Further benefits accrue due to the possibility for improved spatial resolution 

 (fig. 3-3). The resolution of a telescope in orbit is limited by the quality of its 

 optics rather than by the stability of the atmosphere. Also, in the near-weightless 

 environment of low Earth orbit, larger collecting areas are possible, providing 

 higher spatial resolution and greater sensitivity. Spatial resolution can also be 

 improved by constructing interferometers with much longer baselines than are 

 possible on the ground. Sensitivity at wavelengths for which thermal emission 

 is important can be improved in Earth orbit relative to the ground by utilizing 

 cryogenically cooled telescopes and receivers. The decreased levels of such 

 human-caused "noise" as light pollution and radio frequency interference (RFI ) 

 also can lead to improved sensitivity in orbit. Note that many of the improve- 

 ments gained by placing a telescope in low Earth orbit are magnified by going to 

 a lunar base, particularly if it is located on the Moon's far side. 



Although gamma-ray measurements are useful for tracing the broad distri- 

 bution of interstellar matter and cosmic rays in the galactic plane, and although 

 X-ray measurements are useful for studying the very active phases in the devel- 

 opment of recently formed stars, they preferentially sample the hottest and 

 most energetic environments, as revealed by electronic transitions within atomic 

 species. Since these wavelengths cannot directly investigate the biogenic ele- 

 ments in their more complex molecular configurations, their utility for exobiol- 

 ogy is less immediate than other wavelengths. Absorption line studies in the 

 ultraviolet of regions rich in molecules and dust particles are impossible at large 

 distances because of extinction by dust. Even with the high-resolution spectro- 

 graph on the Space Telescope, only relatively transparent regions with visual 

 extinctions A v < ~4 magnitudes (corresponding to foreground hydrogen column 

 densities < ~7X10 21 cm" 2 ) will be accessible in the ultraviolet at high resolu- 

 tion. Even so, these thin regions can yield valuable information about elemental 

 abundances, depletion of elements from the gas onto solids, and the chemistry 

 of small molecules. Of greatest value to exobiology will be the exploitation of 

 those parts of the spectrum in the infrared, submillimeter, and millimeter regions 

 that are inaccessible from Earth. These spectral regions typically bracket the 

 condition where the excitation temperature of the molecular transitions can be 

 provided by the ambient radiation field (h^ = kT ex ) for many chemical species 

 of exobiological interest for a wide variety of environments. Thus both absorp- 

 tion and emission can be observed, depending on the physical conditions. The 

 infrared region from 2 to 20 /urn has many diagnostic advantages for studying 

 dilute matter. Chemically important, nonpolar molecules such as methane, 

 acetylene, and carbon dioxide, which lack strongly allowed radio spectra, have 

 strong vibrational transitions in this region of the spectrum. They are among the 

 simplest molecular forms of carbon, the central element of exobiology, and thus 

 they are missing links in our understanding of the chemical evolution of inter- 

 stellar and circumstellar matter. Observation at high spectral resolution can 

 provide the rotational structure of a vibrational band. It is thus possible to 



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