Further as a Constellation-Enabled Science Mission? The committee concluded that the Kilometer Baseline Far-Infrared/Submillimeter Interferometer mission concept does not deserve further study as a Constellation-enabled science mission. This mission concept currently appears to be within the capabilities of the EELV family. A FIR interferometer is a long-term goal for this important wavelength range, and that goal is unlikely to be diminished by developments in the next two decades. It is not
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Launching Science: Science Opportunities Provided by NASA’s Constellation System FIGURE B.3 Artist’s illustration of the Single Aperture Far Infrared Telescope. SOURCE: Courtesy of John Frassanito and Associates and Northrop Grumman Space Technology. known how difficult it will be to develop the contributing technologies necessary to make this mission viable, but each of the elements has plausible development paths with a modicum of investment over the next decade. SINGLE APERTURE FAR INFRARED (SAFIR) TELESCOPE9 Scientific Objectives of the Mission Concept The Single Aperture Far Infrared Telescope mission is a 10-m telescope cooled to a few degrees above absolute zero (goal of 4 K) for observations between the thermal infrared (~20 μm) and the submillimeter (Figure B.3). It would operate at the Sun-Earth L2 point. As designed, it will improve the sensitivity of far-infrared observations by two to four orders of magnitude and the angular resolution by more than a factor of 10 compared with the sensitivity and resolution of previous missions. More importantly, the combination of sensitivity and resolution will allow the SAFIR Telescope to have a major impact on almost every major subfield of astronomy at distances stretching from the closest stars to the most distant observable galaxies. It achieves the threshold angular resolution of 0.5 arcsec in the thermal infrared needed to isolate galaxies at very high redshifts (z > 2), making it uniquely capable of addressing problems involving observations of the early universe at wavelengths longer than 20 μm. The SAFIR Telescope is also a steppingstone to the long-range goal of very high angular resolution for far infrared astronomy. The instrument suite will also include low-resolution spectrographs. The thermal and FIR wavelength range is most sensitive to cool regions in the interstellar and intergalactic medium at temperatures from a few to a few hundred kelvin. In galaxies, interstellar dust and gas typically absorb and reradiate much of the light from stars and active galactic nuclei, assumed to be black holes, meaning that the FIR radiation often contains the majority of the energy budget. Far infrared lines provide the best way to study 9 The first two subsections in this section are based on D. Lester et al., “Science Promise and Conceptual Mission Design Study for SAFIR—The Single Aperture Far Infrared Observatory,” June 2005; and D. Lester, University of Texas, “Constellation Architecture and the Single Aperture Far Infrared Telescope (SAFIR),” presentation to the Committee on Science Opportunities Enabled by NASA’s Constellation System, February 20, 2008. See footnote 2 above.
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Launching Science: Science Opportunities Provided by NASA’s Constellation System the gas free from the obscuring effects of dust at ultraviolet (UV) and optical wavelengths. These observations provide a unique complement to optical, x-ray, and radio observations, and it is safe to say that researchers cannot understand the early evolution of stars, the creation of metals, and the creation of dust without FIR observations at angular resolutions at least as high as those proposed for the SAFIR Telescope. The extragalactic case for a facility such as the SAFIR Telescope is unique and is unlikely to be eroded by other observatories in the next two decades. Far infrared observations of galactic clouds of gas and dust are also essential to an understanding of the physics of star formation. Circumstellar disks around young and old stars alike, indicative of planet formation in the former case, emit the bulk of their luminosity at thermal and FIR wavelengths, making the SAFIR Telescope an important facility to study planet formation and making it an essential complement to the ALMA, as well as the JWST and the Stratospheric Observatory for Infrared Astronomy (SOFIA). There are a variety of novel uses of FIR observations for studying the interstellar medium and other circumstellar regions that highlight the importance of this wavelength region to an understanding of how the interstellar medium behaves and takes part in the life cycle of new stars and planets. The key science objectives as outlined in the proposal are the following: Probe the earliest epochs of metal enrichment and see the galaxy-forming universe before metals are created. Understand the origin of dust grains in the universe. Resolve the far-infrared cosmic background—trace the formation and evolution of star-forming and active galaxies since the dawn of the universe, and measure the history of star formation. Explore the connection between embedded nuclear black holes and their host galaxies. Understand the relationship of active nuclei to galaxy formation. Track the chemistry of life. Follow prebiotic molecules, ices, and minerals from clouds to nascent solar systems. Identify young solar systems from debris disk structure and map the birth of planetary systems from deep within obscuring envelopes. Assess the degree of bombardment that they face, and the degree of habitability. A fundamental difficulty with observations at wavelengths longer than about 50 μm is that the relatively low angular resolution combined with zodiacal light and the light from many distant galaxies creates the primary noise source; that is, the sensitivity is limited by foreground emission and confusion. However, because the SAFIR Telescope will still gain orders of magnitude in observing capability over all existing and planned facilities in this important wavelength range, its science case is very strong. The committee believes that this mission does offer a significant advance in its scientific field. Characteristics of the Mission Concept as Developed to Date The SAFIR Telescope would build on the technological improvements developed for the James Webb Space Telescope and would appear to be a logical follow-on to that observatory (subject to evaluation by the decadal survey). It is 1.5 times larger than JWST but with far less stringent requirements on surface accuracy. It has much more demanding requirements for instrument cooling (4 K for SAFIR against 40 K for JWST). Because the longest wavelengths are confusion-limited by distant galaxies, even modest gains in cooling over JWST—to 20 K, for example—will satisfy many of the science goals, allowing some margin in this challenging technological development. The SAFIR Telescope would need improved FIR detectors (by about an order of magnitude), requiring unknown cost, but it seems likely that great improvements could be made with a modest investment. In all other respects, the concept is relatively mature. Relative Technical Feasibility of the Mission Concept The current SAFIR Telescope concept arises from the heritage of JWST, and therefore it appears technically feasible with the proviso that the SAFIR Telescope would have to achieve lower temperatures under passive cooling
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Launching Science: Science Opportunities Provided by NASA’s Constellation System than those of any telescope to date. Of the different design concepts considered by the mission team, the current one most closely derives from already-developed technology. The SAFIR Telescope does not propose enormous technological advances over JWST, although the cooling to 4 K is likely to be a major challenge. General Cost Category in Which This Mission Concept Is Likely to Fall The SAFIR Telescope mission is likely to be similar in cost to JWST ($4.5 billion). It uses similar technology, although it will have a 50 percent larger mirror with more demanding thermal requirements but less demanding surface accuracy, wavefront control, and pointing. It is not known how much investment is needed to improve FIR detectors to take full advantage of the SAFIR Telescope. The full execution of this mission concept will likely cost more than $5 billion. Benefits of Using the Constellation System’s Unique Capabilities Relative to Alternative Implementation Approaches The Ares V capabilities are not required for the SAFIR Telescope mission concept as proposed. The size of the telescope in the Vision Mission study was chosen as one that can fit in an EELV using at least some JWST heritage, and at this size it would permit significant scientific advances to be achieved. Assuming that JWST is successfully developed, SAFIR should not be inherently more difficult to design and build. However, if JWST proves to pose major design challenges, then large telescopes such as SAFIR could potentially benefit from having a larger shroud on a more capable launch vehicle, easing packaging and integration of the telescope. Should This Concept Be Studied Further as a Constellation-Enabled Science Mission? The committee concluded that the SAFIR Telescope mission concept does not deserve further study as a Constellation-enabled science mission. The SAFIR Telescope is likely to be very attractive as a Delta IV-class mission and does not require the increased capabilities of the Ares V launch vehicle. The SAFIR Telescope could be considered as a candidate stand-alone Ares V mission, but the heavy launch vehicle is not required. The SAFIR Telescope received a high rank in the 2001 astronomy and astrophysics decadal survey,10 and the more developed concept presented in the Vision Mission study should make it equally attractive in the next decadal survey. SOLAR SYSTEM EXPLORATION/ASTROBIOLOGY VISION MISSION: PALMER QUEST11 Scientific Objectives of the Mission Concept Palmer Quest, a proposal for a mission to the North Polar Cap of Mars, would include a nuclear reactor-powered “cryobot” to drill down through the polar ice to its bottom and determine if life once existed on Mars (Figure B.4). The main science goals of Palmer Quest are to assess the possibility of the presence of life on Mars and to evaluate the habitability of the basal domain of the martian polar caps. Science questions for Palmer Quest are as follows: Does life currently exist on Mars? Did life ever exist there? How hospitable was and is Mars to life? 10 National Research Council, Astronomy and Astrophysics in the New Millennium, National Academy Press, Washington, D.C., 2001. 11 The first two subsections in this section are based on F.D. Carsey, Jet Propulsion Laboratory, and the Palmer Quest Team, “Palmer Quest, the Search for Life at the Bed of the Mars Polar Cap,” July 2005; and L. Beegle, Jet Propulsion Laboratory, “Palmer Quest: A Mission to the Martian Polar Cap,” presentation to the Committee on Science Opportunities Enabled by NASA’s Constellation System, February 21, 2008. See footnote 2, above.
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Launching Science: Science Opportunities Provided by NASA’s Constellation System FIGURE B.4 Illustration of the Palmer Quest spacecraft elements, including a cryobot powered by a nuclear reactor, and the surface station and rover. NOTE: FRAM, Far Ranging Arctic Mission; RPS, radioisotope power source. SOURCE: Courtesy of NASA, Jet Propulsion Laboratory, California Institute of Technology. The Palmer Quest objectives include looking for the presence of microbial life, amino acids, nutrients, and geochemical heterogeneity in the ice sheet; quantifying and characterizing the provenance of the amino acids in Mars’s ice; assessing the stratification of outcropped units for indications of habitable zones; and determining the accumulation of ice, mineralogical material, and amino acids in Mars ice caps over the present epoch. The Palmer Quest mission would also address several objectives from decadal reports of the planetary community, including developing an understanding of the current state and evolution of the atmosphere, surface, and interior of Mars; determining if life exists or has ever existed on Mars; and developing an understanding of Mars in support of possible future human exploration. The science to be addressed by Palmer Quest has the potential for paradigm-altering discoveries related to life on Mars. The mission would launch in August 2022 on a Delta 4050 heavy launch vehicle and a probe would descend through the ice sheet of the Mars North Polar Cap to search for life at its bed. It could be extended to quantify surface fluxes of biochemicals, nutrients, and water ice over the annual cycle and to examine outcropped ice cap strata and basal units from a surface rover. The mission would land a surface package that included a thermal drill (the cryobot, powered by a nuclear reactor), a mobile vehicle (the Far Ranging Arctic Mission [FRAM], powered by a nuclear thermal source), and a surface observation system (the surface station, powered by the nuclear reactor). The drill would pass through approximately 2 km of dusty ice while simultaneously acquiring data. There
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Launching Science: Science Opportunities Provided by NASA’s Constellation System would also be data recorded with regard to the outcropping at the contact of the ice cap and the bed in addition to the sedimentary record stored in the polar layered deposits. The probe would also observe the seasonal cycle of the accumulation of water and CO2 ice, dust, and organics on the polar cap surface. The development of the thermal drill may be useful in future missions involving drilling through the ice surface layer of Jupiter’s moon Europa and Saturn’s moon Enceladus. The spacecraft mass (wet) is approximately 4,900 kg. The committee believes that this mission does offer a significant advance in its scientific field. However, the mission has not been endorsed by any recent NRC reports and does not appear in the 2003 solar system exploration decadal survey.12 Characteristics of the Mission Concept as Developed to Date The maturity of the Palmer Quest mission concept is low. Some of the scientific instruments follow in the footsteps of current and planned Mars missions such as Phoenix Lander and the Mars Science Laboratory. However, the use of a thermal drill, FRAM rovers, and a substation all powered by nuclear fission are unique to this mission. The challenges and technical feasibility associated with these components, illustrating the low maturity of the mission, are described below. Relative Technical Feasibility of the Mission Concept The Palmer Quest mission concept is based on past successful Mars missions while incorporating novel technologies yet to be developed and that are not yet launch-ready. The technological complexity of the mission concepts is high. Three areas of technology development required by this mission and described in the presentation to the committee are thermal issues, radiation protection, and instrumentation capabilities. Data provided as part of this presentation include a listing of the high-level risks associated with each of the components. The cryobot is a tool designed to melt its way through the “ice” and provide a variety of measurements related to water and biological-related chemical components. The experimenters have listed the risks associated with the work of the cryobot as (a) breaking of the tether in the ice, (b) clogging of the filter system, (c) failure of pumps and valves that supply samples to the instruments, and (d) low technology readiness level (TRL) of instruments for life detection. The rover, the workhorse of the experiments that moves equipment and carries out exploration traverses of some 300 km, has listed as possible risks (a) failure of the inflatable wheels, (b) failure to successfully deploy the rover, (c) difficulties during towing of the surface station, and (d) survivability over the martian northern winter. The surface station, whose role is in situ determination of rates of accumulation of gases and dust, the estimation of annual net gases and dust accumulation, and the determination of some properties of fine-scale morphology and structure related to current polar climate, has listed as risks (a) getting stuck on the main lander body during deployment; (b) the assumption of a solid, flat surface for the deployment; and (c) physically altering the environment especially during and after winter. Mass is limited by the aeroshell. There is little room for mass growth in this mission without a major new aeroshell development; therefore, Ares V offers no improvement without a complete redesign of the spacecraft and mission concept. The biological assay involved in the mission is not described in enough detail to determine its technical feasibility. The use of nuclear fission and the complexity of this mission place it in a high-risk category. 12 National Research Council, New Frontiers in the Solar System: An Integrated Exploration Strategy, The National Academies Press, Washington, D.C., 2003.
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Launching Science: Science Opportunities Provided by NASA’s Constellation System General Cost Category in Which This Mission Concept Is Likely to Fall Although the mission costs were not directly calculated in the written report presented to the committee, a rough cost in the $3 billion to $5 billion range was mentioned in the oral presentation to the committee. This cost did not include the technology development of the more expensive mission elements, including the nuclear reactor, Mach 3 parachute, and biology-focused instrumentation. The development, required safety elements, and use of a nuclear-powered device are major cost contributors in this mission. Additional costs would be required for the new technology developments needed by this mission. Therefore, the committee places the cost estimate for the Palmer Quest mission at greater than $5 billion. Benefits of Using the Constellation System’s Unique Capabilities Relative to Alternative Implementation Approaches The Constellation System is not required for this mission because the entry mass at Mars limits the mission size. The payload volume and weight requirements are met by the Delta IV Heavy launch vehicle. Although the Ares V could launch far more payload to Mars than any of its predecessor launch vehicles could, there remains a severe limitation on how much mass can be delivered to the Mars surface. Increasing that mass delivery capability would require an entirely new aeroshell design and associated technology development and testing. However, the Constellation System could possibly provide increased safety for the nuclear components during launch. Should This Concept Be Studied Further as a Constellation-Enabled Science Mission? The committee concluded that the Palmer Quest mission concept does not deserve further study as a Constellation-enabled science mission. This is an expensive, complex, and high-risk mission that does not require the Constellation System and could use an existing launch vehicle such as the Delta IV. The Palmer Quest mission has not appeared in any previous NRC reports, including the solar system exploration decadal survey13 and the recent report An Astrobiology Strategy for the Exploration of Mars.14 In addition, the technical maturity of this mission is low. SUPER-EUSO MISSION15 Scientific Objectives of the Mission Concept The purpose of the Super-EUSO (Extreme Universe Space Observatory) mission is to open a new window into high-energy particle astronomy through the study of Ultra High Energy Cosmic Particles (UHECP). These are charged particles, photons, neutrinos, or yet-undiscovered particles with energies in excess of E ≈ 1019 eV, which enter Earth’s atmosphere, creating an extensive air shower (EAS). The flux is very low (less than one particle per square kilometer per century, at energies E ≥ 1020 eV). To detect such rare events, an extremely large detector volume is required (Figure B.5). Each EAS event is the result of a cascade of nuclear interactions in the atmosphere initiated by a single cosmic-ray particle. An EAS consists of hundreds of billions of electrons, positrons, and photons, all traveling together at nearly the speed of light. This flux of particles excites nitrogen atoms in the atmosphere which de-excite either by collisions with other atoms or by emitting light. This fluorescent light is emitted isotropically, and because all of the particles in the EAS are exceeding the speed of light in air, the EAS also produces Cherenkov radiation that 13 National Research Council, New Frontiers in the Solar System: An Integrated Exploration Strategy, The National Academies Press, Washington, D.C., 2003. 14 National Research Council, An Astrobiology Strategy for the Exploration of Mars, The National Academies Press, Washington, D.C., 2007. 15 The first two subsections in this section are based on J.H. Adams, Jr., NASA Marshall Space Flight Center, “Super EUSO Mission,” in response to request for information by the Committee on Science Opportunities Enabled by NASA’s Constellation System, May 2008. See footnote 2 above.
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Launching Science: Science Opportunities Provided by NASA’s Constellation System FIGURE B.5 Illustration of the layout for the overall Super-EUSO (Extreme Universe Space Observatory) spacecraft. SOURCE: Courtesy of NASA and the Orbiting Wide-angle Light-collector (OWL) Collaboration. is strongly forward-beamed. The fluorescent light appears primarily in three emission lines, at 337, 357, and 391 nm. The Cherenkov light is continuous, and its intensity decreases with wavelength, λ, as 1/λ2. Plans for the proposed Super-EUSO would be for it to observe EASs from space. It would accurately measure the nature, energy, and arrival direction of the primary particles using a target volume of air far greater than is possible from the ground (Figure B.6). The primary science goals are to determine the origin of UHE particles, learn about the propagation environment from the source to Earth, and study particle physics at energies well beyond those achievable in human-made accelerators. The proposed investigation here is intended to address the goals identified by the Science Program for NASA’s Astronomy and Physics Division (2006) and the need for research on high-energy cosmic rays to be detected by space instruments that look back down on Earth and prove more effective than ground-based observatories. Characteristics of the Mission Concept as Developed to Date The mission concept presented for Super-EUSO is based on earlier concepts such as the Orbiting Wide-angle Light-collector (OWL), selected for a NASA Cross-Enterprise Technology Development Program (CETDP) grant and recommended as high priority for the 2008-2013 time frame; and the Extreme Universe Space Observatory (EUSO), first selected by ESA for a concept study before the Columbia accident, which prevented it from being transported to the International Space Station (ISS). Super-EUSO was proposed to ESA’s Cosmic Vision solicita-
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Launching Science: Science Opportunities Provided by NASA’s Constellation System FIGURE B.6 Time-history of the light signal from an extensive air shower seen from a 400-km orbit. SOURCE: Courtesy of NASA. tion and selected for technology development, but the review committee believed that the mission was not technically mature enough to proceed to flight at the time. The mission proposed here is an enlarged and improved version of the OWL and EUSO mission concepts, improving on the performance by exploiting novel technologies. The design of the experimental apparatus exploits the results of the detailed studies carried on during the phase A of EUSO mission in order to redesign the apparatus in such a way as to fully satisfy the scientific requirements. To investigate and identify the sources of (ultrahigh energy) cosmic rays in the nearby universe and enable extreme energy neutrino astronomy will require very large collecting areas such as that proposed by Super-EUSO. The current scenario calls for an experiment able to detect events with an energy threshold of 1019 eV and the ability to view very large volumes of Earth’s atmosphere. Additionally, the experiment needs to have a long enough operating lifetime to collect many samples of the rare ultrahigh events and to detect weakly interacting particles. Relative Technical Feasibility of the Mission Concept ESA’s reviewers classified the readiness level of the current design optics for the Super-EUSO, critical for the mission, as TRL 2 and their design to be very preliminary. The reviewers also thought that deployability of a large folded Schmidt telescope corrector plate (proposed so
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Launching Science: Science Opportunities Provided by NASA’s Constellation System that Super-EUSO could fit in a smaller fairing) was very challenging as well (Figure B.7). Thermal design appeared to be at an early stage but is likely a solvable problem. A rigid (one-piece) corrector plate could be launched using Ares V. Feasibility, manufacturing, and launch of the large, single element optics remains a concept only. No one has attempted such a project at even half the size as proposed by Super-EUSO. Gap detectors for the focal surface were also classified with a TRL 2 because the response in the UV is not yet acceptable or demonstrated. The focal plane is large and complex but technically feasible. The required data-handling system with 1 × 106 of channels is considered immature and was classified with a TRL 3; the power budget (5 kW) was also regarded as challenging. General Cost Category in Which This Mission Concept Is Likely to Fall The Super-EUSO proposal was subjected to a detailed cost and technology readiness review by ESA. ESA concluded that the mission would cost $1.7 billion. This estimate is no doubt very low, as nearly all the technical challenges required to be solved for Super-EUSO are in early stages of development. The committee judges that the cost of Super-EUSO as described would fall into the $1 billion to $5 billion range. Benefits of Using the Constellation System’s Unique Capabilities Relative to Alternative Implementation Approaches The proposed Super-EUSO experiment requires observation of a large volume (owing to the rarity of ultrahigh cosmic-ray events) as well as a large collecting area owing to the low flux levels expected. These requirements drive the project to need a very large aperture, wide-angle telescope to be placed in orbit. High optical throughput requires the telescope to have a fast focal ratio. The baseline telescope at present is an 11-m primary Schmidt design with a 6.7-m corrector lens. The size and mass of such a telescope (with a deployable primary mirror) would require an Ares V launch vehicle, as it offers a sufficiently large fairing to launch this experiment. Should This Concept Be Studied Further as a Constellation-Enabled Science Mission? The current mission design is very early in the planning stages for all the critical components. The technology readiness level is quite low (TRL-2 to TRL-3), according to the proposers. The optics are a particular concern, FIGURE B.7 Illustration of potential methods for folding the optics for the Super-EUSO mission. SOURCES: Left: Courtesy of Carlo Gavazzi Space SpA; middle and right: Courtesy of NASA and the Orbiting Wide-angle Light-collector (OWL) Collaboration.
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Launching Science: Science Opportunities Provided by NASA’s Constellation System FIGURE B.8 Artist’s conception of the Super-EUSO spacecraft from the revised version of the concept. SOURCE: Courtesy of NASA. as there seems to be no technical solution for their manufacturing or launch or a technical path to such development. Other, more feasible mission concepts, such as multiple smaller telescopes or different wide-angle telescope designs that can be manufactured, as well as ground-based expansions of systems such as the Pierre Auger Observatory, should be considered as a more cost-effective means of reaching the scientific goals. The committee determined that the Super-EUSO mission concept, as presented to the committee, does not deserve further study as a Constellation-enabled science mission. Additional note: The proposers submitted a revised and updated version of the Super-EUSO concept to the committee several months after making their initial proposal and presentation. The revised proposal appears to answer some of the concerns about fitting the optics into the Ares V shroud without the need of folding and deployment. (See Figure B.8.) It also appears to address concerns raised by the original proposal about the large corrective optics required. However, the data provided were very limited, and insufficient for the committee to assess the relative TRL for the technology solutions. Furthermore, the revised proposal still does not provide a clear rationale explaining why an Ares V-sized mission is necessary compared with smaller and more affordable solutions, including ground-based solutions.