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On the Space Infrared Telescope Facility and the Stratospheric Observatory for Infrared Astronomy
On the Space Infrared Telescope Facility
and the Stratospheric Observatory
for Infrared Astronomy
On April 21, 1994, Space Studies Board Chair Louis Lanzerotti sent the following
letter to Dr. Wesley Huntress, associate administrator for NASA's Office of Space Science.
In your letter to Prof. Marc Davis, Chair of the Committee on Astronomy and
Astrophysics (CAA), dated November 9, 1993, you requested that the National Research
Council (NRC) conduct an assessment of scientific capability of the rescoped Space Infrared
Telescope Facility (SIRTF) and the Stratospheric Observatory for Infrared Astronomy
(SOFIA) in the light of previous NRC recommendations for space and airborne astronomy.
The CAA, a joint committee of the Space Studies Board and the Board on Physics and
Astronomy, established a Task Group on SIRTF and SOFIA to perform this study. I am
pleased to enclose the Task Group's report.
Please contact me if you have any questions about the report.
REPORT OF THE TASK GROUP ON
THE SPACE INFRARED TELESCOPE FACILITY AND
THE STRATOSPHERIC OBSERVATORY FOR INFRARED ASTRONOMY
I. INTRODUCTION
In the 1991 National Research Council report, The Decade of Discovery in
Astronomy and Astrophysics, the Astronomy and Astrophysics Survey Committee
characterized the 1990s as "the Decade of the Infrared." The Bahcall report (after the
Committee Chair, John Bahcall) expected that the ongoing revolution in the technology for
detecting infrared and submillimeter radiation would lead to major advances in our
understanding of fundamental astronomical problems ranging from solar system studies to
cosmology. To this end, the report (pp. 75-80) strongly recommended three new infrared
equipment initiatives:
The Space Infrared Telescope Facility (SIRTF)-a 0.9-m-diameter, liquid-helium-
cooled telescope with unprecedented sensitivity for imaging and moderate-resolution
spectroscopy between 2 and 700 m, to be launched by a Titan IV-Centaur into a high
Earth orbit (altitude 100,000 km);
An 8-m-diameter telescope, optimized for low-background, diffraction-limited
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On the Space Infrared Telescope Facility and the Stratospheric Observatory for Infrared Astronomy
operation between 2 and 10 m and equipped with adaptive optics, to be built on Mauna
Kea, Hawaii; and
The Stratospheric Observatory for Infrared Astronomy (SOFIA)-a 2.5-m-diameter
telescope mounted in a Boeing 747 aircraft and optimized for diffraction-limited imaging and
high-resolution spectroscopy from 30 m to submillimeter wavelengths.
SIRTF and the 8-m ground-based telescope were the highest-priority large, new
initiatives in, respectively, the space- and ground-based categories. SOFIA was one of the
highest-rated moderate initiatives. The report stressed that the combination of these three
instruments provided enormous potential for discovery in the large and relatively unexplored
wavelength band between 1 and 1000 m-an especially relevant spectral region for studies
of cosmology, galaxy evolution, star-forming regions, and planetary systems.
Since the report's release in 1991, NASA's ability to undertake new missions,
particularly large missions, has become increasingly constrained. The constraints have
arisen not only from budget restrictions, but also from concerns about the risks associated
with large, complex missions. NASA planners are now rescoping proposed initiatives to
comply with new guidelines for the development of scientific missions. NASA's Associate
Administrator for Space Science, Wesley T. Huntress, Jr., has requested that the Committee
for Astronomy and Astrophysics (CAA)1 assess the effects of proposed changes to the
SIRTF and SOFIA programs on their respective abilities to achieve the scientific goals that
justified their high rankings in the Bahcall report.
In response, the CAA established a task group with CAA members Doyal Harper
(University of Chicago) as chair and Anneila Sargent (California Institute of Technology) as
vice chair to review the current status of SIRTF and SOFIA. Members of the Task Group on
SIRTF and SOFIA (TGSS) are listed in Appendix A [not provided]. Their charge was to
"determine whether the rescoped Space Infrared Telescope Facility (SIRTF) and the
Stratospheric Observatory for Infrared Astronomy (SOFIA) missions remain responsive to
the principal scientific objectives identified in the report The Decade of Discovery in
Astronomy and Astrophysics (the Bahcall report) for infrared astronomy and [to] previous
recommendations of the Space Studies Board's Committee on Space Astronomy and
Astrophysics and earlier astronomy and astrophysics survey committee reports." The charge
specified further that "[t]he TGSS's determination will be based on an evaluation of technical
information about rescopings of these two major NASA programs."
The TGSS met at NASA's Ames Research Center on February 17 and 18, 1994, and
heard presentations from representatives of both SIRTF and SOFIA. Project Scientists
Michael Werner (JPL, SIRTF) and Edwin Erickson (NASA-Ames, SOFIA) described the
status of their respective missions, including the scientific and technical rationale behind the
redesign of the mission elements and expected costs. The scientific aims of SIRTF and
SOFIA were amplified by science team members George Rieke (University of Arizona) and
David Hollenbach (NASA-Ames), respectively; SOFIA Deputy Project Scientist Edward
Dunham (NASA-Ames) addressed the particular capabilities of SOFIA for planetary science,
while the Project Manager for SIRTF, Lawrence Simmons (JPL), elaborated on the details of
its extensive technical redesign. The TGSS's assessment of the current state of the
missions is based on these presentations.
The TGSS concludes that, despite reductions in scientific scope that have resulted
from NASA's current cost ceiling for new science missions, SIRTF remains unparalleled in
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On the Space Infrared Telescope Facility and the Stratospheric Observatory for Infrared Astronomy
its potential for addressing the major questions of modern astrophysics highlighted in
Chapter 2 of the Bahcall report. The TGSS is unanimous in its opinion that SIRTF still merits
the high-priority ranking it received in the Bahcall report. The task group also concludes that
the SOFIA scientific capabilities are unchanged from those that contributed to its high
ranking among the moderate missions in the report. As a result, the TGSS discusses SIRTF
more extensively than SOFIA. The task group notes, however, that SIRTF's redefinition
renders the rationale for complementary SOFIA (and ground-based, IR-optimized 8-m)
observations even more compelling. An account of the TGSS's deliberations follows.
II. SIRTF
1. Technical Status
The goal of the SIRTF redesign was to reduce the mission cost from the $1.3B
(FY90; equivalent to $1.5B FY94) estimated for the version considered by the Bahcall
committee to below NASA's guideline of $388M (FY94), exclusive of launch vehicle costs.
All aspects of the mission have been profoundly affected by this major restructuring. The
SIRTF team now focuses its scientific program on four areas identified in the Bahcall report
as being of major importance in modern astrophysics. This scientific program exploits
SIRTF's unique strengths and (along with corresponding cost-benefit trade-offs) has
motivated and constrained the redesign of the mission elements as described below.
Conceptually, the major aspects of the rescoped mission appear to be well understood,
although they are as yet incomplete in detail. The current JPL estimate of the development
cost for the project as described is $310M (FY94), which includes a $68M reserve, and is
$78M less than the NASA guideline.
A. Orbit
A solar orbit rather than a high Earth orbit is now planned for the spacecraft. The
advantages and feasibility of such an orbit have only recently been recognized. It allows
greater launch vehicle flexibility, a substantially improved thermal environment, and
enhanced sky coverage for observations. Spacecraft control and scheduling of observations
will be simplified. The spacecraft will, however, move significantly farther from the Earth and
reach ~ 0.3 AU after 2.5 yrs. Communications will require the use of NASA's Deep Space
Network (DSN).
B. Spacecraft
The rescoped SIRTF incorporates a cryogenically cooled, 85-cm-diameter telescope
with performance over the 3- to 180- m range limited only by the natural background
radiation. The estimated mass of the redefined spacecraft is only 1000 kg, which is less than
that of the highly successful Infrared Astronomical Satellite (IRAS), launched in 1984, and
only about half that of the Cosmic Background Explorer (COBE), launched in 1989. This
very substantial reduction in mass results from modifications in virtually all areas. Liquid
helium requirements are much lower because of the improved thermal environment in solar
orbit, the significant improvements in telescope and instrument power dissipation, and a
decrease in planned facility lifetime from 5 to 2.5 years. Moreover, the telescope will be
launched warm, with a potential for cost savings not only in dewar design and fabrication but
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On the Space Infrared Telescope Facility and the Stratospheric Observatory for Infrared Astronomy
also in testing and integration. After launch, the telescope will first cool radiatively and,
subsequently, via enthalpy of the gas escaping from the liquid helium dewar that cools the
scientific instruments.
C. Launch Vehicle
Due to the significantly reduced spacecraft mass and the solar orbit, a much less
expensive launch vehicle can be employed. The revised SIRTF will be able to use either an
Atlas II or Delta 7925 vehicle, rather than requiring a Titan IV-Centaur.
D. Scientific Instruments
The redefined SIRTF scientific instrument payload incorporates 11 larger-format
detector arrays (down from 19 in the previous concept). Three arrays use InSb detector
material, three use Si:As IBC (impurity band conductor), and three use Si:Sb IBC; the
remaining two use Ge:Ga and stressed lattice Ge:Ga (see Table 1). The number of
cryogenic mechanisms has decreased from 23 to 1, leading to substantial reductions in
power dissipation. The decreased complexity of the payload minimizes risk as well as cost.
The lower number of observing modes combined with the increased pointing flexibility in the
solar orbit should result in very high observing efficiency.
TABLE 1
SIRTF Capabilities: Current Concept
Imaging
Wavelength Detector Detector Pixel Field of
Format Technology Size View
256 x 256 InSb 1.2" 5' x 5'
3.5 m
256 x 256 InSb 1.2" 5' x 5'
4.5 m
128 x 128 Si:As 2.4" 5' x 5'
8 m
128 x 128 Si:Sb 2.4" 5' x 5'
30 m
32 x 32 Ge:Ga 9.6" 5' x 5'
70 m
1 x 16 Ge:Ga (stressed) 19.2" 5' x 20'
160 m
Spectroscopy
Wavelength Resolving Detector
Range Power (array sizes as
listed above)
100 InSb
4 - 5.3 m
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100 Si:As
5 - 15 m
100 Si:Sb
15 - 40 m
600 Si:As
12 - 24 m
600 Si:Sb
20 - 40 m
20 Ge:Ga
55 - 100 m
TABLE 2
Comparison of Titan SIRTF and Current Concept
Parameter Titan Version Current Concept
(Bahcall Report)
Wavelength range 2 - 700 m 3 - 180 m
Lifetime 5 yrs 2.5 yrs
Aperture 92 cm 85 cm
Pointing stability 0.15 arcsec 0.25 arcsec
Secondary mirror position 6 degrees of freedom Focus only
Diffraction-limited wavelengths >3 m >6.5 m
(0.9" @ 3 m) (2" @ 6.5 m)
Planetary tracking High-speed, continuous Stepwise
Average data rate 120 kbps 40 kbps
Mode Full observatory Key project
Important Simplifications
Parameter Titan Version Current Concept
(Bahcall Report)
0.8 m3 0.2 m3
Cooled instrument volume
Cryogenic mechanisms 23 1
Number of detector arrays 19 11
Cryogenic instrument mass 200 kg 50 kg
Cryogenic instrument 17 mW 10 mW
heat dissipation
Warm electronics (mass/volume/power) 97 kg/0.5 m3/150 W 75 kg/0.08 m3/75 W
Fine guidance Internal External
The simplification has been achieved through significant reduction in capabilities.
Diffraction-limited imaging in the 3- m region, polarimetry, 2.5- to 4- m spectroscopy, and
high-resolution spectroscopy in the 4- to 13- m range and longward of 40 m are no
longer possible. In addition, there will be no bolometers for imaging longward of 200 m.
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Since the filter wheels associated with the imagers have been eliminated, narrow-band
imaging will be less efficient, though still viable (by spatial scanning perpendicular to the slits
in the spectrographic modes). The technical changes in the currently envisaged SIRTF
mission are compared (Table 2) to the earlier version considered by the Bahcall report.
However, there have been significant gains in performance in other areas. Detector
technology has matured considerably since the time of the Bahcall report, particularly in the
key 27- to 40- m region. Here, high-quantum-efficiency, low-noise, 128 x 128 Si:Sb IBC
arrays have replaced lower-efficiency 16 x 16 extrinsic Ge arrays. At other wavelengths,
combinations of array size and performance that were only predicted in 1990 have now
been realized in the laboratory. The detector performance is now such that SIRTF
observations will be limited only by the fundamental photon noise of the extraterrestrial sky
brightness (principally thermal emission from zodiacal dust from our vantage point within the
inner solar system), not only for broad-band imaging around 3.5, 4.5, 8, 30, 70, and 160
m, but also for spectroscopy in the bands from 4 to 40 m, 13 to 40 m, and 55 to 100
m with spectral resolving power of 100, 600, and 20, respectively. Table 3 summarizes the
capabilities that have been lost in the new SIRTF concept as well as the gains.
TABLE 3
Changes in SIRTF Capabilities
Deleted SIRTF Capabilities
Improved SIRTF Capabilities
Availability of Si:Sb arrays-improved quantum efficiency,
larger format in the 20- to 40- m range
Greater reliability through simplified hardware
Solar orbit instead of high Earth orbit results in
a. greater observing efficiency-shorter life
b. better sky access-improved response to targets
of opportunity
c. a more stable thermal environment-simpler
attitude control
Deleted SIRTF Capabilities
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On the Space Infrared Telescope Facility and the Stratospheric Observatory for Infrared Astronomy
Imaging
Narrow-band imaging (filter wheels)
Sub-millimeter imaging (180 - 700 m)
Short-wavelength imaging (2 - 3 m)
High-resolution imaging (2 - 5.5 m)
Polarimetry
Spectroscopy
Long-wavelength spectroscopy (40 - 200 m)
Short-wavelength spectroscopy (2.5 - 4 m)
High-resolution spectroscopy (4 - 13 m)
E. Ground Operations
The solar orbit simplifies ground operations and, with the streamlined
instrumentation concept, will provide very high observing efficiency, possibly around 75%,
but will require support of the DSN. However, the reduced data rate and shorter lifetime
demand careful approaches to planning and executing the science program in order to
maximize scientific productivity while assuring community involvement. The traditional
"observatory" paradigm originally envisaged for SIRTF, in which scientific programs evolve
as a wide spectrum of users learn and test the capabilities of the system, is no longer
applicable. The SIRTF team now favors an approach whereby much of the observing time is
devoted to large-scale projects (Key Projects) that will include large imaging and
spectroscopic surveys. In order to ensure optimum scientific returns, the broader
astronomical community will be actively encouraged to participate in the definition of these
Key Projects well before launch. To enable follow-up activities by the community during the
shorter lifetime, Key Project data will be nonproprietary. Very early release of processed and
calibrated data products is planned. Such programmatic changes should help counteract the
loss of science output due to the shorter mission, particularly in view of the increased
coverage of the sky afforded by the solar orbit.
2. Scientific Capabilities
The SIRTF redefinition and operations are driven by four scientific programs: (1)
preplanetary and planetary debris disks, (2) brown dwarfs and superplanets, (3)
ultraluminous galaxies and active galactic nuclei, and (4) deep surveys of the early universe.
By focusing on these important areas in which SIRTF observations can make unique
contributions, the SIRTF team has greatly simplified the instrument design and operating
modes and has vastly reduced mission costs. The four programs provide a sharp scientific
focus that is entirely consistent with the high-priority objectives identified in the Bahcall
report. Scientific research conducted since the report's publication has served only to
emphasize that these programs encompass some of the most compelling problems in
modern astronomy. In addition, as a consequence of the unprecedented sensitivity across
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On the Space Infrared Telescope Facility and the Stratospheric Observatory for Infrared Astronomy
the whole 3- to 180- m band, SIRTF will have strong capabilities for addressing a wide
range of other astronomical problems.
Due largely to its advanced detector arrays, the redefined SIRTF retains much of its
original scientific capability and preserves its major advantage over other instruments-
unprecedented sensitivity in the large, relatively unexplored, and astrophysically important
region of the spectrum between 3 and 180 m. Again, the TGSS stresses that the
sensitivity is now limited only by the natural extraterrestrial sky brightness. Moreover, the
large-format arrays allow full sampling of the diffraction disk beyond 6 m, a capability that
is essential for minimizing the effects of source confusion in very deep integrations.
The powerful focal-plane arrays have a profound impact on all of the science
programs. Photometric and spectroscopic surveys will substantially extend the range of
preplanetary and disk characteristics known from IRAS. Imaging programs that can reach
much fainter systems will strongly constrain disk models. Targeted searches of nearby stars
and young clusters for brown dwarf candidates and surveys for planetesimals in the Kuiper
Belt will be facilitated. Studies of active galaxies and the early universe will benefit
enormously from the high signal-to-noise ratio and dense spatial sampling that, coupled with
sophisticated extraction techniques, will enable deep searches at unprecedented sensitivity.
Observations of ultraluminous galaxies out to redshifts of z ~ 10 will be possible.
Measurements of the contribution from faint galaxies will be an important complement to
COBE measurements of the cosmic background. The TGSS notes that SIRTF's greatest
asset is likely to be its potential for discovery. Like IRAS, the task group expects it to open
new areas that will then be studied at other wavelengths and at higher spatial and spectral
resolution with the upcoming generation of large ground-based telescopes such as Keck,
Gemini, and the European Southern Observatory's Very Large Telescope, with airborne
instruments like SOFIA, and with future space-based or lunar telescopes.
Although the redesign of SIRTF has been guided predominantly by the needs of the
four programs described above, the new instrument will make major contributions in other
astronomical areas. Nevertheless, there has been some unavoidable loss of scientific
opportunity. The restricted technical capabilities will preclude a number of the programs
originally proposed. Eliminating the submillimeter bolometer system will prevent
cosmological observations involving the Sunyaev-Zel'dovich effect and the cosmic
background anisotropy. Without the far-infrared spectroscopic capability, studies of
important cooling lines in the interstellar medium of our own and other galaxies will not be
possible. In addition, a number of goals of the planetary program are now unattainable. In
particular, investigations of planetary atmospheres that rely strongly on imaging in the near
infrared and on high-resolution spectroscopy between 4 and 13 m cannot be carried out.
In deep searches for distant galaxies, for example, SIRTF will provide orders-of-
magnitude improvement over ISO. Figure 1 is a comparison of the relative astronomical
capabilities of the rescoped SIRTF and ISO and, when compared with Figure 4.2 in the
Bahcall report, highlights the dramatic improvement in SIRTF's detection capability since the
time of that report's release. The relative astronomical capability is a figure of merit
combining point-source sensitivity, array size, facility lifetime, and efficiency in the following
relation:
(facility lifetime) x (number of array pixels) x efficiency
Relative astronomical capability = ___________________________________________
(limiting flux density)2
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Roughly speaking, this expression gives the number of resolution elements on the sky that
can be measured to a given flux level by a facility during its lifetime (see p. 78 of the Bahcall
report). Depending on wavelength, the relative astronomical capability of SIRTF will exceed
that of ISO by factors of 103 to 108.
FIGURE 1 Relative astronomical capability of SIRTF and ISO.
3. Conclusions
The TGSS fully endorses the Bahcall Committee's ranking of SIRTF. The proposed
rescoped mission remains responsive to the principal scientific objectives of the Bahcall
report. In terms of cost, SIRTF has moved into the moderate mission category while
retaining much of its scientific capability. The mission has also been much simplified,
significantly reducing risk factors. The revised observing program has been tailored to focus
on a few well-defined, high-priority objectives that include some of the most important
problems in modern astrophysics, but the instrument remains a powerful tool for a variety of
other studies. Despite drastic rescoping, SIRTF has maintained an exceptionally high level
of scientific potential, largely as a result of dramatic technological advances in the area of
infrared detector arrays. The interaction of university-based scientists and U.S. industry in
this endeavor has been remarkably successful; the sensitivity of SIRTF observations is now
limited only by background photon noise. The TGSS believes that it is imperative that NASA
and the astronomy community capitalize on this investment. It appears to the TGSS that the
proposed Key Projects program is an excellent way of involving the whole astronomical
community in SIRTF. This program and other mechanisms for promoting and coordinating
participation by a broad user community are essential for maximizing scientific returns from
a shorter mission.
III. SOFIA
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1. Technical Status
The current estimate of the cost of SOFIA program development to NASA's
Astrophysics Division is $178M (FY94), including vehicle procurement, airframe modification
and refurbishment, ground support systems, systems integration and testing, and a $42M
reserve. For comparison, the corresponding cost projected in the Bahcall report was $230M
(FY90; equivalent to $276M FY94). Neither figure includes the cost of the telescope itself
since foreign participation was already assumed at the time of the Bahcall report.
Participation in SOFIA is a high priority for the German space agency, DARA, which
anticipates supplying the telescope system and ongoing operational support in return for
access to approximately 20% of the science flights.
A major portion of the cost reduction has been realized through a redesign in which
the telescope system was shifted from a location forward of the wing (the scheme employed
in the currently operating Kuiper Airborne Observatory, KAO) to a position between the wing
and tail section, allowing important simplifications in the required aircraft modifications. An
aft location requires construction of only one new pressure bulkhead, rather than two, and
far fewer of the aircraft control systems have to be rerouted around the telescope cavity
door. Since the time of the Bahcall report, there has also been a significant decline in the
price of used Boeing 747 aircraft.
A series of engineering studies covering a broad range of factors, including
aerodynamics, aircraft structural analysis, aero-optics, and telescope design, have reduced
uncertainties in the revised concept. Important issues in moving the telescope to the aircraft
tail were the effect of the thicker boundary layer on image quality and the magnitude of
scattered infrared radiation from the jet engines and hot exhaust gases. These questions
have been addressed with both theoretical simulations and in-flight tests. The KAO was
used for measurements of seeing and to test a passive boundary-layer control system.
Airflow around the telescope cavity has been studied using computational fluid dynamics
and wind-tunnel tests on a scale model of a Boeing 747. In-flight vibration tests and
measurements of infrared emission from jet engines and exhausts were made using actual
747 aircraft. An aft-mounted telescope appears to meet all of the performance specifications
and scientific objectives envisioned for SOFIA at the time of the Bahcall report.
The SOFIA project team has identified several additional studies that are needed
prior to final selection of the model of 747 aircraft and its procurement (in particular, further
wind-tunnel tests of aft-mounted cavity configurations), but overall the program seems well
considered and ready to proceed to Phase C/D development. Ames Research Center now
plans to undertake a larger fraction of the SOFIA development in-house. This should
minimize programmatic risks by building on the unique expertise of Ames personnel in
aerodynamics (especially in the area of boundary-layer control) and in operating science
platforms on aircraft.
2. Scientific Capabilities
The Bahcall report emphasized the value of SOFIA for opening up to routine
observations the wavelength range from 30 to 350 mm, for training new generations of
experimentalists, and for developing and testing new instruments. It also stressed that
SOFIA's capability for diffraction-limited imaging and high-resolution spectroscopy at
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wavelengths inaccessible from the ground would complement SIRTF's great sensitivity.
The report's conclusions regarding SOFIA are rendered more compelling with the
elimination of SIRTF's very long wavelength, high spectral resolution, and polarimetric
capabilities, and the reduction in its operational lifetime. The angular resolution afforded by
SOFIA's large aperture (~ 2.5 m) and the possibility of achieving high spectral resolutions,
with corresponding velocity resolutions of up to 1 km s-1, are of particular importance. Both
capabilities will enhance dynamical studies of the high-density, moderate-temperature cloud
cores where stars form, of the primitive nebulae around newly formed stars, and of the
nuclei of infrared-luminous galaxies. They are also crucial for studies of the atmospheres of
the giant planets. SOFIA will also provide an important ongoing capability for monitoring time-
variable phenomena and responding to "targets of opportunity" such as supernovae,
comets, and occultations.
SOFIA's capabilities for developing new instrumental technology and training
experimentalists remain strong. The airborne astronomy program has already begun to
address the Bahcall Committee's concerns about strengthening the contributions of
astronomy to society by establishing the KAO outreach program, FOSTER (Flight
Opportunities for Science Teacher Enrichment). The SOFIA team plans to build on and
expand this burgeoning program that offers high school teachers first-hand experience with
observational research.
3. Conclusions
Cost reductions in the SOFIA program have been less radical than those required to
rescope SIRTF from a major to a moderate mission, but they have been significant and have
been realized with essentially no decrease in scientific capability. The price of used Boeing
747 aircraft has decreased, and moving the telescope to a location aft of the wing has
enabled major simplifications in the required modifications to the aircraft. Program risks have
also been reduced by a series of ongoing tests and studies, and a plan has been formulated
for much of the development to be done in-house at Ames Research Center. SOFIA has
strong capabilities at wavelengths longward of 180 m and at high spectral resolutions. The
TGSS believes that the absence of these capabilities in the current SIRTF concept makes
the scientific case for SOFIA more compelling. The TGSS concludes that SOFIA, with
frequent flight opportunities for a broad range of state-of-the-art instrumentation programs,
remains a uniquely powerful facility for science and continues the airborne program's role of
developing technology for future space missions, for training experimentalists, and for
educational outreach, as envisaged in the Bahcall report.
1The CAA is a joint activity of the National Research Council's Space Studies Board and the
Board on Physics and Astronomy.
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