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August 2007 Program Summary Article (Becklin, Tielens, Gehrz & Callis, SPIE proceedings) (pdf file)

2005 SOFIA Science Cases

Overview of SOFIA

Observatory Capabilities and Characteristics

The Stratospheric Observatory for Infrared Astronomy (SOFIA) is a 2.5-meter telescope carried aboard a Boeing 747-SP aircraft. It is the successor to the much smaller Kuiper Airborne Observatory (KAO), which was operated by NASA from 1974 to 1996. A collaborative project between NASA and the German Space Agency (DLR), SOFIA is operated by USRA, and the aircraft is currently based at the Dryden Flight Research Center in Edwards, CA. The SOFIA Science Center is located at the Ames Research Center in Mountain View, CA. The aircraft is maintained and flown by NASA. The telescope and scientific instruments have been designed to make sensitive, high-angular resolution measurements of astronomical objects over a wide wavelength range (0.3 microns - 1.6 mm). SOFIA will be NASA's and DLR's premier observatory for infrared and submillimeter astronomical observations for the next decade.

The aircraft will fly at altitudes of approximately 41000 feet, where the atmosphere is much more transparent than at ground level. The telescope is located in a open cavity in the aft section of the aircraft and has a view out of the port side of the aircraft. It telescope has a limited range of motion: Observations are restricted to elevations between +20 and +60 degrees. The telescope is inertially stabilized and the pointing accuracy is expected to be better than 1 arcsec and tracking should be better than 0.5 arcsec. Typical seeing is expected to be diffraction limited at all wavelengths longward of 15 microns. Because of the telescope motion limitations and the fact that SOFIA is a flying observatory, planning observations is far more difficult than at ground-based observatories. Flight restrictions constrain the amount of time possible to spend observing any given object, and moving to a new object requires turning the aircraft and embarking on a new flight path. The observatory will be flown three times per week (Mon-Thurs), for approximately eight hours each flight, and 44 weeks each year. SOFIA is expected to have a lifetime of 20 years.

Instrumentation

The first generation of science instruments for SOFIA consists of 9 imagers and spectrographs spanning the entire wavelength range. The instruments are divided into 3 classes: Facility Science Instruments (FSIs), which are available for use by any observer (General Investigator); Principal Investigator Science Instruments (PSIs), which are built and maintained by a PI team, who have exclusive use of them; and a Special Purpose Principal Investigator Science Instrument (SSI). An observer who wishes to use a PSI or SSI must be collaborating with and have the approval of the instrument team, as the latter will be responsible for providing support. Except for one combination of instruments, only one science instrument will be flown at any given time. Science instruments will be changed at most only once every other week. In addition to these instruments, SOFIA will carry a water vapor monitor, which will report the precipitable water vapor level along a fixed line of sight. The resulting opacity values will be used to calibrate data acquired with the science instruments.

Observing Program

Observing time is awarded by a peer review process. The US and international astronomical communities are granted 80% of the science observing time; the remaining 20% of the time will be allocated to the German astronomical community. Most observations will be made in either of two modes: visitor and queue. Visitor observing mode entails observations made by a General Investigator (or the Principal Investigator team for one of the instruments) aboard the aircraft. Queue mode entails observations made by members of the SOFIA staff for an observer who is not present on the aircraft. There will also be some limited time available for Service Mode observations, which consist of very short observations (<3 hours in length) taken by the observatory staff. Some limited amount of time will also be available for Target-of-Opportunity proposals.

Scientific Promise and Potential

SOFIA will open a new era in astronomy: it will offer astronomers a unique platform, providing regular access to the entire MIR/FIR wavelength range between 5µm and 300 µm, a large part of which is completely inaccessible from the ground. Offering both imaging and spectroscopy, SOFIA is designed to complement both large ground-based telescopes (e.g., Keck, Gemini, JCMT) and the current and future generation of space-based telescopes (Spitzer, Herschel, JWST). For example, SOFIA allows access to wavelengths beyond the ~200 micron limits of the Infrared Space Observatory (ISO) and Spitzer Space Telescope. Although SOFIA will not have the sensitivity of Spitzer, it will image to the "confusion limit" of Spitzer at 160 microns in one hour with a beam area that is a factor of 10 smaller. SOFIA's spatial resolution in the far-infrared matches that of 10-15 meter ground-based submillimeter telescopes (e.g., CSO and JCMT). Hence SOFIA will be used to follow up ground-based and Spitzer observations. Many of the instruments used on SOFIA will be forerunners of those on board Herschel, and therefore SOFIA will be employed to prepare for, or follow up, Herschel observations. Since SOFIA's 20 year lifetime is substantially longer than that of either Spitzer or Herschel, follow up studies with SOFIA will continue long after these satellite missions have ended.

As demonstrated by the KAO (Kuiper Airborne Observatory), IRAS (Infrared Astronomical Satellite), ISO (Infrared Space Observatory), and Spitzer, infrared radiation characterizes a multitude of rich and varied physical processes, and reveals astronomical phenomena occurring in otherwise hidden regions of the cosmos. SOFIA will extend this scientific legacy by means of sensitive, high spectral and spatial resolution observations spanning the infrared domain. Topics to be addressed by SOFIA observations include:

• Interstellar cloud physics and star formation in our galaxy.
• Proto-planetary disks and planet formation in nearby star systems.
• Origin and evolution of biogenic atoms, molecules, and solids.
• Composition and structure of planetary atmospheres and rings, and comets.
• Star formation, dynamics, and chemical content of other galaxies.
• The dynamic activity in the center of the Milky Way.
• Ultra-luminous IR Galaxies (ULIRGS) as a key component of the early universe.

In addition, SOFIA will have a major role in the development of observational techniques and new instrumentation and in the education of young scientists and teachers in the discipline of infrared astronomy.

Various members of the SOFIA science team and astronomical community have generated representative science programs that could be carried out with SOFIA.

A compelling scientific case can be made for flying SOFIA during the post-Herschel era. The complementarities and synergies between these two observatories are discussed in the attached White Paper.

Additional Information

Additional information regarding the SOFIA program and instruments is available on the Selected Publications page.

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