Imaging remote sensing as applied to Earth goes back to the middle of the last century, when the first photos were taken from balloons. As applied to the rest of the solar system, one must look to the first observations (documented by sketches) made by Galileo in 1610 when he turned a telescope to the heavens and caught a glimpse of the surface complexities exhibited on our nearest neighbor, the Moon, and then later confirmed the Copernican Revolution with his discoveries of moons, or orbiting satellites, around Jupiter. Since then, first with telescopes and, after the opening of the Space Age, with orbiting spacecraft, flyby, probe, and lander missions to the Moon and the planets, most of the same instruments that survey the electromagnetic spectrum interacting with the Earth have been the principal tools used in exploring our planetary associates and beyond - searching well into outer space to look at stars and other members of the Universe. This is evident from this list of remote sensing methods depending on EM spectral measurements that have provided exceptional information about planetary surfaces, atmospheres, and, indirectly, interiors: *

* Adapted from Billy P. Glass, Introduction to Planetary Geology, 1982, Cambridge University, Press

This list is incomplete but is still highly representative. The Command and Service Module (CSM) on the Apollo lunar missions carried a complement of other instruments including alpha-particle spectrometers, mass spectrometers, magnetometers, far UV spectrometers, scintillometers, and others designed to make geochemical and geophysical measurements. The astronauts also deployed on-surface instruments for specific studies; among these were seismometers, magnetometers, gravimeters, solar wind gauges, cosmic ray detectors, heat flow probes, and laser ranging retroreflectors. However, in retrospect, sensors that produce images, especially photographs and the like, have provided the most direct and readily interpretible sets of data, and will continue to be a mainstay of future missions.

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