2
Analysis of Vision Mission Studies

In 2004 NASA commissioned a series of 14 studies of possible robotic missions in the time period 2020-2035. Based on a series of roadmaps conducted by NASA in 2003, the studies were told to assume using the Atlas V or Delta IV families of launch vehicles (collectively known as the Evolved Expendable Launch Vehicle, or EELV, system). The studies were labeled the “Vision Mission” studies. Of the 14 studies, 12 resulted in final reports covering 11 mission concepts (2 concerning Neptune missions), most produced by 2006.

The National Research Council’s Committee on Science Opportunities Enabled by NASA’s Constellation System asked the studies’ principal investigators, or other representatives, to present them to the committee at its first meeting. The principal investigators were asked to speculate about how their existing studies might benefit from the Constellation capabilities—primarily the larger payload and shroud dimensions of the Ares V rocket, and the possibility of human servicing of spacecraft. The presenters had only limited time to assess how the Constellation capabilities might affect their proposals. Some indicated that the Constellation System would have no appreciable effect on their concepts, and others indicated that it would serve as a substitute for required technology development. Some of the proposers also indicated that their science objectives might be enlarged and expanded if they had additional mass and volume available.

The committee analyzed each of the missions according to a standard format based on the committee’s statement of task, which specified analysis of the following information for each mission concept considered:

  1. Scientific objectives of the mission concept;

  2. A characterization of the mission concept insofar as the maturity of studies to date have developed it;

  3. The relative technical feasibility of the mission concepts compared to each other;

  4. The general cost category into which each mission concept is likely to fall;

  5. Benefits of using the Constellation System’s unique capabilities relative to alternative implementation approaches; and

  6. Identification of the mission concept(s) most deserving of future study.

For this interim report, the committee chose to consider both the Vision Mission final reports and the presentations to the committee during its February 2008 meeting. In some cases there were no or only minor differences between the reports and the presentations. In others the differences primarily concerned speculation about how the mission would benefit from the Constellation capabilities. For the Interstellar Probe concept, the presentation was made by the team that did not produce a final report. However, the science objectives of both Interstellar Probe studies were identical, and the presenter focused primarily on how the mission concept would benefit from the Constellation System.



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2 Analysis of Vision Mission Studies In 2004 NASA commissioned a series of 14 studies of possible robotic missions in the time period 2020-2035. Based on a series of roadmaps conducted by NASA in 2003, the studies were told to assume using the Atlas V or Delta IV families of launch vehicles (collectively known as the Evolved Expendable Launch Vehicle, or EELV, system). The studies were labeled the “Vision Mission” studies. Of the 14 studies, 12 resulted in final reports covering 11 mission concepts (2 concerning Neptune missions), most produced by 2006. The National Research Council’s Committee on Science Opportunities Enabled by NASA’s Constellation System asked the studies’ principal investigators, or other representatives, to present them to the committee at its first meeting. The principal investigators were asked to speculate about how their existing studies might benefit from the Constellation capabilities⎯primarily the larger payload and shroud dimensions of the Ares V rocket, and the possibility of human servicing of spacecraft. The presenters had only limited time to assess how the Constellation capabilities might affect their proposals. Some indicated that the Constellation System would have no appreciable effect on their concepts, and others indicated that it would serve as a substitute for required technology development. Some of the proposers also indicated that their science objectives might be enlarged and expanded if they had additional mass and volume available. The committee analyzed each of the missions according to a standard format based on the committee’s statement of task, which specified analysis of the following information for each mission concept considered: 1. Scientific objectives of the mission concept; 2. A characterization of the mission concept insofar as the maturity of studies to date have developed it; 3. The relative technical feasibility of the mission concepts compared to each other; 4. The general cost category into which each mission concept is likely to fall; 5. Benefits of using the Constellation System’s unique capabilities relative to alternative implementation approaches; and 6. Identification of the mission concept(s) most deserving of future study. For this interim report, the committee chose to consider both the Vision Mission final reports and the presentations to the committee during its February 2008 meeting. In some cases there were no or only minor differences between the reports and the presentations. In others the differences primarily concerned speculation about how the mission would benefit from the Constellation capabilities. For the Interstellar Probe concept, the presentation was made by the team that did not produce a final report. However, the science objectives of both Interstellar Probe studies were identical, and the presenter focused primarily on how the mission concept would benefit from the Constellation System. 11

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ASTRONOMY AND ASTROPHYSICS VISION MISSIONS Advanced Compton Telescope1 FIGURE 2.1 Artist’s illustration of the Advanced Compton Telescope in both mission configuration and stowed within a Delta IV payload shroud. SOURCE: Courtesy of S. Boggs, NASA. Scientific Objectives of the Mission Concept The Advanced Compton Telescope (ACT) 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 (Figure 2.1).2 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, the ACT maps our galaxy in a broad range of nuclear line emission from radioactive decays, nuclear de-excitations and matter-antimatter annihilations. It measures 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 1 ACT, Advanced Compton Telescope: Witness to the Fires of Creation, NASA Vision Mission Concept Study Report, December 2005, available at ; Jim Kurfess, “The Advanced Compton Telescope Mission,” presentation to the Committee on Science Opportunities Enabled by NASA’s Constellation System, February 2008. 2 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. 12

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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 increases 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 allows 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 2.1 provides estimates of potential ACT observations compared to those of the COMPTEL. The ACT science is complementary to the Gamma-ray Large Area Space Telescope (GLAST) science.3 GLAST addresses the high-energy gamma rays from ~20 MeV to 300 GeV, and ACT covers the region from 0.2 MeV to 10 MeV. GLAST also includes a 10-keV to 30-MeV gamma-ray burst detector. TABLE 2.1 Estimates of Potential ACT Observations/Detections of Various High-Energy Events Compared to Those with the COMPTEL Sources (5 year) COMPTEL ACT Supernovae 1 100-200 Active galactic nuclei 15 200-500 and blazars Galactic 23 300-500 Gamma-ray bursts 31 1000-1500 Novae 0 25-50 NOTE: ACT, Advanced Compton Telescope; COMPTEL, Compton Telescope on CGRO. SOURCE: 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 multi-channel gamma-ray detectors, surrounded by anti-coincidence (ACD) shields on all sides, mounted on a zenith- pointing spacecraft. The 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 Mbps. The ACT concept fits within the Delta IVs 4-meter faring. The ACT mission is not specifically identified in any decadal survey. 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 the Universe Strategic Roadmap. 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. 3 GLAST is scheduled for launch aboard a Delta II rocket from Cape Canaveral in May 2008. 13

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Characteristics of the Mission Concept as Developed to Date The ACT concept has matured to the point that flight detector technology development and selection appears to be 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 LEO, <10 degrees) are within the capabilities of the Delta IV and Ares I. The European Space Agency’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 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 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 has been implemented. Should This Concept Be Studied Further as a Constellation-Enabled Science Mission? 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 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. 14

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Generation-X4 FIGURE 2.2 Artist’s conception of one possible version of the Generation-X x-ray telescope. SOURCE: Courtesy of Roger Brissenden, Smithsonian Astrophysical Observatory. Scientific Objectives of the Mission Concept The proposed Generation-X (Gen-X) space-based x-ray telescope (Figure 2.2) complements all the other wavelength future-generation telescopes, such as the Atacama Large Millimeter Array (ALMA), James Webb Space Telescope (JWST), Square Kilometre Array (SKA), and TMT (Thirty Meter Telescope). This telescope is designed to study the new frontier of astrophysics: the birth and evolution of the first stars, galaxies, and black holes in the early universe. X-ray astronomy offers an opportunity to detect these via the activity of the black holes, and the supernova explosions and gamma-ray burst afterglows of massive stars. The Generation-X Vision Mission is based on an x-ray observatory with 50 m2 collecting area at 1 keV (1,000 times larger than the Chandra x-ray telescope) and 0.1-arc-second angular resolution (several times better than Chandra and 50 times better than the Constellation-X resolution goal). Such a high-energy observatory will be capable of detecting the earliest black holes and galaxies in the universe and will study the chemical evolution of the universe and extremes of density, gravity, magnetic fields, and kinetic energy which cannot be created in laboratories. The goal of Gen-X is to observe the first black holes and stars at redshift z ~ 10-20. The idea is to search for the effects of the early black holes on the formation of galaxies and to trace through time the evolution of galaxies, black holes, and the chemical elements. This mission is intended to address the science goals outlined in the 2001 NRC astronomy and astrophysics decadal survey5 and is the logical follow-on to the Constellation-X mission, an x-ray mission that is proposed for the next decade. The committee believes that this mission does offer a significant advance in its scientific field. 4 R. Brissenden et al., Generation-X Vision Mission Study Report, March 2006; R. Brissenden, “The Generation-X Vision Mission,” presentation to the Committee on Science Opportunities Enabled by NASA’s Constellation System, February 21, 2008. 5 National Research Council, Astronomy and Astrophysics in the New Millennium, National Academy Press, Washington, D.C., 2001. 15

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Characteristics of Mission Concept as Developed to Date Various configurations have been studied for Generation-X: a single 20-meter mirror, six 8-meter mirrors, and a single 16-meter mirror. The committee considers the 20-meter concept and the six 8-meter mirror concepts to be exceedingly complex and decided to evaluate only the 16-meter mirror configuration utilizing the Ares V launch vehicle.6 The Generation-X mission operates at Earth-Sun L2. A 16-meter telescope will require either robotic or human-assisted in-flight assembly. The required effective area, 50 square meters, implies that extremely lightweight grazing incidence x-ray optics must be developed. To achieve the required areal density, at least 100 times lower than in Chandra, technology development is needed to produce 0.2-mm- thick mirrors (approximately the width of two human hairs) that have active on-orbit figure control. The suite of detectors includes a large-field-of-view high-angular-resolution imager (wide field imager), a cryogenic imaging spectrometer (micro-calorimeter), and a reflection grating spectrometer. The technology relating to the mirrors would clearly require early development and investment. The study report proposes the delivery of a detailed technology development plan for the next decade that will get key optics and detector technologies evolved to technology readiness level (TRL) 6. The report suggests the need for a decade of development effort in order that the mission could be realized in the following decade. The use of “two-hair-thick” mirrors with active control is a technological challenge. The success of the mission depends on this development effort. Initial adjustment and alignment, and then the maintaining of the alignment, will be challenging. The space assembly of the 16-meter mirror has not been defined in detail. The proposers have suggested using robotic assembly, but human assembly remains another possibility. Relative Technical Feasibility of Mission Concept The Chandra X-ray Observatory has a collecting area of 0.04 m2 and uses glass shell technology. Gen-X has a 50 m2 collecting area and uses thin foils with active control of shape to reduce the mass. The Ares V launch vehicle would simplify the mission concept because of the large payload fairing and mass to L2 capability. It has the potential of further simplifying the mirror development by allowing thicker mirror shells than were studied for the earlier configurations. The technical challenges are the mirror development and the in-space assembly. General Cost Category in Which This Mission Concept Is Likely to Fall A November 2007 Gen-X proposal that would utilize the Ares V estimated a $4 billion cost (excluding launch costs).7 The committee believes that this is a conservative estimate. The technology development of the mirrors is the large unknown in the cost estimates. 6 There are numerous problems for the other configurations. For instance, the six 8-meter mirror concept would require six launches of Delta IV heavy rockets. The Delta IV heavy requires 3 months of ground preparation prior to launch, and therefore merely the launches alone would require 18 months of preparation during which no other Delta IV rockets—necessary for national security payloads—could be launched. 7 “A Concept Study of the Technology Required for Generation-X: A Large Area and High Angular Resolution X-ray Observatory to Study the Early Universe,” submitted in response to NNH07ZDA001N-ASMCS, November 20, 2007. 16

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Benefits of Using the Constellation System’s Unique Capabilities Relative to Alternative Implementation Approaches The Ares V capability results in a simplified and more cost-effective baseline mission concept. It would be able to deliver Gen-X directly to Earth-Sun L2 point. A single launch would allow a 16-meter- diameter mirror to be placed into position, would permit use of piezo-electric control of the mirror’s optical surface for final figure on-orbit to achieve ~0.1-arc-second angular resolution, and would enable placing into position a 60-meter extendable optical bench between the optics and the science instrumentation package. The estimated mass for the Gen-X Ares V configuration is 22,000 kg. The Ares V is capable of delivering 55,900 kg to translunar injection trajectory (essentially that required for Gen-X). This mass margin possibly could be used to reduce the risk in the mirror development by enabling a thicker mirror. Should This Concept Be Studied Further as a Constellation-Enabled Science Mission? The Gen-X mission concept is worthy of further study as a Constellation-enabled science mission. The major issue is the mirror development. The extra mass capability of the Ares V may enable the use of thicker mirrors and therefore significantly reduce risk. Single Aperture Far Infrared Telescope 8 FIGURE 2.3 Artist’s illustration of the SAFIR telescope. SOURCE: Image courtesy of John Frassanito and Associates and Northrop Grumman Space Technology. Scientific Objectives of the Mission Concept The Single Aperture Far Infrared (SAFIR) mission is a 10-meter 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 2.3). It would operate at the Earth-Sun L2 point. As designed, it will improve the sensitivity of far infrared (FIR) observations by two to four orders of magnitude and the angular 8 D. Lester et al., Science Promise and Conceptual Mission Design Study for SAFIR⎯The Single Aperture Far Infrared Observatory, June 2005; D. Lester, “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. 17

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resolution by more than a factor of 10 compared to previous missions. More importantly, the combination of sensitivity and resolution will allow SAFIR 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. SAFIR 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 far infrared 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 far infrared radiation often contains the majority of the energy budget. Far infrared lines provide the best way to study the gas free from the obscuring effects of dust at ultraviolet 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 far infrared observations at angular resolutions at least as high as those proposed for SAFIR. The extragalactic case for a facility like SAFIR 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 understand 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 far infrared wavelengths, making SAFIR an important facility to study planet formation and making it an essential complement to the Atacama Large Millimeter Array (ALMA), as well as JWST and SOFIA. There are a variety of novel uses of far infrared observations to study the interstellar medium and other circumstellar regions that highlight the importance of this wavelength region to understand 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: • 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 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 SAFIR 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. 18

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Characteristics of the Mission Concept as Developed to Date SAFIR 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. SAFIR would need improved far infrared 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 concept arises from the heritage of JWST, and therefore appears technically feasible with the proviso that SAFIR would have to achieve lower temperatures under passive cooling that any telescope to date. Of the different design concepts considered by the team, the current one most closely derives from already-developed technology. SAFIR 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 mission is likely to be similar in cost to JWST ($4.5 billion). It uses similar technology, but 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 far infrared detectors to take full advantage of the SAFIR telescope. Benefits of Using the Constellation System’s Unique Capabilities Relative to Alternative Implementation Approaches The Ares V capabilities are not required for this 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 enable significant scientific advances to be achieved. Assuming that the 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 SAFIR mission concept does not deserve further study as a Constellation-enabled science mission. SAFIR 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. SAFIR could be considered as a candidate stand- alone Ares V mission, but the heavy launch vehicle is not required. SAFIR received a high rank in the 19

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2001 astronomy and astrophysics decadal survey,9 and the more developed concept presented in the Vision Mission study should make it equally attractive in the next decadal survey. Kilometer-Baseline Far-Infrared/Submillimeter Interferometer10 FIGURE 2.4 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. 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-meter telescopes separated by up to 1 km (Figure 2.4) to provide angular resolutions of a few tens of milliseconds of arc, similar to Hubble, JWST, and ALMA at far infrared and submillimeter wavelengths. The spacecraft would be located at the Earth-Sun L2 point. 9 National Research Council, Astronomy and Astrophysics in the New Millennium, National Academy Press, Washington, D.C., 2001. 10 M. Harwit et al., Kilometer-Baseline Far-Infrared/Submillimeter Interferometer, Vision Mission Final Report, May 2005; “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 2008. 20

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Far infrared (>40 µm) observations of distant galaxies, the intergalactic and interstellar medium, and circumstellar winds and disks are considered necessary to understand 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 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 background from the zodiacal cloud, galactic cirrus, and extragalactic light from unresolved galaxies. The poor angular resolution is especially problematic compared to 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 SAFIR. 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 of order 0.1 arc second 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 far infrared 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 far infrared observations at the resolution provided by a kilometer-baseline interferometer will be both essential and unique for solving a wide suite of astrophysical problems. It is for this reason that the 2000 astronomy and astrophysics decadal survey11 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. A European Space Agency 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. Characteristics of the Mission Concept as Developed to Date This mission pushes far infrared technology to new limits. No large far infrared space telescope has been operated in space, and no interferometer of the proposed scale has ever been tested. The challenges of beam combination and aperture synthesis, particularly with the current generation of far infrared detectors, are daunting. Several levels of technology development will be needed to bring this mission to maturity, including: 1. Large, cooled telescopes operating in space (these might derive from JWST heritage); 2. Controlled tethered telescopes or formation flying to maintain baseline control; 3. Advanced far infrared array detectors and heterodyne detectors working efficiently at the quantum limit; and 4. Large, rapid, cooled delay lines operating in space. 11 National Research Council, Astronomy and Astrophysics in the New Millennium, National Academy Press, Washington, D.C., 2001. 21

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Program: A Midterm Review.24 A comprehensive study of an ice giant would be a logical mission to follow the detailed explorations of Jupiter by Galileo (mid-1990s) and of Saturn by Cassini-Huygens (ongoing). The committee believes that this mission does offer a significant advance in its scientific field. Characteristics of the Mission Concept as Developed to Date Neptune is 30 times as far from the Sun as Earth, and as such any mission will take many years to reach its target; for example, the most energetically efficient orbit takes 30 years. An EELV-based mission using chemical propulsion both to reach Neptune and to decelerate into orbit provides either too little payload mass or too long a flight time. In order to reduce flight times to an acceptable duration, technology is called upon to slow down the spacecraft on approach to its target. In the proposed non- nuclear version of this mission, solar-electric propulsion is employed to speed the interplanetary transit, and then aerocapture reduces the orbital energy, allowing an elliptical orbit about Neptune. In the alternative version, nuclear-electric propulsion is used to accelerate and then decelerate the spacecraft to allow a conventional powered capture. Both of the proposed Neptune missions, with launch in 2016, perform a Jupiter flyby to arrive at Neptune in 2029. Early on, just before and just after capture of the mother spacecraft into a Neptune orbit, a probe is released for ultimate insertion into Neptune’s atmosphere (one at the equator, and one at high latitude). Four years later the spacecraft will rendezvous with Triton on its retrograde orbit in order to allow deployment of an orbiter or a lander on the satellite. The mission ends in 2033. Since the gravity assist by Jupiter is critical, similar missions can be accomplished only every dozen years, meaning that another opportunity will occur late in the 2020s with the mission’s end happening about 2045. Any Neptune orbiter mission requires strategic investments in power sources, transportation, communication, and sensor technology. In order to have reasonable trip times while carrying a comprehensive payload, some new technology is required, either a significantly powered flight (via nuclear-electric propulsion) or aerocapture. Maneuvering within the Neptune system is accomplished in one case by solely using gravity assists by Triton; a mission carrying nuclear-electric propulsion would be able to use that capability to supplement gravity assists when switching orbits. Of the two missions reviewed by the committee, only the non-nuclear version can fly on a Delta IV heavy launch vehicle, according to the current Delta IV Payload Guide (PPG). Using the performance characteristics given in the 2002 PPG, the proposers concluded that, if a lander were to be deployed, a Delta IV heavy would be inadequate. However, the more recent PPG claims an increased payload capability, and this should be sufficient for all stated mission options. This proposed mission utilizes both solar-electric propulsion and aerocapture, the latter representing a significant development challenge. Currently NASA has no plans to develop or demonstrate aerocapture and has recently determined that this technology will not be developed further. The Neptune mission would carry at least two entry probes. In order to constrain formation models of Neptune, the probes would measure the elemental abundances of He, Ne, Ar, Kr, Xe, C, and S, and the isotopic ratios 15N/14N and D/H. By combining these data with similar available information for Jupiter and possibly Saturn, it should be possible to determine whether the gas giants and the ice giants originated out of the same material in the same general locale. One of the probes would carry a gas chromatograph mass spectrometer (GCMS) that is critical to the mission goals. The probes would study temperature and abundance data from Neptune’s stratosphere to ~100-200 bar. Complementary measurements of winds, structure, composition, cloud-particle size, and lightning would also be obtained. The orbiter would carry the usual broad array of instruments to observe the planet, its satellites, rings, and magnetosphere. These include imaging instruments at ultraviolet, optical, infrared, and radio 24 National Research Council, Grading NASA’s Solar System Exploration Program: A Midterm Review, The National Academies Press, Washington, D.C., 2007. 37

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wavelengths, as well as measurements of the magnetosphere both in situ and remotely. Dust and plasma wave detectors might be carried. Any Neptune orbiter would also have the ability to provide global mapping of Triton’s surface and atmosphere, in order to establish the composition and age of the surface, the inventory of volatiles, the satellite’s current geologic activity, and the atmosphere’s character. A Triton surface lander will enable direct measurement and sampling of the atmosphere and surface, to study surface-atmosphere interactions, and may allow direct seismic probing of Triton’s interior. Relative Technical Feasibility of the Mission Concept The technical maturity of the instruments is high, but the maturity of propulsion options is low. Each version of the basic Neptune mission with probes concept (i.e., aerocapture or nuclear-electric propulsion) has an aspect that would require significant further development. At present, neither is expected to be available in the foreseeable future. General Cost Category into Which This Mission Concept Is Likely to Fall The Cassini-Huygens mission, an ongoing comprehensive exploration of the Saturn system that dropped a single probe into Titan’s atmosphere in early 2005, is an acceptably close analog to this proposed flight. That mission cost $3 billion to $4 billion. Given inflation and their somewhat more complex mission profiles, the proposed Neptune missions likely would cost more than $5 billion in contemporary dollars. Benefits of Using the Constellation System’s Unique Capabilities Relative to Alternative Implementation Approaches The nuclear-electric propulsion mission concept significantly exceeds the capabilities of the largest rocket currently in any nation’s inventory. The Ares V would be able to lift the 36 metric tons identified for this concept. However, the nuclear-electric propulsion required for this mission represents significant technical challenges. In the case of the second mission profile, an Ares V rocket would likely allow such a mission to be flown without the requirement for aerocapture at Neptune. Because there is currently no development program for either aerocapture or nuclear reactors, the chemical propulsion option my provide the most mature option. Should This Concept Be Studied Further as a Constellation-Enabled Science Mission? The Neptune Orbiter with Probes mission concept is worthy of further study as a Constellation- enabled science mission. The Ares V launch vehicle could eliminate the need for aerocapture in Neptune’s atmosphere. The study of an ice giant planet is pivotal to understanding the origins of the solar system. Studying Neptune and Triton is also important for comparative planetary studies, in particular for the study of what drives the activity of both bodies in the frigid outer solar system. Finally, understanding an ice giant would be useful for the study of extrasolar planets. 38

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Titan Explorer25 FIGURE 2.10 Illustration of the deployment sequence for the Titan Explorer airship. SOURCE: Courtesy of Joel S. Levine, NASA. Scientific Objectives of the Mission Concept Knowledge and understanding of Titan, Saturn’s largest moon, have increased significantly as a result of measurements obtained from the Cassini spacecraft following its orbital insertion around Saturn on June 30, 2004, and more recently with measurements from the descent of the Huygens probe through the atmosphere and onto the surface of Titan on January 14, 2005. More than 3 years of analysis of the Huygens data and nearly 4 years of analysis of the Cassini data have dramatically improved understanding of Saturn and its moons, particularly Titan and Enceladus. In the search for life in the solar system, Titan is in a unique position. Its density suggests that it is composed of a mixture of rock and ice in almost equal amounts, and Titan’s atmosphere may reveal answers to questions about chemical evolution on the early Earth. The Titan Explorer mission focuses on the following scientific questions: 1. What is the chemical composition of the atmosphere, including the trace gases? 2. What is the isotopic ratio of the gases in the atmosphere? 25 J.S. Levine, Titan Explorer: The Next Step in the Exploration of a Mysterious World, June 2005; J.S. Levine, “Titan Explorer: The Next Step in the Exploration of a Mysterious World,” presentation to the Committee on Science Opportunities Enabled by NASA’s Constellation System, February 2008. Note: This study should not be confused with the Titan Explorer study conducted for NASA during 2007. However, the two studies had similar science goals. That study can be found at http://www.lpi.usra.edu/opag/Titan_Explorer_Public_Report.pdf. 39

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3. What prebiological chemistry is occurring in the atmosphere/surface of Titan today, and what is its relevance to the origin of life on Earth? 4. What is the nature, origin, and composition of the clouds and haze layers? 5. What is the nature and composition of the surface? 6. Are there oceans of liquid hydrocarbons on the surface of Titan? 7. What is the nature of the meteorology and dynamics of the atmosphere? 8. What processes control the meteorology and circulation of the atmosphere? 9. What is the nature of the hydrocarbon “hydrological cycle” on Titan? 10. What are the rates of escape of atomic and molecular hydrogen from the upper atmosphere of Titan, and what impact does this escape have on atmospheric chemistry? 11. How does the atmosphere of Titan interact with the solar wind and Saturn itself? 12. How have the atmosphere and surface of Titan evolved over its history? 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 Titan Explorer mission (Figure 2.10), with a launch mass of 5,961 kg, would launch on a Delta IV-heavy rocket. The spacecraft consists of a Titan orbiter and a Titan airship that will traverse the atmosphere of Titan and can land on its surface. The airship is designed to have an operational lifetime of 4 months after entry, and the orbiter is designed to have a lifetime of 40 months after arriving in orbit around Titan. To answer the questions posed above, instruments on the Titan orbiter include: a. Solar occultation spectrometer to measure atmospheric composition and isotopic ratios, b. Radar mapper to measure the nature of Titan’s surface, c. Magnetometer to search both for a planetary dipole field and surface magnetism, d. Ultraviolet spectrometer to measure the escape of gases from Titan’s upper atmosphere, and e. Visual and infrared mapping spectrometer to measure cloud and haze layers and the nature of the satellite’s surface. Instruments on the Titan Explorer airship include an imager, mass spectrometer, surface composition spectrometer, haze and cloud particle detector, Sun-seeking spectrometer, and a surface science package similar to that flown on the Huygens mission. Titan’s surface will be explored from an airship, the preferred platform. Success with this technique requires the development of cryogenically capable envelope materials for the airship. This requires technology development. Relative Technical Feasibility of the Mission Concept Approximately 1,057 kg of supercritical xenon is used as the solar-electric propulsion module propellant. There is a question whether sufficient xenon is available. The airship instruments (the imager, the mass spectrometer, and the surface composition spectrometer) and the orbiter instruments (the magnetometer and the radar mapper) all need additional development to result in improved resolution. Electrical power via radioisotope thermoelectric generators (RTGs), an enabling technology, for the orbiter would be a second-generation multi-mission RTG, and for the airship a second-generation Sterling radioisotope generator. Development efforts still are needed. 40

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The spacecraft will be captured into orbit about Titan via aerocapture technology. Aerocapture is one of the key enabling technologies for the Titan Explorer mission. Some of the major features of this mission including solar-electric propulsion, an enhancing technology, and next-generation ion engines, need major attention. General Cost Category in Which This Mission Concept Is Likely to Fall The project as described in the final Vision Mission report, would have a total cost in FY 2008 dollars in the range of $2.8 billion to $3.5 billion. The mission was planned around the use of an existing expendable launch vehicle (Delta IV heavy). Benefits of Using the Constellation System’s Unique Capabilities Relative to Alternative Implementation Approaches The Ares V with a dual-engine Centaur has significant excess capability to enable various mission design options. It has more than enough capability to deliver the Titan Explorer, without the use of Earth gravitational assist or the solar-electric propulsion module, within the proposed 6-year mission timeline outlined in the baseline mission architecture. The excess capacity (nearly 10 metric tons) could be used to carry a conventional propulsion system to reduce or eliminate reliance on aerocapture. Further, the elimination of the solar-electric propulsion module is projected to save ~$500 million, offsetting much of the cost differential between the Delta IV and Ares V launch systems. Alternatively, the basic Ares V (without the Centaur) could potentially eliminate the need for Earth gravity assist. It may also be possible to eliminate or dramatically reduce the reliance on the solar-electric propulsion module. Further study of the various mission design options is warranted. Should This Concept Be Studied Further as a Constellation-Enabled Science Mission? The Titan Explorer mission concept is worthy of further study as a Constellation-enabled science mission. The use of an Ares V launch vehicle could eliminate the requirement for aerocapture. The Titan Explorer will provide a significant improvement in knowledge of the evolution of prebiological chemistry. The committee notes that NASA is already conducting ongoing studies of possible Titan missions, although the most recent one studied utilizes a balloon instead of an airship. 41

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SOLAR SYSTEM EXPLORATION/ASTROBIOLOGY VISION MISSION Palmer Quest26 FIGURE 2.11 Illustration of the Palmer Quest spacecraft elements, including a cryobot powered by a nuclear reactor, and the surface station and rover. SOURCE: Courtesy of NASA, Jet Propulsion Laboratory, California Institute of Technology. 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 2.11). The main science goals of Palmer Quest are to assess 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? 26 F.D. Carsey et al., Palmer Quest, the Search for Life at the Bed of the Mars Polar Cap, July 2005; L. Beegle, “Palmer Quest: A Mission to the Martian Polar Cap,” presentation to the Committee on Science Opportunities Enabled by NASA’s Constellation System, February 2008. 42

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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 will also address several objectives from decadal reports of the planetary community, including understanding 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 includes a thermal drill (the cryobot, powered by a nuclear reactor), a mobile vehicle (the FRAM [Far Ranging Arctic Mission], 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 would also be data recorded of 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 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.27 Characteristics of the Mission Concept as Developed to Date The maturity of this mission 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 this mission, are described below. Relative Technical Feasibility of the Mission Concept This mission 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. 1. The Cryobot, 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 27 National Research Council, New Frontiers in the Solar System: An Integrated Exploration Strategy, The National Academies Press, Washington, D.C., 2003. 43

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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. 2. 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. 3. 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. 4. 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. 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. 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 will 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 dollars. 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 previous launch vehicle, 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 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 utilize 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 44

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survey28 or the recent report An Astrobiology Strategy for the Exploration of Mars.29 In addition, the technical maturity of this mission is low. OVERALL VISION MISSION ASSESSMENT CONCLUSIONS AND RECOMMENDATIONS The mission concepts reviewed by the committee represent a substantial amount of time and effort invested by their study teams. If implemented, they would likely result in significant advances in several fields of space science. The committee hopes that NASA will build on these studies. Finding 3. The following Vision Mission studies might benefit from the opportunities enabled by the Constellation System and are therefore considered more deserving of future study: Generation- X, Modern Universe Space Telescope, Stellar Imager, Interstellar Probe, Solar Polar Imager, Neptune Orbiter with Probes, and Titan Explorer. The committee did not assess the relative scientific priority of the missions within this group. In the final report, these mission concepts will be compared to additional mission concepts (collected in response to the committee’s request for information) that the committee determines to be more deserving of future study, and the committee will produce a consolidated list. Recommendation 1. NASA should conduct further studies of the scientific benefits as well as the technical benefits to mission execution, such as reduction of mission complexity and risk, enabled by the Constellation System for the following missions: Generation-X, Modern Universe Space Telescope, Stellar Imager, Interstellar Probe, Solar Polar Imager, Neptune Orbiter with Probes, and Titan Explorer. The committee did not conduct detailed cost assessments of any of the Vision Missions. Instead, the committee used the cost estimates provided by the mission proposers (not adjusted for inflation), and supplemented by the experience of the committee members and comparisons with similar missions. In most cases, the proposers were basing their cost estimates on an EELV launch vehicle family and not on the Constellation launch vehicles or architecture, particularly Ares V. Finding 4. There are uncertainties in the cost estimates associated with the Vision Missions listed above when flown on the Ares V vehicle. Transitioning of payloads to a new, more capable vehicle would require reconsideration of several items: complexity of instrument packaging and deployment, need for auxiliary propulsion, and so on, all of which would affect mission costs. The committee was informed by NASA that no reliable cost estimates of the Ares V are available. Recommendation 2. NASA should perform cost analysis for the missions that the committee determined could benefit from the Ares V capability (Generation-X, Modern Universe Space Telescope, Stellar Imager, Interstellar Probe, Solar Polar Imager, Neptune Orbiter with Probes, and Titan Explorer). This analysis should use the Ares V technical capabilities together with appropriate upper stages as a baseline. 28 National Research Council, New Frontiers in the Solar System: An Integrated Exploration Strategy, The National Academies Press, Washington, D.C., 2003. 29 National Research Council, An Astrobiology Strategy for the Exploration of Mars, The National Academies Press, Washington, D.C., 2007. 45

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The original NASA solicitation for the Vision Missions did not require respondents to address opportunities for international cooperation. In reviewing many of the individual Vision Missions, however, the committee noted parallel or complementary activities on the part of potential international partners. For example, the Advanced Compton Telescope, which does not require an Ares-V-class launch vehicle, does require a low-inclination orbit such as could be provided by the European Space Agency’s (ESA’s) Ariane V launch vehicle. Similarly, both the Interstellar Probe and the Solar Polar Imager correspond to missions that are under study by the ESA. It is beyond the scope of this interim report to make recommendations regarding international cooperation, but the committee nevertheless considers it important to point out that due to cost, many of these extremely large missions would probably require some degree of international cooperation or collaboration in order to be feasible. The cost, schedule, and performance implications of such international cooperation are not considered in this report. Finding 5. International cooperation could provide access to international scientific expertise and technology useful for these missions, and could reduce costs through provision of foreign instruments and infrastructure. During its review, the committee determined that a major constraint affecting the viability of nearly all of these missions as originally conceived was the lack of availability of certain technologies that currently exist only as concepts. In some cases the technologies, such as advanced sensors, are required for the spacecraft to accomplish its mission. However, the committee noted that several technologies were required for several mission concepts and were essentially “mission enabling.” These include propulsion technologies that might allow an alternative to the use of a heavy-lift launch vehicle like the Ares V and are applicable to multiple missions (for example, aerocapture, which can be used at Venus, Mars, Titan, and Neptune, and solar sails, which can be used for the Solar Polar Imager and Interstellar Probe missions).30 The committee learned during its review that a substantial fraction of the mission concepts would benefit from human assembly and servicing, assuming that this capability is built into the Constellation System. Because of the limited nature of the information presented to the committee during its first meeting in preparation for this report, the committee chose to defer consideration of these technology issues and the potential impact of human servicing and to discuss them in its final report. The committee also learned that some of the missions anticipate upgrades to the Deep Space Network (DSN). The committee expects to gather further data from NASA on the agency’s plans for upgrading the DSN and will address these missions’ interactions with and expectations for the DSN more fully in the final report. The overall technology requirements are summarized in Table 2.2. Finding 6. The committee identified the following technology issues as meriting further attention. Some of these technologies are of a basic, mission-enabling nature; others provide options that can be traded for alternative mission architectures. • Basic enabling technologies ⎯Free-flying constellations ⎯Tethered flight ⎯Next-generation Deep Space Network ⎯Space nuclear reactors 30 The committee did note that the principal investigators of the outer planet missions and the Interstellar Probe mission assume the availability of nuclear (Pu-238) power supplies to be able to execute these missions. No new technology other than that already in development by NASA is considered necessary for these missions, although there is insufficient plutonium currently available to fly these missions. The committee understands that a separate NRC study will address the issue of further plutonium sources for future deep-space missions. 46

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• Technologies enabling alternatives to Ares V ⎯Aerocapture ⎯Solar sails ⎯Solar-electric propulsion ⎯Nuclear-electric propulsion ⎯Robotic assembly and servicing • Technologies enhancing Constellation capabilities ⎯Human assembly and servicing TABLE 2.2 Correlation of Technology Needs with the Vision Missions Analyzed by the Committee Technologies Enhancing Constellation Basic Enabling Technologies Technologies Enabling Alternatives to Ares V Capabilities Space Solar/ Robotic Nuclear Nuclear Assembly Human Next Reactors, Electric and Assembly Free-flying Tethered Gen Solar Mission NEP Propulsion Servicing and Servicing DSN Aerocapture Sails Constellations Flight Advanced Compton Telescope Generation-X √ √ Interstellar Probe √ √ √ √ √a Kilometer- √ √ Baseline Far- Infrared/ Submillimeter Interferometer LUVO-MUST √ √ Neptune Orbiter √ √ √ √ with Probes √a Palmer Quest √ Mars Mission SAFIR √ √b Solar Polar √ √ √ Imager Stellar Imager √ √a Titan Explorer √ √ NOTE: Basic enabling technologies are those technologies that exist in concept at present but whose development is needed for some of the missions. Technologies enabling alternatives to Ares are those whose availability would provide options for the missions currently conceived as being executed without requiring an Ares V. Human assembly and servicing would enhance the capabilities of these missions. Some of the missions, like the Advanced Compton Telescope, would not directly benefit from these technology developments. DSN, Deep Space Network; LUVO, Large UV/Optical Telescope; MUST, Modern Universe Space Telescope; NEP, nuclear-electric propulsion; SAFIR, Single Aperture Far Infrared Observatory. a Significant mission enhancement. b Only if solar sail is implemented. 47