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Scientific Assessment of NASA's SMEX-MIDEX Space Physics Mission Selections (Chapter 3) Scientific Assessment of NASA's SMEX-MIDEX Space Physics Mission Selections 3 The Newest Space Physics Explorers: TRACE and IMAGE The most recently selected space physics Explorers are TRACE, a SMEX, and IMAGE, one of the first MIDEX missions. These missions were conceived under the new, more highly constrained guidelines for Explorers. Although it has not yet flown, TRACE is well into development and therefore illustrates the scientific potential that may be possible under the new guidelines. IMAGE, the newest space physics MIDEX, illustrates the scope of what might be termed a maximum science agenda deemed achievable within the current Explorer program. Of course, any test of achievement must await the mission itself. REPORT MENU NOTICE TRACE MEMBERSHIP FOREWORD The TRACE Small Explorer was selected in September 1993 and is EXECUTIVE SUMMARY scheduled for launch in the fourth quarter of 1997 on a Pegasus-XL. This 224-kg CHAPTER 1 spacecraft, destined for a 600- to 700-km dawn-to-dusk, Sun-synchronous orbit, CHAPTER 2 is essentially dedicated to a single high-resolution telescope for continuously CHAPTER 3 CHAPTER 4 observing the Sun at ultraviolet (UV) and extreme ultraviolet wavelengths (EUV). CHAPTER 5 TRACE represents a low-risk, cost-effective approach to addressing important REFERENCES solar physics concerns identified in the NRC Space Strategy report. In planned APPENDIX conjunction with the operating experiments on the Solar and Heliospheric Observatory (SOHO), TRACE will provide key data to help us understand the processes that lead to solar variability. TRACE's open data policy and Yohkoh- based data dissemination system should ensure an immediate impact within the space physics community. Data from TRACE will produce a set of solar images of a portion of the solar surface coaligned through filters that select different spectral features. By comparing the temporal evolution of events as seen through different filters, investigators will gain critical information about the origin and evolution of local file:///C|/SSB_old_web/smexch3.html (1 of 5) [6/18/2004 1:43:34 PM]
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Scientific Assessment of NASA's SMEX-MIDEX Space Physics Mission Selections (Chapter 3) energy-release processes and rearrangement of coronal structures like coronal holes. Because the spectral features measured by TRACE span the major parts of the transition region between the chromosphere and corona, the direction in which energy releases move can be determined from the time lags. The science capability of TRACE will also be greatly enhanced by the availability of simultaneous SOHO data, including SOHO's global perspectives. Coordinated ground-based and suborbital observations, though not part of the TRACE mission, are expected to enhance the TRACE science accomplishments. While recognizing the value of the TRACE mission, the committees note that some major objectives in the area of solar physics will remain unsatisfied. For example, the TRACE/SOHO combination does not measure vector magnetic fields, and the longitudinal magnetic field measurements are not made continuously. The spatial resolution in the white light band is modest, and the study of small magnetic structures cannot be carried out with this instrument. Although the data access will be open, the problems investigated will be constrained by how observing sequences are defined, and it is unclear how the community will participate in those decisions. Nevertheless, TRACE is a substantial step forward in the study of the mechanisms of solar variability. The management experience from the TRACE perspective seems positive. The GSFC SMEX team was responsive to the novel requirement for the guide telescope to drive the attitude control system. The parts program and quality assurance were left largely up to the experiment team. However, cost constraints allowed for little redundancy. Fortunately, the instruments for the TRACE mission required little development because of extensive heritage from the rocket program and from SOHO (especially the experiments with the Extreme Imaging Telescope [EIT] Michaelson-Doppler Interferometer [MDI]). The data system draws heavily on Yohkoh heritage. Without this heritage (and its spare parts), TRACE could not have been done as a SMEX (with a $35 million funding cap), nor could it have been carried out on such a short schedule. It should also be recognized that the nominal cost of the mission does not reflect the full cost associated with Goddard civil service salaries, which is also true for SAMPEX and FAST. IMAGE The IMAGE mission was selected as one of two prime missions in the recent NASA MIDEX selection. As mentioned previously, MIDEX is a new flight opportunity program within the Explorer line. Capped at $70 million per mission, it represents the largest flight program routinely available to the space physics community. The MIDEX program has an aggressive schedule with a relatively rapid hardware development phase of about 36 months. The IMAGE spacecraft is being procured through the PI's institution, Southwest Research Institute (SRI), and is scheduled for launch in January 2000 on a Med-Lite launcher (Delta). file:///C|/SSB_old_web/smexch3.html (2 of 5) [6/18/2004 1:43:34 PM]
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Scientific Assessment of NASA's SMEX-MIDEX Space Physics Mission Selections (Chapter 3) IMAGE has a 2-year prime mission lifetime. Like FAST, IMAGE features a fully integrated payload. However, a recent change from a Taurus to a Delta launch has ameliorated the need for the expensive FAST power system. It will also allow a higher perigee and a longer lifetime through solar maximum, thereby significantly enhancing the expected science return. The scientific goals of IMAGE are to do the following: (1) identify the method of entry of solar wind plasma into the magnetosphere; (2) determine the extent and location of ionospheric plasma sources; (3) discover how and where energetic plasmas are accelerated, transported, and lost during substorms and storms; (4) determine the direct effects of solar wind forcing on the magnetosphere; and (5) measure coronal mass ejection (CME)-related neutral atom fluxes and radio emissions as forecasting tools for geomagnetic storms (Burch, 1995, pp. 1–2). Accomplishing these goals will require instrumentation measuring radio waves, ultraviolet photons, and neutral atoms to construct images of magnetospheric plasmas. It includes not only core magnetospheric plasmas but also the sharp density gradients at the several plasma/field boundary layers. The IMAGE spacecraft will be placed into a 500 km x 7 Earth Radii (RE) altitude polar orbit that will permit the imagers to view the magnetosphere globally and thereby reconstruct three-dimensional images. The IMAGE mission has a clear scientific focus highly relevant to the third research topic identified in the NRC Science Strategy report, magnetospheric- ionospheric physics. The second-highest priority in that research topic (after reaping the full scientific potential of the International Solar–Terrestrial Physics [ISTP] program) was to "simultaneously image the plasma and energetic particle populations in the aurora, plasmasphere, ring current, and inner plasma sheet to study the global structure and large-scale interactions of magnetospheric and ionospheric regions during different levels of geomagnetic activity" (SSB, 1995, p. 74). This high-priority research activity is precisely the area targeted by the IMAGE mission, which is thus highly relevant to a principal scientific goal set forth in the report. Moreover, the second-highest priority, cross-cutting emphasis in the NRC Science Strategy report, involved the use of existing technologies to obtain rapid results cost effectively. To a large degree, the IMAGE instrumentation, though fresh in design, is based on similar currently available experiments. Several of these concepts were developed in the course of the definition of the Inner Magnetospheric Imager (IMI) mission, originally intended as one of NASA's first Solar-Terrestrial Probe missions. IMAGE will address the need outlined in the NRC Science Strategy report for "spacecraft and instrument technology . . . to complement point-by-point in situ measurements of the magnetosphere with global imaging" (SSB, 1995, p. 77). In addition, the IMAGE mission incorporates the modeling required to interpret the measured images, consistent with recommendations made in the NRC Science Strategy report. file:///C|/SSB_old_web/smexch3.html (3 of 5) [6/18/2004 1:43:34 PM]
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Scientific Assessment of NASA's SMEX-MIDEX Space Physics Mission Selections (Chapter 3) As mentioned previously, IMAGE benefits to a certain extent from heritage derived from IMI development efforts. This raises an interesting paradox: Ideally, an Explorer mission should be a pathfinder for a subsequent, more comprehensive, second-generation mission such as IMI. (This was expressed as the third-priority emphasis in the NRC Science Strategy report—namely, the development of new technologies where second-generation instruments could be identified for magnetospheric imaging with greater sensitivity and greater energy, angle, and mass resolution.) However, the selection of IMAGE as an Explorer has essentially eliminated IMI from the Solar-Terrestrial Probe queue. It was unclear from the presentation made to the committees how much of the IMI science could be accomplished with the smaller, cheaper, faster IMAGE version. Overall, IMAGE expects to provide a low-cost, fast-track mission for global imaging, one of the highest-priority science goals of the NRC Science Strategy report. At the same time, it raises the broader issue of how much "smaller, cheaper, faster" methods can replace a more comprehensive approach to the science (e.g., with a solar-terrestrial probe mission). file:///C|/SSB_old_web/smexch3.html (4 of 5) [6/18/2004 1:43:34 PM]