Station would relieve any requirement that it be transported to orbit in a fully 

 integrated way. Finally, the Space Station will provide a stable platform from 

 which to conduct observations for the decades that are required to accomplish 

 a systematic astrometric search of a large number of stars. Because the stellar 

 position is measured only relative to the reference stars in the surrounding field, 

 and not absolutely, an astrometric telescope is uniquely immune to nearly all of 

 the mechanical perturbations introduced by manned use of the Station. It is 

 therefore one of the few astronomical tools that can be considered for an 

 attached payload— the most cost-efficient way to interact with the Space 

 Station. 



In the hopeful event that planets begin to be identified around nearby stars, 

 the next task will be to attempt to discern whether some form of life might 

 exist there. New technologies and a new generation of dedicated orbital instru- 

 mentation would be required to tackle this problem. Some very preliminary 

 suggestions have been made; they require either an interferometer or a very large 

 single aperture operating at visible wavelengths. If the separation of the 

 ~1-m elements of the interferometer can be maintained to within 10~ 6 cm 

 for periods of ~100 hours, then it should be possible to image the atmosphere 

 surrounding the distant planet and spectroscopically detect the ozone band near 

 6800 A, assuming the concentration of ozone is similar to the very large non- 

 equilibrium (biologically generated) value of the terrestrial atmosphere. Require- 

 ments on the surface accuracy of a large monolithic mirror greatly exceed those 

 achieved for the HST. If such a mirror quality can be achieved and combined 

 with occulting masks that suppress the bright stellar image, similar searches for 

 2 and ozone could be conducted. Near-term ground-based studies should begin 

 on these or any other approaches to explore the potential for developing the 

 technologies required to meet what today appear to be impossible specifications. 



Suggestions for Further Reading 



Black, D. C; and Brunk, W. C, eds.: An Assessment of Ground-Based Tech- 

 niques for Detecting Other Planetary Systems. NASA CP-21 24, 1 986. 



Tarter, J. C; Black, D. C; and Billingham, J.: Review of Methodology Avail- 

 able for the Detection of Extrasolar Planetary Systems. J. British Interplanetary 

 Soc, vol. 39, 1986, p. 41 8. 



3.9 Molecules in Space 



There are two reasons for the relevance of interstellar chemistry to exobiol- 

 ogy. First, the presence of large amounts of fairly complex organic molecules 

 (up to at least 1 3 atoms in size) in interstellar clouds raises the question of what 

 the limit is to the complexity attainable by interstellar molecules. Second, if 



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