FIGURE 6.1 An artist's conception of an interferometer consisting of three telescopes operating at optical and infrared wavelengths on the moon. Light from the three telescopes is combined in the central building.

5- to 100-millionths-of-an-arcsecond resolution in the visible and near-infrared wavelengths, 0.2 to 5 µm. Astrometric observations could be made with high precision over this wavelength region.

Five passively cooled, 1.5-m telescopes operating together in this wavelength region could revolutionize many research topics in astronomy by combining high sensitivity with unprecedented spatial resolution (Figure 6.1). Such an instrument could map protoplanetary disks around young stars in the constellation of Taurus with a resolution better than 0.004 of the earth-sun separation, more than enough resolution to find gaps in the disks indicative of the presence of forming planets, to detect planets around stars out to a few thousand light-years by astrometric motions, to measure distances and motions of stars and star-forming regions in nearby galaxies, and to resolve the environment around the energy sources of quasars.

The full power of such an instrument would be difficult to realize on the earth because of the effects of the terrestrial atmosphere, and difficult to achieve in orbit because of the precise separations and orientations that would