program for ground-based astronomy. The committee notes that the NSF Subcommittee on Ground-based Optical and Infrared Astronomy, of the Advisory Committee for Astronomical Sciences, has highlighted digital archiving as a key recommendation.

COMPUTERS AND THEORETICAL ASTROPHYSICS

The light from many astronomical objects is produced by violent, complicated, and quickly evolving phenomena. Sophisticated simulations are often needed to understand, for example, shock waves around protostars and jets from the cores of active galaxies, but these simulations are frequently oversimplified due to a lack of sufficient computer resources. In the coming decade, more realistic simulations will play an essential role in understanding the underlying physics of these phenomena. The 1990s will be the decade in which a number of long-standing astrophysical problems will be solved, and computers will play an important role in these solutions.

Astrophysics depends on theory and modeling to a greater degree than most other physical sciences, because astronomers can observe only remotely. Moreover, the observed phenomena, the photons and fast particles that escape from astrophysical objects, are typically the result of complicated interactions among nonlinear processes. It is often necessary to construct elaborate models to achieve a satisfactory interpretation of the observations. More powerful computers and computer programs that incorporate realistic physics will greatly increase the ability of astrophysicists to extract physical insight from their observational data.

The committee estimates that 10 percent of practicing astronomers are engaged in theoretical simulation of astrophysical phenomena. Some of this computational astrophysics uses local workstations and mini-supercomputers. Roughly 10 percent of the time devoted to scientific computing at the NSF Supercomputer Centers is used by astrophysicists who are using supercomputers to solve problems that push the system to the limits of today's software and hardware capabilities (Plate 5.1 and Plate 5.2).

The observational community has made concerted efforts at developing community software; commendable examples include NSF's development at its national observatories of AIPS, IRAF, and the Flexible Image Transport System (FITS) image format. Comparable efforts should be made in the area of theoretical astrophysics. A wide range of theoretical techniques must be developed to meet different problems in astrophysics, but it is difficult to predict a priori which techniques will be most useful. Large programs for stellar evolution, radiative transport, magnetohydrodynamic simulations, and characterization of material properties such as equations of state and grain opacities can take tens of person-years to develop. Community modeling tools



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