obtain in a single measurement complete information on energy-level popula- 

 tion distributions and hence to infer densities, temperatures, and total abun- 

 dances accurately for material that is either sufficiently warm to excite these 

 transitions or that lies in front of a strong continuum source. Such information 

 is more difficult to obtain from isolated measurements of individual rotational 

 transitions in the radio regime, although this approach is necessary for the typi- 

 cally cold interstellar clouds. In the infrared, submillimeter, and millimeter spec- 

 tral regions, it is possible from space to observe chemically important species like 

 oxygen and water that cannot readily be studied from Earth because their spec- 

 tra are obliterated by atmospheric absorption. 



Spectrometers of high spectral resolution (with resolving power X/AX ~ 10 5 ) 

 are needed for observations of molecular lines to obtain information of diagnos- 

 tic value. High spatial resolution is also desirable to investigate phenomena 

 occurring both at distant locations within the Milky Way and in other galaxies. 

 The study of protoplanetary systems and accretion shocks surrounding them in 

 star-forming regions provides a particularly stringent requirement on spatial 

 resolution: ~0.01 arcsec (see Appendix C). For the immediate future, this may 

 not be available. To achieve both high spectral and high spatial resolution simul- 

 taneously requires extremely large apertures (and/or interferometers) to collect 

 enough photons. 



The orbital observatories will permit access to the far-infrared and submilli- 

 meter regions of the spectrum that have previously been mostly unexplored. The 

 required data base of fundamental frequencies and preferred molecular config- 

 urations does not now exist to guide the conduct of observations or the inter- 

 pretation of data from this wavelength regime. In order to take full advantage of 

 orbital observation opportunities, a strong ground-based laboratory research 

 program must be pursued. The infrared and submillimeter spectra of many 

 biogenic compounds are extremely rich, and a very high degree of accuracy is 

 required to allow chemical specificity. Laboratory studies at low pressures and 

 temperatures representative of the interstellar environment can provide definite 

 predictions of spectral characteristics. However, theoretical calculations are also 

 required to deduce the frequencies of the most probable transitions for mole- 

 cules and radicals which are too reactive or otherwise not amenable to ordinary 

 laboratory techniques. This becomes both increasingly more difficult and more 

 important with increasing molecular weight. Laboratory study is also needed to 

 understand the properties of the dust grains that were cycled through many 

 phases of the stellar and interstellar media before being incorporated into the 

 solid bodies of the solar system. These grains may be a primary source of carbon, 

 and in addition the molecular mantles manufactured on the grain surfaces in the 

 molecular cloud complexes may provide a significant source of highly processed 

 organic compounds that could find their way into other protostellar nebulae and 

 strongly influence their future evolution toward life. 



28 



