space science. In contrast to the fruitful 1970s, only two American astronomical satellites were launched in the 1980s, and leadership in areas the United States had pioneered, such as x-ray astronomy, moved to Europe, the Soviet Union, and Japan. The currently planned program in space astronomy, described in the Strategic Plan (NASA, 1988, 1989) for NASA's Office of Space Science and Applications, can reverse this trend. NASA's plan includes telescopes that range from small payloads like the Submillimeter Wave Astronomy Satellite to be launched in 1995, to the four Great Observatories (discussed below). This section describes the ongoing program in space astronomy that underpins the recommendations for new projects made in Chapter 1.

The Great Observatories

The first Great Observatory, the Hubble Space Telescope (HST), was launched in April 1990. The second, the Gamma Ray Observatory (GRO), is currently scheduled for launch in 1991, after the publication of this report. Construction of the third, the Advanced X-ray Astrophysics Facility (AXAF), the highest-priority major new program of the Field Committee in 1982, is under way. As discussed in Chapter 1, this committee endorses the plan to complete the Great Observatories, which are essential for reaching the frontiers of the universe. The Space Infrared Telescope Facility (SIRTF) is the only Great Observatory awaiting initiation and is this committee's highest-priority large equipment initiative for the decade of the 1990s.

HUBBLE SPACE TELESCOPE

A manufacturing flaw in the primary mirror of the HST will prevent it from forming images of faint objects with a resolution greater than about 1 arcsecond, until after the installation of a second generation of instruments with correcting optics. But important observations will be possible with HST even with reduced performance. The General Observer program for using the HST observatory, which is conducted by the Space Telescope Science Institute, makes possible the use of HST's frontier astronomical facilities by U.S. and foreign scientists. The Hubble Fellowship Program for recent PhDs helps to train some of the best young researchers in space astronomy.

At this stage, it appears likely that close to the full potential of HST can be achieved by installing either new instruments or corrective optical elements, although there is no guarantee that all technical and practical problems will be overcome. A replacement, with corrective optics, for the general-purpose Wide-Field/Planetary Camera (WF/PC) is under construction and is scheduled for installation in 1993; the replacement camera is essential to carry out the fundamental goals of this observatory.

In addition to the improved camera, NASA selected in 1988 two other new



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