Lunar scientists took the opportunity, and the flowing money associated with Apollo, to endow the SIM bay with other capabilities in addition to its photographic coverage. The surface
A frame from Apollo 15's panoramic camera of Hadley Rille.
crews were able to provide detailed knowledge of a small area whose 'ground truth' could be used to calibrate and contextualise measurements of the Moon's composition taken across a wide area from an orbiting spacecraft.
Methods of determining the composition of distant astronomical bodies were worked out by astronomers in previous centuries. In simple terms, they relied on the property of substances to radiate or absorb light in precise wavelengths, or energies. To the eye, each substance appeared to have a characteristic colour which, when spread out into a spectrum by a spectrograph, revealed patterns of lines that acted as a fingerprint of that substance. Spectra for common elements were obtained beforehand in a laboratory. When the same patterns were measured in the light from distant bodies, researchers could be certain of the constituent elements in that body. All you needed was a spectrograph to break light into its separate colours and you could see the patterns of radiation or absorption that corresponded to each substance.
Using appropriate instruments, this basic technique could be expanded beyond the narrow range of light wavelengths that we see with our eyes to include the wider electromagnetic spectrum and the various particles associated with ionising radiation. As the CSM flew over the Moon, instruments in the SIM bay took advantage of the complete lack of a worthwhile atmosphere to determine the makeup of that small planet using a varied suite of techniques.
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