scientific results, including the discovery of celestial x-ray sources, the measurement of the dipole anisotropy of the microwave background, discovery of variable gamma-ray emission from the galactic center, and critical infrared and gamma-ray measurements of Supernova 1987A, have come from the suborbital program.

The Kuiper Airborne Observatory (KAO), a 0.9-m telescope operating for 15 years in a C-141 aircraft, opened up far-infrared and submillimeter wavelengths to scientific investigation, produced over 700 scientific papers, and trained 40 PhD students. The importance of the suborbital program for the training of instrumentalists is exemplified by the fact that 80 percent of the U.S. science team on the successful IRAS program had worked previously on the KAO. As discussed in Chapter 1 and Chapter 4, one of the major recommendations of this report is to fund the Stratospheric Observatory for Far-Infrared Astronomy (SOFIA) as a successor to the KAO.

International Collaborations

Astronomy has traditionally been an international enterprise, and as space missions have become more complex, collaborations with foreign colleagues have made possible important programs that otherwise would have been unaffordable. NASA has used resources from the Explorer program to support U.S. scientists to fly instruments on foreign spacecraft in the absence of flight opportunities on American spacecraft. The ROSAT x-ray telescope was launched in 1990 as a collaboration between Germany, the United Kingdom, and the United States. ROSAT 's high-sensitivity survey will produce an all-sky catalog of more than 100,000 galactic and extragalactic x-ray sources. Following the survey, U.S. and European researchers will carry out pointed observations of many known and newly discovered objects. U.S. instruments are also planned for the Japanese-U.S. ASTRO-D mission, the Soviet-French Spectrum X-Gamma, and the ESA's X-ray Multi-Mirror Mission. Other important missions include radio interferometry from space with the Japanese VSOP and the Soviet RadioAstron missions and a proposed NASA-ESA collaboration on an orbiting submillimeter telescope. Participation by U.S. scientists in ESA's Infrared Space Observatory (ISO) will provide valuable astrophysical data and help define SIRTF's scientific program. In Chapter 1, the committee recommends a new budgetary line to cover the costs of international collaborations carrying U.S. instruments, an activity that currently is supported out of the Explorer program.

Shuttle Payloads

After the Challenger accident, NASA revived the mixed-fleet philosophy that utilizes unmanned boosters, like the Deltas. The committee strongly endorses this strategy of making unmanned boosters available as the main

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