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Scientific Assessment of NASA's SMEX-MIDEX Space Physics Mission Selections (Chapter 1) Scientific Assessment of NASA's SMEX-MIDEX Space Physics Mission Selections 1 Introduction NASA's Explorer line has for decades provided opportunities for many small to medium-size, community-inspired missions. The Explorer missions generally supplemented and complemented larger agency or division programs and goals. These opportunities gave rise to such space physics successes as the Atmospheric Explorer (AE), the Interplanetary Monitoring Platforms (IMPs), the International Sun-Earth Explorer (ISEE), the Active Magnetospheric Particle Tracer Explorer (AMPTE), and the Dynamics Explorer (DE) missions of the 1960s, 1970s, and 1980s. The early Explorer missions were all conducted in the mode of open solicitation of individually competed and selected instruments, with individual PIs for each experiment. More recent Explorer missions, such as SAMPEX and FAST, have a single PI for the entire spacecraft project. The Advanced Composition Explorer (ACE), TRACE, and IMAGE are now being readied for flight, all in the one-PI/ spacecraft mode of an integrated suite of REPORT MENU instruments. NOTICE MEMBERSHIP Recent pressures to downsize total spacecraft and mission costs have led FOREWORD to a new focus on the Explorer line. In particular, three cost categories of EXECUTIVE SUMMARY Explorers have now been established, with the MIDEX the newest and largest, CHAPTER 1 CHAPTER 2 capped at $76 million; the SMEX at less than $38 million; and the University CHAPTER 3 Explorer Class (UNEX), similar to the NASA/University Space Research CHAPTER 4 Association (USRA) Student Explorer Demonstration Initiative (STEDI) missions, CHAPTER 5 at less than $4 million. Furthermore, all of these Explorers are now restricted to REFERENCES Med-Lite or smaller launch vehicles. Previously, Explorers costing up to several APPENDIX hundred million dollars were acceptable, with Delta-class launches the norm. The space physics community had hoped that a dedicated Solar- Terrestrial Probe (STP) class of missions would be approved to fill the need for what are now considered larger missions—over $70 million, with more ambitious payloads and scientific goals. Though the Thermosphere-Ionosphere- Mesosphere Explorer (TIMED) is now an approved mission of this class, there is still no dedicated STP line in NASA mission planning. As a result, the Explorer line has become essential for continuing progress in the field. The large number of proposed Explorer missions reflects the richness of the field and the creativity file:///C|/SSB_old_web/smexch1.html (1 of 4) [6/18/2004 1:43:01 PM]

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Scientific Assessment of NASA's SMEX-MIDEX Space Physics Mission Selections (Chapter 1) of its community. However, in the last year, only 2 out of 50 Explorer mission proposals were selected for development. With these considerations in mind, the committees undertook a short study to assess the potential of missions in the Explorer class to execute the high- priority space physics investigations recommended in their report, A Science Strategy for Space Physics (SSB, 1995). Toward this end, the committees were briefed on a number of recent Explorer programs. In addition to the recently selected TRACE (SMEX) and IMAGE (MIDEX), the committees examined the SAMPEX and FAST missions as representative of already launched SMEX Explorers. CSSP/CSTR also considered two space physics-oriented STEDI missions to assess using the future UNEX class to enhance the Explorer line capability in solar and space science. The primary task of this brief study is an assessment, from the perspective of the NRC Science Strategy report, of the science capabilities of the recent Explorer selections TRACE and IMAGE and of expectations of this program in the future. In the strategy report, five general topics in space physics were identified as key areas for study in the next decade. They are listed here, not according to priority: 1. Mechanisms of solar variability; 2. Physics of the solar wind and the heliosphere; 3. Structure and dynamics of magnetospheres and their coupling to adjacent regions; 4. Middle and upper atmospheres and their coupling to regions above and below; and 5. Plasma processes that accelerate highly energetic particles and control their propagation. Prioritized lists of research activities were recommended for each of these topics. In addition, the NRC Science Strategy report promoted four themes within the five topics, listed here in prioritized order: 1. To complete currently approved programs; 2. To exploit existing technologies and opportunities to get new results in a cost-effective manner; 3. To develop the new technology required to advance the frontiers of space physics; and file:///C|/SSB_old_web/smexch1.html (2 of 4) [6/18/2004 1:43:01 PM]

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Scientific Assessment of NASA's SMEX-MIDEX Space Physics Mission Selections (Chapter 1) 4. To support the theory and modeling activities vital to space physics. Below is a summary of the presentations made to the committees and the findings drawn from them and subsequent discussions. These findings include brief descriptions of what was learned from various team representatives about the space physics Explorer mission experience, the Explorer mission science goals in light of the NRC Science Strategy report priorities, and the challenges faced by the newest space physics Explorers. Because of the integral link between the Explorer program's attributes and its scientific productivity, the committees' assessments could not isolate its science aspects. This report is therefore as much a commentary on the Space Physics Explorer Program as an analysis of mission accomplishments. The Space Studies Board conducted a more general review of the overall Explorer program with a complementary emphasis (SSB, 1996). This report includes specific recommendations to NASA based on the committees' findings. Last update 10/5/00 at 8:35 am Site managed by Anne Simmons, Space Studies Board file:///C|/SSB_old_web/smexch1.html (3 of 4) [6/18/2004 1:43:01 PM]