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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
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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).
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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.
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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).
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