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Scientific Assessment of NASA's SMEX-MIDEX Space Physics Mission Selections (Chapter 2) Scientific Assessment of NASA's SMEX-MIDEX Space Physics Mission Selections 2 Recently Launched Space Physics Explorers: SAMPEX and FAST SAMPEX SAMPEX is an approximately 350-kg spacecraft that carries a complement of in situ particles and fields instruments in a high-inclination orbit that ranges between 675 km and 550 km. SAMPEX was selected as a SMEX mission in April 1989. It was launched on July 3, 1992, on a four-stage Scout expendable launch vehicle. The overall SAMPEX science goal is to study highly energetic charged particles of both magnetospheric and cosmic ray origin. This research includes REPORT MENU particles trapped in Earth's magnetosphere and those that enter the NOTICE magnetosphere from interplanetary space over the magnetic poles. One specific MEMBERSHIP SAMPEX science goal addresses the composition and charge state of FOREWORD anomalous cosmic rays (ACRs), so identified because they are not of solar or EXECUTIVE SUMMARY traditional galactic or extragalactic origin and have unusual species abundances CHAPTER 1 and solar cycle dependencies. Using the Earth's magnetic field as a CHAPTER 2 spectrometer, SAMPEX data have been used to determine charge state by CHAPTER 3 accurately measuring masses and energies of cosmic rays as a function of their CHAPTER 4 location in the magnetosphere. Another specific SAMPEX science goal is to CHAPTER 5 measure magnetospheric particle precipitation patterns over the entire globe and REFERENCES to relate these patterns to various atmospheric effects as well as to spacecraft APPENDIX anomalies. SAMPEX recently completed its nominal three-year prime mission and is now in an extended mission phase. SAMPEX science goals touch on many of the objectives noted in the NRC Science Strategy report. For example, SAMPEX has already contributed to a fundamental understanding of anomalous cosmic rays. SAMPEX data show definitively that anomalous cosmic rays are mostly singly ionized. In addition, based on their composition, they are ordered by their first ionization potential in a way that confirms their origin from interstellar neutrals entering the heliosphere. Other studies have shown that the interstellar neutrals approaching the Sun file:///C|/SSB_old_web/smexch2.html (1 of 5) [6/18/2004 1:43:13 PM]
Scientific Assessment of NASA's SMEX-MIDEX Space Physics Mission Selections (Chapter 2) become ionized through charge exchange with solar wind ions, are accelerated at the heliospheric termination shock, and diffuse into the inner solar system. The higher-energy population has been shown to be doubly charged, suggesting that it is a subpopulation that has been charge exchanged a second time after an initial acceleration to approximately 100 keV, eventually attaining about twice the energy of the singly charged component. These results enhance our understanding of astrophysical mechanisms leading to the acceleration of cosmic rays to extremely high energies. Another SAMPEX result bearing on particle acceleration processes is that the relative abundances of various isotopes of, for example, oxygen and neon are different in the lower-energy solar cosmic ray range than in the higher-energy range. SAMPEX has also mapped magnetospheric energetic electron fluxes as a function of the magnetospheric coordinate L, as well as temporal changes and local time. Moreover, SAMPEX data have shown that upper-atmosphere NOx changes with the level of flux of precipitating energetic electrons. The effect is still poorly defined but intriguing in its potential implication for Sun-Earth connections of significance to humankind. With its extended mission, SAMPEX will be able to observe changes associated with the solar cycle. These observations are essential to understand both the NOx result and the implications of the ACRs. The science accomplishments of SAMPEX are a function of both the scope of the instrumentation and the specifics of the orbit. Many of the goals concerning precipitation of particles and ACR charge states require a polar low Earth orbit. (Even with its more advanced complement of instruments, ACE will not be able to achieve many of these measurements from its L1 orbit.) SAMPEX addresses many of the NRC Science Strategy goals in the area of acceleration and propagation of highly energetic charged particles, as well as some areas concerning the effects of these particles on the upper atmosphere and spacecraft. Areas addressed incompletely or not at all include an understanding of the acceleration mechanisms for CRs and ACRs. Though some mechanisms are hinted at and others may perhaps be ruled out for ACRs, these are inferences that are neither definitive nor comprehensive. Similarly, SAMPEX has addressed elemental abundances but only for the more commonly occurring species and over a limited energy range. The issue of how much time has elapsed between element synthesis and acceleration of the particles is largely unaddressed. Finally, although some work has been done on the correspondence between atmospheric chemistry and energetic particle precipitation, progress in this area will not be enough to resolve questions on the consequences. Nevertheless, the SAMPEX mission has been and continues to be highly relevant to the priority goals outlined in the NRC Science Strategy report. The presentation to the committees on SAMPEX indicated that the Explorer process in place at the time had generally worked well. The oversight by NASA/Goddard at the instrument level was modest; coordinated efforts with the Goddard spacecraft engineers were positive and constructive. Notably, the funding was never reduced; SAMPEX was funded in full, as originally proposed. Rapid contracting helped the schedule. The single central data processing unit (DPU) file:///C|/SSB_old_web/smexch2.html (2 of 5) [6/18/2004 1:43:13 PM]
Scientific Assessment of NASA's SMEX-MIDEX Space Physics Mission Selections (Chapter 2) for all instruments designed for SMEX worked to the mission's advantage. Much of the schedule risk was reduced because so much of the instrument payload had been developed and/or used in earlier missions. Several of the instruments flew in 1989 on a shuttle "get away special" (GASCAN), so they were systems that had already been developed, built, and tested. Others had been built for the U.S. spacecraft in the Solar Polar mission, which was cancelled. This high level of heritage was considered critical in meeting both costs (the spacecraft, at $22 million, was well below the SMEX limit) and schedule. Today, the SAMPEX mission operates in an extended mission phase designed to continue exploration of some of the science issues raised here and, in particular, to measure solar activity effects during the approach to the upcoming solar maximum (expected in 2001). A particularly noteworthy attribute of the extended mission is its cost effectiveness: operating costs are only $1.5 million per year for Goddard Space Flight Center (GSFC) operations and $1.2 million per year for science. FAST FAST, the second SMEX Explorer, was launched on a Pegasus-XL in 1996. The spacecraft was designed to explore the microphysics of the auroral acceleration region by making a comprehensive set of particles and fields measurements at unprecedented time resolution. FAST is addressing a specific goal of the NRC Science Strategy report: understanding the small-scale physical processes (microphysics) in the magnetosphere and ionosphere (an essential element of the report's section "Structure and Dynamics of Magnetospheres and Their Coupling to Adjacent Regions"). The 187-kg FAST spacecraft operates only in the auroral zone above approximately 65º and 75º magnetic latitude, where it samples the plasma and field conditions at altitudes of 400 to 4,200 km (above the regions rockets can directly probe). Its particle instruments are designed to determine as full a coverage of the time-dependent distribution functions of the local ions and electrons as possible. Very high temporal resolution is achieved through a specially developed data system with large radiation-hardened memory (1 gigabit) and fast write capability. These high data rates allow detailed analysis of wave-particle interactions and electron gyro-period phenomena. For example, particle distribution functions can be correlated with wave fields seen while in an especially high-rate burst mode of operation, which can be triggered by fluctuating fields. It is hoped that FAST will establish the origins and roles of parallel electric fields in the auroral acceleration regions called inverted Vs; of wave-particle interactions in the creation of upward-moving ion beams of auroral arcs and the fine structure within them; of electrostatic double layers; and of field-aligned currents, auroral kilometric radiation, and kinetic Alfven waves potentially critical in solar wind-magnetosphere-ionosphere coupling. FAST can resolve spatial scales to much smaller dimensions than most imaging systems. For global context, it relies on ground-based observatories, the POLAR satellite, or DOD file:///C|/SSB_old_web/smexch2.html (3 of 5) [6/18/2004 1:43:13 PM]
Scientific Assessment of NASA's SMEX-MIDEX Space Physics Mission Selections (Chapter 2) satellites (the Defense Meteorological Satellite Program [DMSP] or Midcourse Space Experiment [MSX] spacecraft) for simultaneous auroral imaging. FAST also takes advantage of conjunctions with ground-based and suborbital observations for more detailed correlative data analyses. In fact, the FAST prime mission plan includes "campaigns" with these other facilities during which FAST can be commanded into specified mode sequences in real time. The FAST data system required custom design and construction. On board, the data rates are too high for the SMEX standard systems and so, data are simply passed directly to the memory through the instrument DPU. On the ground, data passes through the Goddard Operations Center at University of CaliforniaâBerkeley, which provides most of the data system functions. Launch of FAST was delayed by problems with the Pegasus-XL launcher. FAST hardware was, in fact, ready for launch 2 years before problems with the Pegasus-XL were resolved. FAST investigators told the NRC committees that they were very pleased with the support they received from GSFC. In particular, the new GSFC engineering team took the initiative to provide the special capabilities needed for mission success. The FAST experience with Goddard support was quite positive. FAST instrumentation was originally highly developed under the sounding rocket program. Each of the four instruments also has extensive heritage from a spaceflight program: the electron analyzer from AMPTE, GIOTTO, WIND, MO, and CLUSTER missions (see the appendix for a list of acronyms and abbreviations); the ion mass spectrometer from CLUSTER; the electric field experiment from S3-3, ISEE-1, CRRES, POLAR, and CLUSTER; and the magnetometer from Pioneer Venus, ISEE-1 & -2, and POLAR. This heritage was considered crucial to the mission's success in budgeting, scheduling, and technical risk. FAST is now in orbit with all instruments functioning. Preliminary results suggest that it will fulfill its promise of elucidating the microphysics of Earth's auroral acceleration region. file:///C|/SSB_old_web/smexch2.html (4 of 5) [6/18/2004 1:43:13 PM]
