other stars, interstellar clouds, and distant radio galaxies and quasars. The arcsecond imaging of the VLA will complement the thousandth-of-an-arcsecond capability of the Very Long Baseline Array (VLBA) when the latter is completed in 1992. The VLA extension will add four new telescopes and ancillary hardware to bridge the lack of baselines between telescopes needed for angular resolutions between a tenth and a thousandth of an arcsecond in the performance of the VLA and the VLBA.

Space-based Projects

Dedicated Spacecraft for FUSE. The committee recommends that NASA augment the Explorer program sufficiently to convert the Far Ultraviolet Spectroscopy Explorer into a Delta-launched experiment using its own dedicated spacecraft. In 1989, NASA selected FUSE for development in the Explorer program. This committee, like the Field Committee a decade ago, believes FUSE will produce extraordinary scientific results. The committee is concerned that continued linking of FUSE to the Shuttle program through a reusable Explorer spacecraft will unnecessarily delay the mission and increase its cost. A dedicated spacecraft will enhance the scientific potential of the FUSE mission, providing an optimized orbit, simpler operations, and a longer mission lifetime.

SOFIA. The proposed Stratospheric Observatory for Far-Infrared Astronomy (SOFIA) is a 2.5-m telescope mounted in a Boeing 747 aircraft and optimized to study infrared and submillimeter wavelengths from above most of the earth's water vapor. SOFIA provides the highest-resolution spectroscopy of any planned facility for wavelengths between 30 and 300 microns, and its large aperture will yield high-spatial-resolution observations that will complement SIRTF's capabilities. In addition to its scientific value, SOFIA will be an excellent facility for the training of the next generation of experimentalists and for the rapid development of advanced instrumentation. The predecessor to SOFIA is the Kuiper Airborne Observatory (KAO), which has trained some 40 PhD astronomers in an environment that subjects both students and instrumentation to many of the rigors of space missions. SOFIA represents the natural evolutionary replacement for the KAO and is a joint project with Germany.

Explorers. The committee believes that flying six Delta-class Explorer missions for astrophysics in the next 10 years will play a key role in revitalizing space astronomy. Each of the panels dealing with space observations [Working Papers (NRC, 1991)] identified examples of forefront science that could be carried out with such experiments. The missions should be chosen through the peer review process, with specific emphasis on modest scope, rapid execution, and careful cost control. The active involvement of the selected science team in all phases of the program is critical to the success of these missions.

Astrometric Interferometry Mission. Astrometry, which is concerned with



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