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Executive Summary 2 Of great concern is the drastic decrease in the rate of Explorer launches: from three to four per year in the 1960s to less than one per year in the 1980s for all of the space science disciplines. This decline can be attributed to several causes: increasing weight and sophistication of instruments and space craft, extended development phases, unanticipated technical problems, and a budget ravaged by inflation, which causes extended study and development phases. Some of these causes highlight the true success of the program. Scientific research naturally evolves to face the unanswered questions. Many of the simple measurements have been made. As we build up our understanding of the universe, we address more difficult prob lems, and this can lead to more sophisticated (and costly) experiments. The Committee on Space Astronomy and Astrophysics (CSAA) undertook this study to evaluate and comment on the role of Explorers in astronomy and astrophysics. The scientific guidelines remain those stated in the report of the Astronomy Survey Committee, Astronomy and Astrophysics for the 1980's (National Academy of Sciences, 1982). Broadly-based consultations with scientists, engineers, and administrators have helped the committee to enumerate many considerations that can result in a more effective Explorer Pro gram for Astronomy and Astrophysics. RECOMMENDATIONS There is no doubt that the Explorer Program has resulted in outstanding scientific discoveries and continues to contribute in a vital way to the progress of space research in astronomy and astrophysics. Our discussions have led to five recommen dations to enhance the effectiveness of this program in the coming decades. 1. Astronomy and Astrophysics Explorers have produced spectacular advances in our knowledge of the universe. Major Explorer missions that are approved for future flight, such as the Cosmic Background Explorer (COBE),
Executive Summary 3 Extreme Ultraviolet Explorer (EUVE), and X-ray Timing Explorer {XTE) are likely to lead to similar advances. These missions include survey/discovery missions such as Uhuru and the Infrared Astronomical Satellite (IRAS), and missions devoted to directed detailed study such as the International Ultraviolet Explorer (IUE) and XTE. The Astronomy Survey Committee has identified many Explorer opportunities, and in response to the 1986 Dear Colleague Letter, this committee expects additional Explorer concepts that have similar promise for the future. Therefore, we recommend that Astronomy and Astrophysics Explorers be continued as a vital part of the NASA space astrophysics program. To allow important scientific goals to be achieved in a reasonable time, an allocation of approximately $80 million dollars per year (FY 1986 dollars) for Astronomy and Astrophysics Explorers within the total Explorer budget is reguired. We recommend that the Explorer budget be augmented sufficiently to accommodate this amount. 2. The era of the Space Shuttle and Space Station, and the increased worldwide activity in space, create new types of Explorer opportunities in addition to the traditional launches of dedicated Explorer spacecraft missions. These new opportunities include retrieval and refurbishment of spacecraft for dedicated missions, use of space station related carriers such as Eureca, (European Retrieval Car rier) and provision of instruments for foreign missions. The latter two can increase the frequency of opportunity so important to the Explorer concept. Reuse or duplica tion of spacecraft can lower the costs of spacecraft that are now a substantial fraction of a mission budget. Astronomy and Astrophysics Explorer mis sions should include a mix of dedicated spacecraft, instruments on refurbished spacecraft, and instru ments on foreign missions.
Executive Summary 4 3. The record of the past and plans for the near future tes tify to the high quality of innovative science that is achieved by peer-selected Explorer science. Open competition for selection of new Explorer concepts should occur frequently. These investigations should continue to be selected on the basis of peer review, based on scientific excellence, timeliness, and cost-effectiveness. P art of the Explorer budget should be used, as it now is, to effect small, timely projects that should be subject to the scrutiny of standing committees in the con text of all Explorer options and other flight oppor tunities. 4. Observations from space form an integral part of contem porary astrophysics. Frequent and timely access to space opportunities represents an essential aspect of the Explorer Program-a characteristic that allows the com munity to address forefront scientific questions with well defined experiments, and to attract the many talents of scientific researchers and students. Experiments can be launched by expendable vehicles and by the Space Shut tle. Missions can also be designed for platforms whose payloads are replaced while in orbit. We recommend that NASA take measures to ensure regular access to space for future Explorer payloads. For the scientific goals of astronomy and astrophysics, one Explorer opportunity per year must be available. 5. The Explorer Program can serve science best by permit ting international cooperation in the most flexible manner. For programs of moderate scope, the inclusion of interna tional partners may help to enhance scientific return and reduce the cost to the Explorer Program while maintain ing the desired launch frequency. We encourage NASA, in collaboration with the international science commu nity, to continue to
Executive Summary 5 take advantace of the unique features of the Explorer Procram to foster cooperative scientific missions with space acencies of other nations.
Executive Summary 6 NGC 6624 Ulauru survey in 2-6 keY x-radiation (1Q70-1Q71) showing a profusion SAS- 1 ( U/auru): A remarkable achievement or an astronomical sky survey was the or point cosmic x-ray sources.
IT. THE ROLE OF EXPLORERS IN ASTRONOMY AND ASTROPHYSICS GOALS OF THE EXPLORER PROGRAM The Explorer Program has provided the astronomy and astrophysics community with small and moderate-sized mis sions for a remarkable variety of mission types. Explorer spacecraft have been used as a generally inexpensive resource to survey cosmic objects in new spectral regions or to carry out intensive study of more narrowly defined scientific programs. Explorer science has been achieved in several ways ranging from Principal Investigator (PI) instruments to moderate exper iments open to a Guest Observer community in part or in total. Explorers represent scientific initiatives that are not large enough to justify a major new start for NASA. Although about 70 NASA Explorer spacecraft have been built and launched since the first Explorer launch in 1 958, only eight of these Explorer missions were dedicated to astronomy and astro physics. However, this handful of well-chosen Explorers has created enormous impact on the scientific course of astrophy sics. We shall demonstrate the scientific achievements in Sec tion III. In addition, if adequately funded, the Explorer Pro gram can complement the execution of the scientific programs of NASA and the community in various ways by providing the following: 7
