The following article has been published in the
Proceedings of the Airborne Astronomy Symposium on the Galactic Ecosystem: From Gas to Stars to Dust
edited by M.R. Haas, J.A. Davidson, & E.F. Erickson (San Francisco: ASP)

SOFIA: The Future of Airborne Astronomy

E. F. Erickson
NASA Ames Research Center, MS 245-6, Moffett Field, CA 94035-1000

J. A. Davidson
SETI Institute; NASA Ames Research Center, MS 245-6, Moffett Field, CA 94035-1000

             TABLE OF CONTENTS
 1. Airborne Astronomy - the Legacy
 2. Characteristics and Performance

 3. Science
    (i) The Interstellar Medium
   (ii) Star Formation
  (iii) Bipolar Outflows
   (iv) Circumstellar Disks
    (v) Our Solar System
   (vi) The Sun
  (vii) Stellar Structure
 (viii) Reprocessing of the ISM
   (ix) Other Galaxies
    (x) The Galactic Center
 4. Comparison with Other Missions
 5. Education
 6. Technology
 7. Readiness
 8. Conclusion
 9. Acknowledgments
10. References
Abstract. For the past 20 years, the 91 cm telescope in NASA's Kuiper Airborne Observatory (KAO) has enabled scientists to observe infrared sources which are obscured by the earth's atmosphere at ground-based sites, and to observe transient astronomical events from anywhere in the world. To augment this capability, the United States and German Space Agencies (NASA and DLR) are collaborating in plans to replace the KAO with a 2.5 meter telescope installed in a Boeing 747 aircraft: SOFIA -- The Stratospheric Observatory for Infrared Astronomy. SOFIA's large aperture, wide wavelength coverage, mobility, accessibility, and sophisticated instruments will permit a broad range of scientific studies, some of which are described here. Its unique features complement the capabilities of other future space missions. In addition, SOFIA has important potential as a stimulus for development of new technology and as a national resource for education of K-12 teachers. If started in 1996, SOFIA will be flying in the year 2000.

1. Airborne Astronomy -- the Legacy

For nearly three decades astronomers have been making infrared observations from aircraft based at NASA's Ames Research Center in California. In 1965 Gerard Kuiper used the NASA Convair 990 to show that the clouds of Venus were nearly devoid of water, demonstrating the advantages of airborne observations in the near infrared. In 1968 the Ames Learjet was used by Frank Low to measure far infrared luminosities of Jupiter and Galactic nebulae. In 1974 the Kuiper Airborne Observatory (KAO) started its now 20 year career as a national facility for astronomy. Its users have produced ~1000 refereed publications, and ~50 Ph.D. theses for students at U.S. and foreign universities; it also supports kindergarten through high school teacher outreach programs. A good review of the KAO program and its contributions to science, education, and technology is given by Larson (1994).

Most infrared radiation from astronomical objects which never reaches the ground is detectable from the lower stratosphere. This fact is the principal justification for an airborne telescope. Figure 1 plots computed atmospheric transmission (Traub & Stier 1976) as a function of wavelength for aircraft (14 km) and mountain-top (4 km) altitudes. The absorption is largely due to water vapor, with significant contributions from carbon dioxide and ozone in some wavelength bands. The model assumes overhead water quantities of 2.3 µ m. for the aircraft and 1.2 mm. for the ground-based telescope, and a zenith angle of 60 degrees. From 14 km, the broadband atmospheric transmission is adequate ( > 70%) for photometric observations at most infrared wavelengths, but the emissivity limits detector sensitivity due to the fluctuations in the arrival rate of the photons from the sky. A number of the water lines in the far infrared are still saturated at aircraft altitudes, as shown in the lower panel. Between these lines the transmission can exceed 95%, the emissivity is correspondingly low, and so high resolution spectrometers can achieve sensitivity limited principally by the emission of the telescope.

Despite the numerous saturated atmospheric lines at aircraft altitudes, most important astronomical spectral features can be measured from an airborne observatory. To demonstrate this, we list in Table 1 the spectral features originating in the interstellar medium (ISM) which have been observed from the KAO. These features characterize important phases of material in the ISM: molecules, neutral and ionized atoms, and solids, as discussed in Section 3. Some of the research results from the KAO are presented or reviewed in the Proceedings of the Airborne Astronomy Symposium. The extensive scientific and technical heritage of the airborne program, and particularly our experience with the KAO, provides a solid basis to project the performance and the design of the next generation airborne observatory -- SOFIA.

2. Characteristics and Performance

SOFIA's characteristics are summarized in Table 2 below. The large aperture and wavelength range, routine accessibility to most infrared wavelengths, and mobility are unique features of the observatory (Caroff 1994). Relative to the KAO, SOFIA will be roughly ten times more sensitive for compact sources, enabling observations of fainter objects and measurements at higher spectral resolution. Also, it will have three times the angular resolving power for wavelengths greater than about 10 µ m., permitting more detailed imaging throughout the far infrared.

Table 2
Summary of Basic SOFIA Characteristics

The anticipated performance of SOFIA is indicated as a function of wavelength in Figures 2 through 5.

Figure 2 shows the expected image quality, which is limited by seeing from the air flow over the telescope cavity at visible and near infrared wavelengths, and by diffraction at long wavelengths. The specified performance of the optical system limits the image quality in the ~4 - 10 µ m. range.

Figure 3 shows the anticipated photometric sensitivity per pixel, which is simply scaled from the performance achieved on the KAO. Here ``PSC'' and ``FSC'' refer to the IRAS Point Source and Faint Source Catalogues, respectively. We see that SOFIA would be able to observe any of the compact infrared sources in these catalogues.

High resolution spectrometers are expected to be available for most of the wavelength range of SOFIA, as indicated in Figure 4. Interstellar lines are typically broadened to a km/sec or more, whereas higher resolving power can be useful for study of solar system objects.

Spectroscopic sensitivity, shown in Figure 5, corresponds to the spectral resolving power shown in Figure 4. Narrow lines emitting more than 0.1% of the total continuum emission from the IRAS sources should be detectable. The important cooling lines of neutral oxygen at 63 µ m. and C+ at 158 µ m. from photodissociation regions are typically this strong relative to the continuum. Shocked interstellar gas will produce a higher line to continuum ratio.

(continued in Section 3. SOFIA Science)