5. Calculation, using results from 3 and 4, of a flux rate for xenobionts 

 reaching the Earth's surface, permitting determination of the experimental 

 requirements that would be needed for detecting by direct measurement any 

 putative xenobionts 



One critical problem in carrying out the above scenario is that material col- 

 lected in Earth orbit (Chapter 4) may also contain viable organisms derived from 

 Earth (perhaps ejecta attached to volcanic debris). Thus, it would be extremely 

 important to determine the flux rate of organisms from Earth into space. The 

 final result of such studies might be that viable organisms found in Earth orbit 

 are most likely to have been derived from the Earth's biosphere. A direct micro- 

 biological test of panspermia with existing tools is difficult but possible: for 

 example, one could search for the presence of novel amino acids or nucleosides. 



Microbial survival is one of several parameters that can be used to put proba- 

 bility boundaries on the hypothesis of panspermia. While one can hypothesize 

 that the evolution of life on other planets might have led to organisms more 

 tolerant to the conditions of interstellar space than the terrestrial creatures with 

 which we are familiar, such a hypothesis cannot be tested at present. This dis- 

 cussion is thus organized around the notion that extraterrestrial organisms will 

 be fundamentally similar to those found on Earth. The proposed experiments 

 are concerned with determining the survival potential of terrestrial organisms 

 subjected to conditions characteristic of space. Since the major environmental 

 variables in space are radiation flux (particularly ultraviolet), vacuum condi- 

 tions, and low temperatures, these parameters should be critically examined. The 

 studies can be organized around three general areas or questions: 



1. To what extent does an organism's physiological state affect its survival 

 potential? 



2. What is the range of resistance to ultraviolet and other injurious factors 

 among microbes isolated from terrestrial and aqueous environments? 



3. What are the critical factors that affect any resistance to the conditions 

 found in space? 



Some experimental work has been done in each of these areas, including carrying 

 microbes into space aboard Spacelab, but additional ground-based studies are 

 needed to enable proper planning for further in situ space-based experiments. 

 Ground-based studies have the advantage that they are relatively inexpensive and 

 the experimental design is flexible. Thus, use of space simulations and atmo- 

 spheric controls should allow various hypotheses dealing with space survival to 

 be tested. These hypotheses can be expected to evolve to a point at which they 

 could be developed as experiments or experimental programs suitable for further 

 testing in space. However, since a satisfactory, ground-based simulation of the 

 complex interplay of all of the environmental factors of outer space is difficult, 

 if not impossible, to attain, the performance of experiments on living micro- 

 organisms in Earth orbit is required to properly study the viability of microbes 

 in space. 



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