The SOFIA program: astronomers return to the stratosphere

Abstract

The Stratospheric Observatory for Infrared Astronomy (SOFIA) is the next generation of airborne astronomical observatories. Funded by the US and German space agencies, SOFIA is scheduled for science flights beginning in late 2005. The observatory consists of a 747-SP modified to accommodate a 2.7-m telescope with an open port design. Academic and government laboratories spanning both the US and Germany are developing science instruments for SOFIA. Using state-of-the-art technologies, SOFIA will explore the emission of astronomical sources with an unprecedented level of angular resolution (θ[arc-sec]=0.1 × wavelength [μm]) and spectral line sensitivity at infrared and sub-millimeter wavelengths. The current status of SOFIA is available from the observatory web site at http://sofia.arc.nasa.gov and is updated frequently.

Introduction

The Stratospheric Observatory for Infrared Astronomy (SOFIA) is a joint effort by the US and German space agencies to develop the next generation airborne infrared observatory (Becklin, 1997; Erickson and Davidson, 1995). After almost two decades of study and development, the SOFIA program is on schedule for first light in the fall of 2005. The SOFIA team consists of international participants from government, industry, and academia.

The 21 year history of science missions aboard the Kuiper Airborne Observatory (KAO) demonstrated the unique capabilities of stratospheric observations at wavelengths inaccessible from the ground. SOFIA provides scientists with a powerful suite of state-of-the-art instrumentation for infrared astronomical imaging and spectroscopy. The observatory's nasmyth configuration telescope with an optical quality primary mirror is expected to produce excellent, diffraction limited imaging at mid-infrared wavelengths and longer.

A combined US and German instrument program enables sensitive spectroscopic measurements covering a wide range of spectral resolutions. Observations aboard SOFIA range from high-speed occultation instruments in the optical to bolometers, photoconductors, and heterodyne mixers operating at longer far-infrared and sub-millimeter wavelengths. The top-level characteristics expected for SOFIA in 2005 are listed in Table 1. Compared to space-based missions, the SOFIA program provides researchers with project development time scales suitable for graduate student and postdoctoral research programs.

Astronomical observations at infrared wavelengths reveal an otherwise hidden universe. Celestial sources emit infrared radiation through a wide variety of physical processes that includes thermal continuum emission and the characteristic line emission of atoms, molecules, and larger biogenic macromolecules. SOFIA science topics include the study of interstellar gas and dust in our Galaxy and other more distant galaxies.

As interstellar gas collapses to form stars, observations at infrared wavelengths can pierce the dusty veil of the surrounding gas cloud to view the centrally concentrated collapsing core. Imaging observations place constraints on the frequency, energetics, and spatial extent of these regions. Spectroscopic observations provide details of the collapse kinematics along with the abundance of excited atoms and molecules within the in-falling envelope. The abundance of atomic and molecular species in many cases depends upon the energetics and history of the collapsing gas cloud. In addition to the study of star formation, the SOFIA science program will include the study of planet forming disks, planetary atmospheres, comets, and other solar system type objects.

SOFIA observations should also address studies such as the nature of our own Galactic center where a massive black hole is widely surmised to exist. Research studies can further compare our Galactic center to the central regions of other galaxies where signatures of black hole accretion are considerably more volatile.

With refinements in instrument sensitivities, SOFIA should detect and resolve the faint emission of the first galaxies at sub-millimeter wavelengths. The SOFIA program provides a broadly based scientific tool set for studies of many obscured and yet to be resolved star forming regions (Krabbe and Röser, 1999).