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A Scientific Assessment of a New Technology Orbital Telescope 1 Introduction For decades astronomers have known about the advantages of locating astronomical telescopes in space. From this vantage point it is possible to view the full span of the electromagnetic spectrum. Images of astronomical bodies are not distorted by passage through Earth’s turbulent atmosphere, and, in principle, observations are not limited by vagaries of the weather or the diurnal cycle. The Hubble Space Telescope (HST) has vindicated many of these claims, but only at a very high cost. In the 15 years since the HST was built, enormous advances have been realized in the design, manufacturing, and testing of optical systems. These new technologies allow us to make realistic plans for the next-generation of space telescopes —spacecraft with optical systems that are larger and lighter, yet much cheaper, than the HST’s. New techniques for figuring and testing allow rapid and inexpensive polishing of large mirrors to accuracies rivaling or exceeding the HST’s. The feasibility of using wavefront sensors and active-optics systems to control the figure of mirror surfaces has been proven by many groups. The University of Arizona and the European Southern Observatory, for example, have demonstrated that 4-meter-class mirrors, equipped with multiple actuators, can achieve wavefront errors of better than 53 nm rms (admittedly with substantially more rigid mirrors than those discussed in this report). A necessary component of such systems, the optical sensors used to construct wavefront maps from star measurements, has also been developed and proven by researchers on both sides of the Atlantic Ocean. To be significantly less costly than the HST, a second-generation space telescope not only must employ many of the technologies outlined above, but also must utilize new operational concepts to reduce operating expenses. The use of a very thin, actively supported primary mirror, for example, can significantly reduce spacecraft mass. Similarly, placing the telescope in a high Earth orbit can significantly increase observing efficiency and greatly reduce demanding operational constraints relative to those of a similar instrument in a low Earth orbit, such as the HST. Each of these new concepts, however, carries a significant penalty in terms of increased risk. And, neither NASA nor the astronomical community can afford another expensive and highly visible failure such as the HST’s spherical aberration. Before it rushes to embrace these new technologies, the astronomical community must have some confidence that mission-critical elements will work as advertised. This report discusses the astronomical potential of a military project, which, it is claimed by its proponents, will provide astronomers with this technology validation at a relatively low cost. Although this so-called Advanced Technology Demonstrator (ATD) mission is motivated by future national
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A Scientific Assessment of a New Technology Orbital Telescope security requirements, the task group notes that many of its components and the technologies they embody are similar to those required for an advanced astronomical observatory in space. Thus the ATD hardware may potentially have a role to play as a New Technology Orbital Telescope (NTOT) in fields such as cosmology, astrophysics, and planetary astronomy. Given the dual nature of the mission, the task group henceforth refers to a joint military/astronomy mission exploiting the ATD hardware as the ATD/NTOT. A purely military mission is referred to as the ATD. BMDO’S ADVANCED TECHNOLOGY DEMONSTRATOR With funding from the Strategic Defense Initiative Organization (SDIO), Itek Optical Systems has applied many of the techniques relevant to future space telescopes to create a fully active 4-meter mirror system with a very thin facesheet and lightweight carbon-fiber support structure. It has been proposed that Itek’s optics and other hardware be test flown as part of the ATD program sponsored by the Ballistic Missile Defense Organization (BMDO). The basic philosophy adopted for the as yet unfunded ATD program is not to design hardware to meet requirements set by specific operational (or scientific) objectives. Rather, the ATD is a design-to-cost program in which the specific tasks to be accomplished are designed around the capabilities of existing or minimally modified hardware. Since these components are key to any potential scientific utility, the bulk of this report is devoted to assessing the ability of this hardware to perform a variety of priority astronomical observations. As presented to the Task Group on BMDO New Technology Orbital Observatory by representatives of BMDO, Lockheed, and Itek, DOD’s technological goals for the ATD program include the following:1 Complete resolution of the ability to track and point a laser beam at a missile in the boosting phase in a space environment from the correct orbit geometries and ranges for a space-based laser; Track and gather data on reentry vehicles and decoys at long ranges; Gather imagery in the visible and near-infrared of space objects at geosynchronous orbits; Gather Earth background data at high resolution on the water and carbon dioxide bands in the short- and medium-wave infrared as a data base for future system design; Gather data on the observables of a missile in the boost phase; and Demonstrate the ability to designate targets on the ground from a space platform for the use of laser-guided weapons in a tactical scenario. The task group emphasizes that it has neither the charge nor the expertise to assess the utility, realism, or feasibility of these technology goals. The principal components that must be assembled to undertake such tests are the following: A laser; A telescope; A sensor package to aim at, to track, and to observe targets; An image stabilization system to enable tracking of targets; A spacecraft bus to provide housekeeping functions such as power and communications; and A launch system to place the whole package into an appropriate orbit. The Telescope Perhaps the most important element of the ATD is its telescope. Since 1975, the federal government has invested more than $100 million to fund a series of research and development programs at Itek aimed at the design, production, and testing of large, active-optics technology (see Box 1.1, “Optical Heritage,” for more details). Although the ATD is designed to collimate and direct a high-energy laser beam, the changing world situation now means that it is possible to contemplate nonmilitary applications for the large, lightweight, segmented mirrors developed by these programs.
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A Scientific Assessment of a New Technology Orbital Telescope Box 1.1 Optical Heritage The optical components proposed for use in the ATD/NTOT draw on the heritage of a number of projects, including the Large Advanced Mirror Program of the 1980s and the Adaptive Large Optics Technologies and Large Optical Segment programs of the early 1990s. Large Advanced Mirror Program The Large Advanced Mirror Program (LAMP) was a ground demonstration of a 4-meter, actively controlled, segmented mirror. Each of its seven, 2-meter, quasi-hexagonal segments was approximately 17 mm thick and attached to surface control actuators mounted in a stiff, lightweight graphite-epoxy back-up structure. A total of 312 surface control and 42 segment position actuators enabled the LAMP mirror to be controlled so as to yield a wavefront accuracy of better than 100 nm rms. The LAMP mirror was subsequently installed as the beam expander for a hydrogen fluoride laser at the San Juan Capistrano test range in California. Adaptive Large Optics Technologies The Adaptive Large Optics Technologies (ALOT) project—a 4-meter, lightweight, segmented-mirror telescope—was designed and built by Itek to test the technology for autonomous capture, phasing, and figure control of a fully adaptive primary mirror by sensing an extended scene in real time. The telescope consists of a central 2.6-meter annular mirror, with 144 actuators, surrounded by a number of 0.7 × 2.1-meter radial mirror segments (only one, however, was built) each equipped with 43 actuators (see Figure 1.1). Testing of ALOT in Itek’s thermal-vacuum chamber revealed that the active control system could yield a residual wavefront error of 70 nm rms in the presence of active disturbances to the beam. All of ALOT’s support structures are made of graphite-epoxy composite, including the tripod supporting the secondary mirror, the Cassegrain baffle, and the reaction structure supporting the segmented primary mirror. All components, except the control electronics, are space qualifiable and compatible with launch by a Titan IV booster. The principal modifications required for space qualification of the existing ALOT hardware would be upgrading the electronics, replacing the control computer with one that is space-qualified, modifying the software, and adding thermal insulation and an external shroud. The ATD/NTOT would utilize (or copy) much of the existing ALOT hardware with the exception of its segmented mirror. Large Optical Segment The Large Optical Segment (LOS) project is currently in progress. Its goal is to use the technology developed for LAMP to build two of the thirteen 4-meter segments required for an 11-meter mirror. Unlike the quasi-hexagonal structure of LAMP’s primary, LOS’s will resemble ALOT ’s in that it will consist of a central annular mirror surrounded by a number of radial segments. Each of the segments is ground from a 4-meter blank of Corning ULE glass. The ATD/NTOT would substitute LOS’s 4-meter monolithic central segment (or a copy) for ALOT’s segmented mirror, thus greatly reducing the number of actuators needed to control the figure of the ATD/NTOT’s primary mirror. Sensor Package The same military desires that spawned research into the means to control the targeting of space lasers also inspired research into the design of the instrumentation necessary to assess potential targets and aim weapons. One such activity was the StarLITE Special Studies Task, undertaken by Lockheed,2 which resulted in the design of a low-cost sensor suite, containing a variety of visible and infrared focal-plane arrays, that could acquire, track, and point a simulated laser weapon at a boosting target. Image Stabilization Tracking a boosting rocket at a distance of many hundreds to thousands of kilometers requires that the beam expander maintain a very precise line-of-sight stability. This stability may be achieved by the inclusion of an appropriate on-board inertial reference in the pointing and tracking system. An example of such a device is the Inertial Pseudo Stellar Reference Unit (IPSRU) developed at the Charles Stark Draper Laboratories.
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A Scientific Assessment of a New Technology Orbital Telescope FIGURE 1.1 The 4-meter Advanced Large Optics Technologies (ALOT) telescope is shown in a testing chamber at Itek Optical Systems. The proposed ATD/NTOT project would reuse or replicate much of this hardware, with the exception of ALOT’s 4-meter segmented mirror, which would be replaced by a monolithic, meniscus mirror of equal size similar to the central, annular segment of the Large Optical Segment project. (Courtesy of Itek.) Spacecraft Bus Custom-designing satellites to meet the requirements of each particular payload is inherently expensive. Commercial opportunities, such as meeting the demand for the many tens or hundreds of satellites required by, for example, global cellular telecommunications systems such as Iridium and Teledesic, have prompted many aerospace concerns to develop generic buses capable of supporting a variety of payloads. Lockheed’s F-SAT (Frugal satellite), for example, can provide power, attitude control, computing, communications, and propulsion for payloads in the 1- to 5-ton class. Launch System The decline in superpower rivalry that has made once-classified defense technology available for civilian use has also opened other possibilities. Among these is access to Russia’s stable of large, reliable, and relatively cheap booster rockets. Indeed, access to the highly capable Proton launcher is now marketed commercially by a consortium including Lockheed and several Russian enterprises. Two launches have been booked to date, with options taken out for an additional 13. The first flight is scheduled for March 1996.
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A Scientific Assessment of a New Technology Orbital Telescope The proposed costs for the major components of the ATD, together with their construction schedules, are summarized in Box 1.2. POTENTIAL VALUE OF THE ATD/NTOT TO SPACE ASTRONOMY At first glance, the ATD/NTOT promises to be an evolutionary step in advancing technology in exactly the directions needed to eventually undertake some of the most challenging missions in space astronomy. One of the most fundamental questions is whether life exists elsewhere in the universe. An infrared space telescope capable of detecting Earth-like planets around nearby stars requires new technology such as that to be demonstrated by the proposed ATD/NTOT mission. One possible design for an instrument to detect extrasolar planets would use two cooled mirrors, configured as a Bracewell interferometer to cancel the light of the star. Once a planet is detected, the same telescope could search the infrared spectrum of the planet’s atmosphere for the presence of water, carbon dioxide, and ozone. Ozone at the same high level as seen in Earth’s spectrum would be a strong indicator of life, since ozone is formed from oxygen that is created and maintained only by living organisms. Other fundamental observations, such as the imaging of galaxies from their very beginnings early in the life of the universe, also require large cooled telescopes. The High-Z proposal, for example, developed in response to a 1994 request from NASA for new mission concepts in astrophysics, would attack this problem by deploying an instrument like the ATD/NTOT in a heliocentric orbit ranging from 1 to 3 AU. 3 The natural background at a wavelength of 3 microns is, from this vantage, the darkest of any part of the optical/infrared spectrum. Thus, High-Z could observe the distant universe, as it was near the beginning of time, with greater sensitivity than any other telescope. A somewhat similar concept, the Polar Stratospheric Telescope, envisages deploying a telescope similar to the ATD/NTOT beneath a high-altitude aerostat tethered in Alaska.4 The ATD/NTOT technology is Box 1.2 ATD Costs and Schedule The proposed costs of major elements of the ATD 4-meter space telescope, together with a summary of its construction schedule and the facilities to be used in its testing, are based on information supplied by Lockheed and Itek. Costs of modifications necessary for an astronomy mission are not included. Although the task group did not perform any detailed analyses to verify this information, nothing quoted below seems unreasonable. Costs 4-meter telescope from Itek ~$ 50 M Sensor package from Lockheed ~$ 25 M F-SAT bus ~$ 50 M Payload integration ~$ 40 M Space vehicle integration ~$ 20 M Proton launch vehicle ~$ 60 M Space operations and data reduction ~$ 25 M Cost ~$270 M Management reserve ~$ 80 M Realistic costs ~$350 M Schedule Itek can build 4-meter telescope in 2 years F-SAT bus available in 2 years Sensor suite also takes 2 years Modular approach allows for 12-month integration and testing Available Test Facilities Itek’s test chamber for telescope and sensor suite Lockheed’s acoustic and thermal vacuum chambers for vehicle test
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A Scientific Assessment of a New Technology Orbital Telescope even being considered for moderate-sized, ground-based telescopes, although the problems implicit in this application are quite different from those found in space applications, for example, the reasonably high-frequency response necessary to maintain the shape of the primary mirror in the presence of wind buffeting. TASK GROUP’S EXPERTISE, PHILOSOPHY, AND APPROACH Because it includes people with considerable expertise in the design, construction, and testing of astronomical optics, the task group can make at least reasonable estimates of the likelihood that the ATD/NTOT will achieve the optical performance advertised by its designers. Given this fact and the task group’s charge most of this report is devoted to an analysis of the optical performance and potential of the ATD/NTOT. This focus is most appropriate because the optical components are the key features that allow the ATD/NTOT to be considered as a potentially viable astronomical project. On the other hand, this task group has little or no experience in areas such as spacecraft engineering and structural analysis. Thus, it claims no expertise in judging the likelihood of success in these areas, even though such success is clearly crucial to providing a facility that is astronomically useful for the long term. Yet Lockheed has an extensive and successful record in these areas. The task group has therefore relied, for the most part, on the past performance achieved by Itek and Lockheed in producing many components of the system and on its own estimates of the compounding of various sources of errors and the extent to which they may affect the astronomical performance. Operating Philosophy The basic purpose of the ATD is to demonstrate technologies that may be used in future systems built by DOD. This approach has the advantage that critical components can be tested and proven in a realistic environment before any commitment is made to the development or deployment of inherently expensive systems. In this spirit, the task group adopted as its basic philosophy that any potential involvement of the astronomical community in the ATD/NTOT should, at least initially, be predicated on the assumption that the ATD/NTOT is primarily a test of new technology for astronomy and is not a mission driven by any particular scientific requirements. In this light, the greatest value of the ATD/NTOT for astronomy will be to show whether or not it is possible to break the Hubble paradigm, that is, to demonstrate that it is possible to get large space optics at low cost. While doing so, it may also be able to contribute to major astronomical results. Attention to Possible Mission Enhancements Although the task group’s approach is to evaluate what important scientific observations and tests can be performed during the ATD/NTOT’s baseline mission (as it understands that mission), it notes that there is significant uncertainty as to the precise nature of the baseline mission. Discussions between the task group and project officials have already led to changes that either enhance or do not interfere with the baseline mission and do not have a significant cost impact. Accordingly, the task group also offers its thoughts on possible scientific enhancements to the mission, where appropriate. As the mission’s goals evolve, some of these enhancements may add significantly to the value of the ATD/NTOT for all potential participants. Indeed, the principal purpose of involving astronomers early in the planning for the ATD/NTOT was to ensure that these discussions would be initiated and would continue. Content of This Report In the following chapters the task group examines the ATD/NTOT concept in more detail to determine if the ATD/NTOT is capable of performing useful astronomical observations. More importantly, it investigates whether the ATD/NTOT is an effective stepping-stone to the next generation of space telescopes.
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A Scientific Assessment of a New Technology Orbital Telescope REFERENCES 1. An Advanced Technology Demonstrator for a 4-Meter Space Experiment, LMSC F439223, Lockheed Missiles & Space Company, Sunnyvale, Calif., 1993. 2. ADAPT: StarLITE Special Studies Task Final Report, LMSC P005092, Lockheed Missiles & Space Company, Sunnyvale, Calif., 1993. 3. Stockman, Hervey (Peter) S., High-Z: A Near-IR Space Telescope for Probing the Early Universe, Space Telescope Science Institute, Baltimore, Md., 1994. 4. Ford, H., Bely, P., Bally, J., Crocker, J., Dopita, M., Tilley, J., White, R., Allen, R., Bartko, F., Brown, R., Burg, R., Burrows, C., Clampin, M., Harper, D., Illingworth, G., McCray, R., Meyer, S., Mold, J., and Norman, C., “POST: A Polar Stratospheric Telescope,” SPIE, Vol. 2199, Advanced Technology Optical Telescopes V, p. 298, 1994. (Note, this reference describes an early version of the POST concept.)
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