submillimeter-wave telescopes. However, infrared spectra are especially impor- 

 tant in determining complex molecular signatures; because of atmospheric inter- 

 ference, such space-based observatories as SI RTF and LDR will be crucial in 

 this regard. A high-resolution spectrometer on SI RTF would be especially useful 

 for precise molecular identification. A vigorous program of laboratory spectro- 

 scopic studies is necessary to complement the observational program because the 

 laboratory spectra of many important interstellar molecules have not yet been 

 fully studied. 



The availability and relative abundances of biogenic elements can be deter- 

 mined by observational studies directed at either atoms or molecules. The deter- 

 mination of the relative abundances of carbon and oxygen in various sources is 

 important because it is thought by some investigators that molecular complex- 

 ity can occur only in carbon-rich regions. An inventory of the biogenic elements 

 in external galaxies would be especially interesting and would be aided by 

 space-based observatories in all wavelength regions. 



Finally, the existence of water and organic compounds can be investigated 

 via spectroscopic studies in a variety of wavelength regions, especially the sub- 

 millimeter and infrared, depending upon the excitation conditions in the source. 

 It is obvious that space-based observatories such as HST, LDR, and SI RTF will 

 aid immeasurably in the search for these species. Whether molecules such as 

 these are widespread in our own and other galaxies is intimately related to the 

 probability that life exists elsewhere. 



6.1 .5 Time Scales 



The order in which observations of interest to exobiologists are performed 

 and the pace at which our understanding increases will be determined by the 

 timetables established within NASA and ESA for the development of the orbital 

 facilities identified earlier. The following list represents the current best estimate 

 of the order of implementation of the telescopes judged most useful to observa- 

 tional exobiology. The observations listed under each spacecraft reflect the 

 priorities outlined above. 



Hubble Space Telescope (HST): NASA 1989 



1. Astrometric and coronographic searches for extrasolar-system planets to 

 be augmented by second-generation infrared instrumentation in 1994 



2. Ultraviolet observations of young, stellar objects— determination of the 

 radiation environment in early stages of planetary systems 



3. Visible and near-infrared observations of grains in stellar atmospheres 



4. High-resolution spectroscopy (ultraviolet/visible) of planetary atmo- 

 spheres, comets, and asteroids; imaging of planets at wavelengths of specific 

 molecular bands 



105 



