##

Astronomers from ancient times until Galileo knew almost nothing about stars and planets except that the latter moved in a regular fashion through the skies. One of the first examples of remote sensing was Galileo’s use of a primitive telescope to discover the Moon’s craters and the moons of Jupiter. In the early 20th century, Hubble learned that most stars were actually galaxies - clusters of billions of stars. But it took the space program, with its probes to and orbiters around the planets, to open up the other planetary bodies in the Solar System to a systematic examination. The wealth of knowledge this has brought, largely through the remote sensing devices carried on the spacecraft, has given astronomers today remarkable insights into the nature and history of the planets. This Section will convincingly underwrite that statement. The first page reviews the different sensors and parts of the spectrum used in these great advances.

## PLANETARY REMOTE SENSING

### Remote Sensing Techniques

Remote sensing by imaging, as applied to Earth, goes back to the middle of the last century, when balloonists took the first photos. As applied to the rest of the solar system, we 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 on our nearest neighbor, the Moon. Later, he confirmed the Copernican theories with his discoveries of moons, or orbiting satellites, around Jupiter. Since then, we have many observations of our solar system neighbors, first with telescopes and, after the opening of the Space Age, with orbiting spacecraft, flyby, probe, and lander missions. Nowhere else in the diversified and imaginative programs of NASA and other space agencies from different nations has there been such a plethora of observational and scientific triumphs as the exploration of the planets and the Cosmos beyond.

Most of the same instruments that survey the electromagnetic spectrum (EM) around Earth have been the principal tools for exploring our planetary associates and beyond; searching well into outer space at stars and other members of the Universe. Here is a list of remote sensing methods using EM spectral measurements that have provided exceptional information about planetary surfaces, atmospheres, and, indirectly, interiors: *

METHOD | EM SPECTRUM | INFORMATION | INTERPRETATION | MISSION
Gamma-Ray Spectroscopy | Gamma rays | Gamma spectrum | K, U, Th Abundances | Apollo 15, 16: Venera
X-ray Fluorescence spectrometry | X-rays | Characteristic Wavelengths | Surface mineral/ chemical comp. | Apollo; Viking Landers
Ultraviolet Spectrometry | UV | Spectrum of Reflected sunlight | Atmospheric Composition: H,He,CO2 | Mariner; Pioneer; voyager
Photometry | UV, Visible | Albedo | Nature of Surface; Composition | Earth Telescopes; Pioneer
Multispectral Imagers | UV, Visible, IR | Spectral and Spatial | Surface Features; Composition | On most missions
Reflectance Spectrometers | Visible, IR | Spectral intensities of reflected solar radiation | Surface Chemistry; mineralogy; processes | Telescopes; Apollo
Laser Altimeter | Visible | Time delay between emitted and reflected pulses | Surface Relief | Apollo 15,16,17
Polarimeter | Visible | Surface Polarization | Surface Texture; Composition | Pioneer; Voyager
Infrared Radiometer (includes scanners) | Infrared | Thermal radiant intensities | Surface and atmospheric temperatures; compos. | Apollo; Mariner; Viking; Voyager
Microwave Radiometer | Microwave | Passive microwave emission | Atmosphere/Surface temperatures; structure | Mariner; Pioneer Venus
Bistatic Radar | Microwave | Surface reflection profiles | Surface Heights; roughness | Apollo 14,15,16; Viking
Imaging Radar | Microwave | Reflections from swath | Topography and roughness | Magellan; Earth systems
Lunar Sounder | Radar | Multifrequency Doppler Shifts | Surface Profiling and imaging; conductivity | Apollo 17
S-Band Transponder | Radio | Doppler shift single frequency | Gravity data | Apollo
Radio Occultation | Radio | Frequency and intensity change | Atmospheric density and pressure | Flybys and Orbiters

* 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 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 measure geochemical and geophysical properties. The astronauts also deployed, on the 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 similar items, have provided the most direct and readily interpretible sets of data, and will continue to be a mainstay of future missions.

Primary Author: Nicholas M. Short, Sr. email: nmshort@ptd.net

Collaborators: Code 935 NASA GSFC, GST, USAF Academy
Contributor Information