system operating in space, where no limits to the coherence size, angle, or time are imposed by the terrestrial atmosphere. NASA has been studying a number of possible space missions as a first step in interferometry from space. A concept that appears particularly appealing is to use interferometric techniques to achieve a 1,000-fold improvement in our ability to measure celestial positions. The mission requirement would be to measure positions of widely separated objects to a visual magnitude of 20 with a precision of 30 millionths of an arcsecond; a more challenging goal would be to measure positions with a precision of 3 millionths of an arcsecond. The Astrometric Interferometric Mission (AIM) would permit definitive searches for planets around stars as far away as 500 light-years through the wobbles of the parent star, trigonometric determination of distances throughout the galaxy, and the study of the mass distributions of nearby galaxies from stellar orbits. AIM would demonstrate the technology required for future space interferometry missions.

Large Earth-based Solar Telescope

The Large Earth-based Solar Telescope is a solar telescope with a 2.4-m aperture that would use adaptive optics to increase the spatial resolution of solar observations. It would be the premier terrestrial telescope for high-resolution solar observations at optical and near-infrared wavelengths. With LEST, it will be possible to investigate in unprecedented detail the interactions of magnetic fields and turbulent motions under way in the solar surface and overlying atmosphere, which are responsible for the hot solar corona, the solar wind, solar flares, and solar-terrestrial phenomena.

The program will be a cooperative international venture among nine countries. It will combine U.S. expertise in adaptive optics and instrument design with European contributions of an outstanding site in the Canary Islands and a major share in the costs of construction. The committee endorses a plan in which the United States would pay one-third of the construction and operation costs of LEST in return for a proportionate share of the observing time.

VLA Extension

The Very Large Array can currently produce images with better-than-arcsecond resolution. This will be complemented by the better-than-a-thousandth-of-an-arcsecond resolution of the Very Long Baseline Array (VLBA) when it is completed in 1992. There will still remain, however, a gap between the capabilities of the VLA and the VLBA that will restrict the performance of the combined instruments at intermediate resolutions between about 0.01 and 0.1 arcsecond. A plan to bridge this gap would be carried out in three phases: (1) supply the VLA with VLBA tape recorders, (2) build four new antennas to provide intermediate spacings between the VLA and VLBA, and (3)



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