Orbital Sensing Instruments
Eight types of instrument are under consideration for the remote-sensing activity. Spectrometers measuring gamma rays, x-rays and alpha particles emerging from the lunar surface would be able to detect several elements. The gamma-ray instrument could ascertain the amounts of iron, potassium, thorium and uranium in the top foot of the lunar surface. The x-ray instrument would receive radiation excited by the sun in a very thin surface layer and give information on the concentration of major elements such as silicon, magnesium and aluminum. The alpha instrument would reveal if there were any extensive leakage of radon gas from the lunar interior, such as often accompanies volcanic or hot-spring activity on the earth. An infrared radiometer would measure infrared emission from the surface and thus would be able to find hot spots and volcanic activity. A gas mass spectrometer would measure the number and type of atoms around the service module, thereby determining the density and composition of the vanishingly small amounts of gas at lunar orbital altitudes. An electromagnetic sounder would bounce pulses of radio waves (10 kilohertz to 100 megahertz) off the moon and measure how much came back, thereby finding out about subsurface layering and determining whether there is chemical differentiation or even possibly a layer of ice. A metric camera would photograph most of the moon with good geometric control in order to determine how out-of-round the moon is and whether the centers of maria are lower than the edges. A laser altimeter would bounce a light beam off the lunar surface to measure altitude accurately; such measurements, taken together with orbital data and information from the metric camera, would help to determine the moon's shape.
This kind of broad coverage would mesh well with the detailed coverage astronauts on the surface would make of small areas. Each landing would provide a standard for the orbital experiments by measuring in detail what the instruments should see from orbit. The orbiting instruments then would yield a far broader coverage of surface characteristics than could be obtained from manned landings alone.
Attainment of the goals we have described will still leave several exciting frontiers for lunar exploration. They include visits to Mare Orientale, the polar region and the far side of the moon. Such visits will require the development of a new technology.
Mare Orientale, the huge "bull's-eye" feature discovered in Lunar Orbiter photographs, is on the far western edge of the moon as viewed from the earth. It is a splendidly preserved, concentrically layered feature probably formed by the impact of a giant meteorite. The feature offers an unparalleled challenge for exploration, but it also presents large operational difficulties for a landing. The Cordillera Mountains, which ring the Orientale basin to form a circular outer scarp some 960 kilometers in diameter, are among the most massive on the moon, rising some 18,000 feet above the adjacent terrain. Perhaps this site, of all the possible ones on the moon, offers the best opportunity for studying the evolution and history of the moon.
A polar landing is a particularly fascinating prospect. Areas near the poles are in permanent shade, so that one might hope to find frozen ammonia, carbon dioxide, water and similar volatiles that otherwise would have escaped from the moon long ago.