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s Practical Considerations The task group believes that the program outlined above is visionary in its scientific implications, and yet remains feasible. There are, of course, a multitude of budgetary, management, op- erational, and technological issues that will have to be resolved in order to execute this program. Below, the task group has at- tempted to assess the most important of these issues using the projected program of Chapter 4 as a base. BUDGETARY REQUIREMENTS The task group has attempted to estimate the financial re- sources necessary to accomplish this program. The basis for its estimate is straightforward: . The task group assumes that the Support Research and Technology (SR&T) and Explorer-type programs will be main- tained at about the current level (about $100 million per year). Every effort should be made to increase these budgets because the future initiatives proposed in this report may depend critically on developing new technologies and approaches to conducting exper- iments in space. . In addition, the task group takes the cost of developing, maintaining, servicing, replacing, and upgrading the current or 67

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68 planned observatory-level facilities to correspond to the recom- mendations made by the NRC's Astronomy Survey Committee in 1982 at current budgetary estimates. . For each major wavelength regime, the task group assumes that one and only one observatory-cIass facility will operate at a time. . The task group has assumed that in each wavelength band a new facility will be developed soon enough to prevent the oc- currence of substantial gaps in observational capabilities due to facility deterioration or obsolescence. . In all cases the task group assumed that the old facility is replaced by a new one at about twice the cost of the facility that is replaced. . The cost of new initiatives such as optical interferometry and radio interferometry has been estimated to be comparable with that of current observatory-cIass missions. The task group considers its estimates to be on the high rather than low side if more of the expected efficiencies in development and operation should materialize. The task group concludes that the base program (Astronomy Survey Committee recommendations and replacements plus SR&T and Explorers) could be carried out with a modest annual real- dolIar increase of the NASA fiscal year 1985 astrophysics budget at 2.3 percent per year. By 2015 this budget would be $1 billion per year. Such a program would mainly utilize the Space Station capabilities for service and maintenance of co-orbiting platforms and free flyers. A further increase in the rate of growth of the current fiscal year 1985 NASA astrophysics budget at 3.7 percent per year would be required to pursue vigorously the new opportunities afforded by the Space Station for in-orbit assembly and deployment of large structures. The budget for astrophysics would then be of the order of $1.5 billion per year by 2015. INTERNATIONAL COLLABORATION International collaboration can occur on at least two levels. On one level scientists from other nations may collaborate as part of a mission team. Alternatively, they might compete by providing their own proposals for space on a U.S. satellite. This

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69 level of collaboration has provided the United States with an important source of new ideas and challenges. It has also provided the United States with some leverage to influence the development of international space science. At a different level, a joint effort can be envisaged as has oc- curred in the past-where the flight hardware of a whole program or mission is produced as the result of an international effort. Past experience shows that such undertakings are successful only if a number of prerequisites are met: . The parties must be competitive in order to be of interest to each other. assured. Joint planning must occur at an early stage. Clear goals and managerial procedures must be defined. Continuing support of the project by the parties must be The United States now has competitive partners, mainly in Japan and Europe. Further collaboration on major projects will require joint planning. Collaborating nations will have to become accustomed to the idea that leadership on any one project can rest with any one of the parties. The task group envisages a system where missions are managed by one participant, and yet scientific instrumentation and exploitation are open to all. COST-TO-WEIGHT RATIO The cost of a space mission seems to be directly proportional to its weight. Since the facilities being considered for future research are extremely large and heavy, their costs will be high unless we can develop ways to reduce the cost-to-weight ratio across the board in space research. Further, the mass of future instruments could be reduced through the use of sensors and active control systems for aligning large structures to optical tolerances. MANAGEMENT AND OPERATIONS The future research facilities that we are planning are ex- pected to be long-lived, refurbishable while in orbit, and available to the whole astronomical community. The way that these facil- ities are operated and maintained is of crucial importance. We

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70 must study and gain improved understanding of different man- agement approaches (e.g., principal investigators, guest observers, institutes), the costs of operating for Tong durations, scientific strategies for instrument replacement, and usable lifetimes for ob- servatories within some budget constraints. COORDINATED FACILITIES Real advantages could accrue from the ability of several obser- vatory-cIass instruments to share support and refurbishment ca- pability. The advantages and disadvantages of a variety of such arrangements should be studied. SCIENTIFIC INSTRUMENTS TECHNOLOGY Interferometry The need for higher angular resolution, with a microarcsecond as a target, will require examination of the present and projected state of interferometry techniques from radio wavelengths to the ultraviolet. Technical needs of interferometry can be identified in three areas: structural technology, optical technology, and station keeping. Among the subjects needing study in the area of struc- tural technology are the construction, measurement, and control of large precision structures; the precision of control of pointing and momentum exchange; vibration minimization and decoupling; and the general area of metrology for high-precision monitoring of the structures. Optical technology is a central concern, in- cluding the development of active systems, sensors, fiber optics (especially single mode), and the study of image reconstruction. Finally, station-keeping technology needs further study, includ- ing precision position and altitude control, quiet thrusters, orbital analysis, and studies of improved contamination control. Finally, a study is needed of methods of reconstructing images and the closely related question of fringe detection, both as they relate to detectors per se and as they relate to algorithms for tracking - rlnges. A sequence of missions needs to be defined to lead to the achievement of the above goals for resolution. Most likely the early progression will emphasize small instruments. These could

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71 include a quasi-imaging device with 1- to 10-milliarcsecond resolu- tion and incomplete ultraviolet plane coverage. Eventually, larger instruments with higher collecting areas, greater resolution, and complete ultraviolet plane coverage will be possible, based on the experience with smaller instruments. Detectors and Techniques A variety of new detectors and techniques for possible use in cosmic x-ray astronomy and astrophysics are likely to be devel- oped over the next 10 to 15 years and would thus be of interest for the new initiatives described in Chapter 4 in the sections on the Very High Throughput Facility and Hard X-ray Imaging Fa- cility. A particularly exciting development is the x-ray bolometer detector/spectrometer, now being developed as a possible focal- plane instrument for AXAF. This shouicT allow energy resolution of a few electron volts over a broad bandwidth (0.1 to 10 keV) with nearly unit quantum efficiency by making use of cooled (to helium-3 temperatures) silicon or germanium bolometers in which the thermal energy associated with absorption of individual x-ray photons is detected. Small detectors, with limited imaging arrays, are a likely next step for follow-on facilities such as the VHTF. New detector systems with modest energy resolution but very high spatial resolution will also likely become available for both the soft and hard x-ray bands. New techniques for x-ray imaging are being developed for both soft and hard x rays. Two approaches for achieving the very large collection areas required for a VHTF system are being de- veloped for evaluation on Shuttle flights of short duration. One employs thin foils to make highly nested arrays of telescope mir- rors. The other consists of relatively compact arrays of curved glass plates to be assembled in modular arrays of telescopes. At energies below about ~ keV, narrow-bandwidth imaging telescopes operating at nearly normal incidence are being developed for solar applications; scared-up versions of these could be used for high- sensitivity spectral line imaging of cosmic x-ray sources. At hard x-ray energies (above 10 to 30 keV, where grazing-incidence op- tics become impractical), cocied-aperture and Fourier transform imaging techniques are being developed on balloon and Shuttle experiments, and could be applied to the HXIF. A longer-range program to develop direct imaging using Bragg concentrators at

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72 energies up to about 100 keV should also be encouraged. Finally, the development of x-ray lasers is now being pursued for nonastro- nomical purposes. If successful, the applications to future x-ray missions are very promising, from heterodyne interferometry to high-resolution spectroscopy.