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Considerations for an Effective Program 34 HD 36402 HD 5980 v,.. , .... ..., v,.. , .... ..., · 100 0 100 400 · 100 0 100 400 JUE: High resolution ultraviolet absorption line spectra of hot stars i n the Large Magellanic Cloud (LMC) and in the Small Magellanic Cloud (SMC) obtained by IUE provided the first direct evidence Cor the existence of a hot gaseous corona or halo sur rounding the Milky Way galaxy. In the two IUE spectra shown above of selected inter stellar lines , absorption near 0 km/s is associated with the Milky Way (both figures), also near 270 km/s with the LMC (left figure) and near 150 km/s with the SMC (right figures. Absorption in the Milky Way halo by such ions as Si IV, C IV and N V figure). H I 21-cm emission profiles Cor each line of sight are shown at the top of the implies the presence of gas with temperatures in the range of 80,000 to 200,000 K. This gas has been found to extend away from the galactic plane with a scale height of about 3 ,000 pc .
Considerations for an Effective Program 35 1. Instrument Complexity The complexity of the experiments carried aboard Explorer spacecraft has steadily increased over the years. While much of this increase is attributable to the maturing of the various disciplines within astrophysics and space science, some is directly attributable to the infrequency of opportunities for space flight. When flight opportunities in a field occur only once per decade, there is strong motivation to make the com plement of instruments sophisticated enough to address a large fraction of the current problems in that area. The costs associ ated with this complexity decrease the number of initiatives that can be accommodated within the Explorer budget and further reduce the frequency of flight opportunities. A commit ment to frequent flight opportunities should be made, and the scope of each Explorer project precisely defined to address a limited range of important scientific questions. Another aspect of the interaction between delays and experiment complexity is apparent in cases where an approved Explorer is delayed so long that the proposed measurement falls behind the scientific state of the art. As delays occur, instru ment designs are upgraded to avoid obsolescence, and costs rise. To alleviate this problem, it is essential that the time between instrument selection for an Explorer mission and actual flight be held to the minimum required to construct the instrument and spacecraft. The problem of the cost growth associated with the increasing sophistication of forefront experiments in a maturing field of space science is a complex one, requiring accommoda tion of projects of different sizes within the overall NASA pro gram. Certainly various flight opportunities can be identified, many of them associated with the Space Shuttle and poten tially the Space Station. An issue of direct significance for this report, the distribution of Explorer Program resources between missions of significantly different sizes must be addressed. Within the criteria of scientific excellence, we recommend that a variety of experiment sizes should be accommodated while keeping the desired flight frequency of one Astronomy and Astrophysics Explorer opportunity per year.
Considerations for an Effective Program 36 2. Spacecraft Cost The cost of the spacecraft, as well as the integration and testing, represents the major fraction (approximately 50 per cent) of many recent Explorer missions. In astronomy and astrophysics the detailed study of objects frequently requires images or spectra taken with a three-axis stabilized instrument. The pointing accuracy and stability that are necessary increase the cost of a spacecraft over a simple spinning device. In other cases, observations require cooled detectors and telescopes. Cooling increases the demand on weight and power require ments of a spacecraft which in turn leads to a more costly con figuration than an uncooled system. It appears possible to build a small dedicated platform that could meet a set of mission requirements for scientific instruments on pointed spacecraft. For instance, the Multiple Mission Spacecraft (11M:S) is one such configuration. We are particularly impressed with plans to reuse the MMS as a plat form for EUVE and XTE. Other concepts similar to this exist too: Proteus within NASA, and Eureca within ESA. This opportunity may offer necessary relief to the Explorer Program and allow an accelerated start for some Explorer concepts. We encourage progress in this area for those scientific missions that can be accommodated. The use of reproducible "standard" spacecraft or plat forms, e.g. , the MMS, derives from the same philosophy. The sounding rocket program was an outstanding example wherein a few designs accommodated a wide range of users. The existence of duplicate support systems made possible rapid tur naround of experiments. We suggest that this could also reduce costs and enhance flight opportunities. 3 . Project Engineering and Management For the Explorer Program to provide frequent flight opportunities for projects of limited scope, the criterion for suc cess must be perceived as the accomplishment of a well defined, rather limited set of scientific objectives.
Considerations for an Effective Program 37 MMS: The MMS spacecraft now supporting the Solar Maximum Mission shown schematically with a replacement payload here depicted as the Extreme Ultraviolet Explorer. Basing in space of re-usable spacecraft such as this MMS, with the integra tion of new payloads to the spacecraft while in orbit, offers potential for a substantial savings in cost for future Explorer experiments consistent with the accessible orbits - such as EUVE (Extreme Ultraviolet Explorer) and XTE (X-ray Timing Explorer). There are a number of specific engineering and manage ment approaches that can be used to control costs. Of prime importance is detailed definition early in the project of all instrument parameters and requirements that affect the space craft design, and rather rigid adherence to these constraints throughout the project. In addition, adequate contingency
Considerations for an Effective Program 38 should be established for resources such as power, weight, memory, and data rate so that the normal growth that inevit ably occurs in the instrument requirements for these resources can be made without major imp act on costs and schedule. The use of previously designed sp acecraft, where possible, can significantly reduce costs, but re quires close cooperation between the project manager and the Principal Investigator(s ) at an early stage to ensure that the experimental re quirements can be accommodated. Multiple-use spacecraft designs could prove valuable to the Explorer Program. However, individual Explorer projects should not be delayed or made more costly to accommodate specific spacecraft hardware. Explorer costs are also driven by extreme or changing experiment re quirements. For example, fine pointing re quire ments should be carefully evaluated to avoid imposing unneces sarily stringent demands on the spacecraft design. The design and prototyping of scientific instruments to be flown should be fairly advanced at the start of an Explorer project to assure that major changes in the demands on the spacecraft will not occur as the instrument development progresses. To begin Explorer projects with well advanced instrument designs requires a significant, ongoing commitment to laboratory research into space instrumentation, and to suborbital (aircraft, balloon, sounding rocket ) flights of new instruments. The Principal Investigator of an Explorer mission ulti mately carries the responsibility to achieve the scientific goals of the experiment. Management structure must allow for clear authority of the Principal Investigator. Particular care should be taken not to overburden the project with management over head re quirements that inexorably increase costs and technical complexity. The allocation of management responsibilities among the NASA centers should establish simple, well defined lines of authority, with one center providing the prime management function, and teams re quired at other centers reporting directly to that management, not independently to NASA Headquar ters.
Considerations for an Effective Program 39 4. Interface with Other NASA Programs The Explorer Program has a strong record of emphasis on maximizing scientific accomplishments while subordinating other objectives to this primary goal. Requirements that result from the interfacing of the Explorer Program with other NASA projects can significantly increase Explorer costs. For example, the requirement that Explorers be launched from the Space Shuttle imposes significant safety regulations, which are costly to meet. Another example is the requirement that Explorers use the Tracking and Data Relay Satellite System (TDRSS) for communications, when in some cases this is a more costly alter native than direct communication with the NASA tracking net work on the ground. Requirements such as these should be imposed only when they are more cost effective to an Explorer mission than alternative approaches such as a launch with expendable rockets or direct communication with the ground stations. EXPERIMENTS IN COST REDUCTION Current mechanisms for developing instruments may be unduly costly. In part, this may derive from the natural ten dency to try to provide the maximum science capability on each mission in an era of few flight opportunities. The unwil lingness to accept some risk of failure, related in turn, again, to scarcity of opportunity, also plays a role. We would urge NASA to study whether current management, engineering, and acquisition practices provide the highest likelihood of scientific return per dollar invested. In particular, a common, reusable spacecraft could be a cost-effective route to deriving more sci ence from the budget for some Explorer missions. FLEXIBLE MISSION LIFETIMES The manyfold objectives of Explorer missions suggest that a variety of mission durations should be accommodated. In
Considerations for an Effective Program 40 particular, the third-component (specific studies) missions can often be expected to produce rich science for periods signifi cantly longer than the nominal 1 - to 2-year Explorer lifetime. At most wavelength bands, the number and types of astronom ical objects amenable to productive study are usually large, even with a given instrument. These "specific-study" Explorer instruments can have the continuing productivity for years that we expect of modest ground-based telescopes. We note the continued productivity of IUE over a period of 8 years and the high demand for observing time from HEA0-2 and EXO SAT at the end of their ,...... 2-year lifetimes. The extension of certain missions will surely raise costs. However, we know that missions designed for a 2-year lifetime (to avoid increasing the cost) will often operate substantially longer, e. g. 3.9 years for SAS-3. It is important, though, to avoid the inclusion of features that would prematurely limit, a priori, the lifetime. The growing power of small computers and modern networking could make the continued operation of an older and well understood Explorer a relatively small-scale and low-budget process. Thus the comparatively large scientific gain may well offset the modest cost increases in some cases. The vision of several concurrently active U.S. Explorers operat ing at different wavelength bands in conjunction with each other and with the larger "permanent" observatories, is an attractive one indeed. SELECTION PROCEDURES NASA has issued a Dear Colleague Letter dated March 14, 1 986, entitled "Explorer Concept Study Program" to solicit concepts for future scientific missions. This Letter reflects the procedures recommended in A Strategy for the Explorer Pro gram for Solar and Space Physics (National Academy Press 1 984). We endorse the philosophy and the procedures embodied in the Dear Colleague Letter. A large number of important mission concepts in astrophysics will undoubtedly result, including missions identified in the Astronomy Survey J
Considerations for an Effective Program 41 Committee report as well as recently emerging concepts. This new strategy for Explorer selections would enhance scientific accomplishments with the following additional con siderations: ⢠Selection of miSsion concepts should take place fre quently and over-selection should be stringently avoided. ⢠A second level of selection for Phase B studies, as described in the CSSP Report, should draw from the pool of concepts that have been studied during Phase A. This pool of concepts need not be restricted to those selected in previous preliminary rounds, if other missions with sufficient definition are available for consideration. ⢠The new selection procedures should maintain the present flexibility of the Explorer Program in facilitating inter national cooperation. In the past, a small amount ( about 5 percent) of the annual Explorer budget has been used for discretionary / target of-opportunity efforts. We support continuation of this pro cedure at a similar level of expenditure. Target-of-opportunity efforts are a worthwhile component of the Explorer Program that give valuable scientific return and underscores its flexibil ity. We concur with the Space Science Board resolution of November 1 984 that selection for participation in this discre tionary component be made in a broad context. A standing committee should review scientific and programmatic con siderations and evaluate other flight options. Inclusion of dis cretionary efforts in the program should not delay the ongoing Explorer missions. DIVERSITY OF OPPORTUNITIES The decrease in flight opportunities in Spacelab and Explorer Programs has led the community to develop strategies to guide NASA in the selection of new investment areas. Small, ( initially) high-risk experiments, may have been "crowded from the market" of all space opportunities. Considerable
Considerations for an Effective Program 42 improvement would follow naturally from a greater frequency of flight opportunities. Some experiments may be accommo dated by opportunities other than the Explorer Program. We urge that methods for selecting experiments in the Explorer Program allow for different levels of experiments within the guidelines of scientific excellence, timeliness, and cost effectiveness. Encouraging vigor and imagination in the long term demands a flexible approach. INTERNATIONAL COLLABORATIONS In astrophysics, there is a well-established tradition of international collaboration in the space program. The Explorer series has included such outstanding successes as the IRAS, a collaboration with the Dutch and the SERC of the United Kingdom, and the IUE, a collaboration with ESA and the SERC of the United Kingdom. Collaborations for future exper iments include the ROSAT program of the Federal Republic of Germany. Foreign collaborators have shared in the scientific results and shouldered a substantial amount of the cost of these missions, representing 21 percent of the IUE cost and 47 percent of that of IRAS. International collaborative programs have worked well because of the strong scientific liaison among astronomy and astrophysics researchers. Similar levels of technical abilities provide good working relationships and extremely productive scientific experiments. Dependable funding in the Explorer line allows intern a tional commitments to be met. Collaborative scientific mis sions allow complementary programs to be developed. Such joint programs optimize the scarce resources of all participants. We are concerned that a rigid and complicated selection procedure will effectively thwart the best attempts at interna tional collaboration. To establish joint programs, simultaneous agreements among the partners must be obtained, so that costly and inefficient delays can be avoided. Selection pro cedures and phase A and phase B studies must not be awk wardly out of phase. The hurdles of multiple selection can
Considerations for an Effective Program 43 make international collaboration unattractive and nearly impossible for all participants. We urge that NASA and the scientific communities work effectively to eliminate this poten tial problem. Two moderate class missions recommended by the Astron omy Survey Committee of the National Academy of Sciences, appear to be strong candidates for international collaborations: A Far Ultraviolet Spectrograph in Space and a Space VLB Interferometry Antenna. THE COST OF AN ASTRONOMY AND ASTROPHYSICS PROGRAM The current set of obligations in the Explorer Program beginning with FY 1987 totals approximately $170M ( in FY 1 986 dollars ). These missions include COBE, EUVE, and XTE ( assuming a reusable spacecraft such as the MMS ) , as well as the nondedicated missions ROSAT, CRRES, HNC, and HESP*. In addition, ongoing studies and concepts to be initiated with the 1 986 Dear Colleague Letter amount to about $3M per year. If the Explorer line were not augmented from the present level of $48.2M (FY 1 986), and maintained only an inflationary increase, then a minimum of four years would be required to achieve these missions. Since the dedicated missions derived from initial selections in 1 974-1975, this schedule implies that at least 1 5 years were required for completion of the programs! These missions clearly must be completed and launched in the near term. In addition, because of the lack of dedicated Explorer opportunities for two decades, there are many new scientific concepts that must be selected and initiated. The Explorer budget line requires augmentation to continue the innovative programs and outstanding science that are hall marks of Explorer missions. Looking ahead to the needs of the community m *HESP is a Japanese high energy solar physics mission with NASA participation .
Considerations for an Effective Program 44 astronomy and astrophysics and to the recommendations of the Astronomy Survey Committee, we envision a steady state of one Astronomy and Astrophysics Explorer opportunity per year. Over the next 10-year period, the mix of experiments might include (in FY 1986 dollars) the following: 3 moderate missions @ $1 10M average $330M 7 small/joint missions @ $40M average 280M current obligations 170M studies 30M Proposed Astronomy and Astrophysics Decade Total $810M This calculation implies a funding of approximately $80M per year (FY 1986) to accomplish the Astronomy and Astro physics Explorer Program. With this budget, the direction and scope of a healthy Explorer Program can be firmly established in the next decade. During this time, we must overcome the crippling hiatus of launch opportunities, complete the backlog of missions, and rebuild a vigorous steady state in the Explorer Program. These particular circumstances may lead to more than one Explorer launch per year for astronomy and astrophy sics in the immediate future. Other disciplines, such as solar and space physics, with a long prior history of Explorer Program use, have needs that must be accommodated in the Explorer Program as well. When the programs of all communities are considered, a sub stantial augmentation to the Explorer budget line will be neces sary.
45 APPENDIX: ABBREVIATIONS USED IN TEXT ANS Astronomical Netherlands Satellite AXAF Advanced X-ray Astrophysics Facility COBE Cosmic Background Explorer Satellite CRRES Combined Release and Radiation Effects Satellite CSAA Committee on Space Astronomy and Astrophysics CSSP Committee on Solar and Space Physics ESA European Space Agency EURECA European Retrieval Carrier EUV Extreme Ultraviolet EUVE Extreme Ultraviolet Explorer EXOSAT European X-ray Astronomy Satellite GRO Gamma-Ray Observatory HEAO High Energy Astronomical Observatory HESP High Energy Solar Physics Experiment HNC Heavy Nuclei Collector HST Hubble Space Telescope IMP Interplanetary Monitoring Platform IRAS Infrared Astronomical Satellite ISEE Interplanetary Sun-Earth Explorer IUE International Ultraviolet Explorer Satellite LDEF Long Duration Exposure Facility LDR Large Deployable Reflector MMS Multiple Mission Spacecraft NASA National Aeronautics and Space Administration oso Orbiting Solar Observatory RAE Radio Astronomy Explorer Satellite ROSAT Roentgen Satellite SAS Small Astronomy Satellite SERC Science and Engineering Research Council ( United Kingdom) SIRTF Space Infrared Telescope Facility SMM Solar Maximum Mission TDRSS Tracking and Data Relay Satellite System VLB Very Long Baseline XTE X-ray Timing Explorer Satellite
