SOFIA: The Future of Airborne Astronomy
(continued from Section 3: Science )

4. Comparison with Other Missions

Many important problems in modern astronomy require infrared observations with high sensitivity, high spectral resolution, and high angular resolution, or a combination of these capabilities. To fill these needs, the National Academy of Sciences Decade Survey (Bahcall) Committee has recommended both SOFIA and SIRTF (the Space Infrared Telescope Facility) as high priorities for development by NASA during the 1990's. A discussion and individual mission summaries of past and future infrared missions is contained in Session Nine of these proceedings, starting with the discussion summary by Caroff (1994).

Here we present a brief comparison of the principal missions SOFIA, SIRTF, ISO (the Infrared Space Observatory), KAO, and IRAS. Figure 13 and Table3 below depict the features of these missions which underlie the differences in their science goals. Table 3 compares launch/first flight dates, telescope diameters, design lifetimes, instrument complements, mobilities, and sponsors.

 

Table 3
Summary of Infrared Astronomy Missions
KAO IRAS ISO SOFIA SIRTF
1974 1983 1995 2000 2002
0.91 meter 0.60 meter 0.60 meter 2.5 meter 0.85 meter
20+ years 1 year 1.5 year 20 years 2.5 years
12/Evolving 2/Fixed 4/Fixed 15/Evolving 3/Fixed
Deployable Earth Orbit Earth Orbit Deployable Solar Orbit
USA USA+NED+UK ESA+JPN+USA USA+FRG USA

Basically, the cryogenically cooled space missions achieve far higher sensitivity in broad wavelength bands, whereas the airborne facilities permit higher angular resolution and capacity to accommodate new instruments and science programs over a long lifetime. For example, the SOFIA science program will capitalize on its spatial and spectral resolution, for example in revealing how stars form, and on its mobility, which permits optimum observations of ephemeral events such as occultations. The SIRTF science goals exploit its excellent sensitivity and large detector arrays, for example to study galaxy formation at the edge of the visible Universe. SOFIA's targets will typically be galactic objects or nearby galaxies that may be studied in detail not possible with SIRTF, while many of SIRTF's targets will be faint or distant objects that would be undetectable with SOFIA. SIRTF's focal plane instruments are optimized for the highest priority scientific goals, but will be used for a range of scientific investigations. SOFIA's program is broad in scope because of its wide wavelength coverage, long lifetime, and annual opportunities to propose new science investigations and focal plane instruments.

As pointed out by Caroff (1994), in addition to its unique science potential, the airborne program provides unique continuity, training opportunities, and wavelength coverage to the infrared community.

5. Education

Scientific and technical literacy are among the most critical needs of our nation. We benefit not only from brilliant researchers and creative inventors, but also from understanding of and respect for scientific endeavors by the taxpayers who must support them, and who themselves will be working in an increasingly scientifically and technologically advanced world. Thus, education in the fields of science, mathematics, and technology is not simply an investment in future scientists and engineers, but an investment in the appreciation of society for these endeavors. Through its unique scientific mission, SOFIA can increase this appreciation.

For scientists, SOFIA will provide a unique window to view the invisible infrared universe. However, for educators, it will be an exciting and accessible example of leading-edge high technology in the telescope, the scientific instrumentation, and the mission operations systems. For the public, SOFIA will serve as a high visibility, modern scientific facility, {\it epitomizing} the American ideals of innovation, exploration, and achievement.

The extensive participation in SOFIA observations by the science community, and the opportunities for teachers and the media to experience science in action on board, guarantee the potential of SOFIA for education. The rapid response of an airborne observatory to ephemeral astronomical events also helps to attract and focus public attention on science, as was the case for the KAO observations of Supernova 1987A, and for the impact of Comet Shumaker-Levy on Jupiter in 1994. These events frequently require remote deployments, which will expose this modern flagship of astronomy to the public world-wide, amplifying its effectiveness in expanding awareness of science.

The education program on SOFIA will offer to non-scientists a first-hand view of scientific research: its excitement, hardships, challenges, frustrations, teamwork, and discoveries. The intent of the SOFIA educational program is to bring these experiences to American students, teachers and the public routinely and on a significant scale. These outreach efforts will be built into the core program and evolve from the experiences with programs currently conducted with the KAO, such as the Flight Opportunities for Science Teacher EnRichment (FOSTER). SOFIA will be larger and fly more frequently than the KAO, and thus can support an expanded program.

Outreach activities are planned which will serve (1) pre-college students and teachers, (2) undergraduate and graduate students and faculty, and (3) the public and the media. SOFIA will promote excellence in science, mathematics and technology education through direct involvement of non-scientists with the SOFIA investigators, and via workshops, internships, and utilization of existing educational infrastructure such as museums and planetaria. In addition, many people will be able to experience SOFIA research remotely through the Internet and telepresence. Ongoing internal and external evaluation of the program will assure its effectiveness, much as the peer review process will do for the science program. Educational activities on SOFIA will touch the spirit and imagination of many American youth.

6. Technology

The unique observing potential, 20 year lifetime, and frequent opportunities for participation which SOFIA will offer the scientific community assure the development and prompt application of new technologies. Many of these will surely be valuable in future space and ground-based astronomy, as well as in other areas. The history of the airborne astronomy program is a guide to this process: the chopping secondary mirror, a feature of all modern infrared telescopes, was initially developed for the Learjet telescope. This facility also allowed the first ``hands-on'' testing of far-infrared bolometer detectors and a He$^3$ refrigerator in an astronomical application.

KAO investigators have extended this work by making significant contributions to bolometer array and newer refrigerator technologies, which are used on ground-based submillimeter telescopes, as well as on the KAO. Detectors anticipated for use on SIRTF and AXAF are currently being flown in KAO instruments. Experience with KAO focal plane instruments has been applied to the design of the space missions IRAS, COBE, ISO, SWAS, Cassini, AXAF, WIRE, and SIRTF. Germanium photoconductor detectors developed for use on the KAO were actually used on IRAS. We anticipate that nearly all future space IR missions ({\it e.g.,} FIRST and Edison) will reap major benefits from SOFIA-related technology.

Some of the technologies evolved in conjunction with SOFIA may have commercial applications. For example on the KAO, extensive research was done to develop infrared radiometers to measure the atmospheric water column depth overhead; this technique proved useful in detecting clear air turbulence, and the technology is now under review for suitability on commercial aircraft. In conjunction with wind tunnel testing of the SOFIA model, a pressure sensitive paint has been developed to provide very high spatial resolution of the pressure variations on airfoils; this technology has already been applied by major American aircraft companies in new wing designs. Future SOFIA technology could find application in aerodynamic noise reduction for aircraft, automated intelligent systems monitoring of real-time control systems, and mission operations and planning procedures for space flight.

7. Readiness

A configuration with the telescope cavity located behind the wing has been shown in predevelopment studies with aircraft modifiers to be much less expensive than putting the telescope ahead of the wing. Extensive analysis, including infrared observations of the exhaust plumes from NASA's Shuttle Carrier Aircraft (a Boeing 747), has shown that scattered emission from this source is not a concern for all but a very small class of possible science investigations. The aft configuration has been adopted as a cost cutting measure, although it may increase the seeing distortion of the images at near infrared wavelengths (Figure 2) due to the thicker boundary layer in the rear of the plane. Figure 14 below depicts the design with the aft cavity installation of the telescope.

Wind tunnel tests of the airflow over the open-port telescope in an aft cavity have resulted in a quiet, low drag shear-layer control concept for SOFIA, and have demonstrated that the flow reattachment is stable and the control of the aircraft is not affected for expected flight conditions. These tests also provide good estimates of the wind loading on the telescope. Further wind tunnel tests are anticipated to select among the door design concepts currently being considered.

The telescope design (Figure 15) features an airbearing support and numerous other similarities to the KAO telescope, which has achieved sub-arcsecond pointing stability even in light turbulence. Structural and optical analyses indicate that either a metal or composite structure could be used, and that any of several (glass) primary mirror designs would work. The 2.5 meter primary can be as slow as ~f/1.5 with the telescope in the aft location, so that figuring is not a problem; the chopped image quality is better for larger primary f-numbers.

SOFIA definition studies, sponsored jointly by NASA and the German Space Agency DLR, have been completed, and the project has been deemed ready for development by these agencies. If funding is available, NASA and DLR plan to begin the development of SOFIA in 1996, which would permit the first flights to occur in the year 2000.

8. Conclusion

The characteristics of SOFIA -- its astronomical promise, moderate cost, maintainability, and opportunities for broad-based community participation -- will extend the tradition of the airborne program for innovation, education, and exciting science. Its vision will penetrate dark reaches of our own and other galaxies, revealing objects and processes otherwise hidden from view with spatial resolution which will be unmatched until well into the next century. It will elucidate problems ranging from the spectacular death of massive stars to the inconspicuous incubation of low mass stars, from the composition of interstellar dust to the formation of prebiotic materials and protoplanetary systems, and from the enigmatic character of our own Galactic Center to the nature of stupendous luminosity sources in colliding galaxies. SOFIA's image, performance, and accomplishments will be a credit to its heritage.

9. Acknowledgments

It is a pleasure to thank Al Betz, Harold Butner, Dale Cruikshank, Ted Dunham, David Hollenbach, and Dan Lester for contributions to the science section, Edna DeVore for contributions to the education section, and Mike Werner for assistance with the comparison of SOFIA to other missions. The photograph of M 82 was provided by the Mount Wilson and Las Campanas Observatories, Carnegie Institution of Washington. The photograph of Beta Pictoris is credited to Bradford Smith (University of Arizona) and Richard Terrile (JPL). The picture of Uranus and its ring system was painted by Rick Sternbach. Howard Bushouse (Computer Sciences Corporation) provided the CCD image of UGC 12914/12915. The drawings of SOFIA and of the SOFIA telescope were produced by Dan Machak and Will Valloton (respectively) of NASA Ames.

10. References

Caroff, L. J. 1994, in Proc of the Airborne Astronomy Symp on the Galactic Ecosystem: From Gas to Stars to Dust
ed. M.R. Haas, J.A. Davidson, & E.F. Erickson (San Francisco: ASP) , p 647

Larson, H. P. 1994, in Proc of the Airborne Astronomy Symp on the Galactic Ecosystem: From Gas to Stars to Dust
ed. M.R. Haas, J.A. Davidson, & E.F. Erickson (San Francisco: ASP) , p 591

Traub, W. A., and Stier, M., 1976, Applied Optics, 15, 364