Report of the Task Group on
SIRTF [Spitzer] & SOFIA
The Task Group on SIRTF (Space Infrared Telescope Facility)
and SOFIA (Stratospheric Observatory for Infrared Astronomy),
operating under the aegis of the Committee on Astronomy and
Astrophysics of the Space Studies Board and the Board on Physics
and Astronomy, sent the following report to NASA Associate
Administrator for Space Science Wesley T. Huntress, Jr., on April
21, 1994. The letter is reprinted in full, except for figures and
tables and their citations; complete copies of the letter can be
obtained by contacting the Board at 202/334-3477 (fax 202/334-
3701).
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:
- Space Infrared Telescope Facility (SIRTF)
- A 0.9-meter diameter, liquid-helium-cooled telescope with unprecedented sensitivity for imaging
and moderate-resolution spectroscopy between 2 and 700 microns, to be launched by a Titan IV-Centaur into
a high Earth orbit (altitude 100,000 km).
- Infrared Optimized Telescope
- An 8-m-diameter telescope, optimized for low background, diffraction-limited operation between 2
and 10 microns and equipped with adaptive optics, to be built on Mauna Kea, Hawaii.
- Stratospheric Observatory for Infrared Astronomy (SOFIA)
- A 2.5-meter diameter telescope mounted in a Boeing 747 aircraft and optimized for diffraction-
limited imaging and high-resolution spectroscopy from 30 microns 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 microns -- 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)
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. 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 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
contribute 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-micron 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 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. 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.
The simplification has been achieved through significant
reduction in capabilities. Diffraction-limited imaging in the 3-um
region, polarimetry, 2.5- to 4-micron spectroscopy, and high-resolution
spectroscopy in the 4- to 13-micron range and longward of 40 microns are no
longer possible. In addition, there will be no bolometers for
imaging longward of 200 microns. 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 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-
micron region. Here, high-quantum-efficiency, low-noise, 128x128
Si:Sb IBC arrays have replaced lower-efficiency 16x16 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 microns, but also for spectroscopy in the bands from 4
to 40 microns, 13 to 40 microns, and 55 to 100 microns with spectral resolving
power of 100, 600, and 20, respectively. Table 3 of Appendix B
summarizes the capabilities that have been lost in the new SIRTF
concept as well as the gains.
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 increase 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 missions 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 the whole 3- to 180-micron 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
microns. 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 microns, 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 microns cannot be
carried out.
In deep searches for distant galaxies, for example, SIRTF will
provide orders of magnitude improvement over ISO. Figure 1 [not
provided] 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:
Relative astronomical capability =
[(facility lifetime) x (# of array pixels) x efficiency] /
[(limiting flux density)**2]
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 10^3 to 10^8.
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 and 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
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
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 microns,
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 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, 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
had 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 microns 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 is 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.
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