PLANETARY REMOTE SENSING

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@nationi.net
Collaborators: Code 935 NASA GSFC, GST, USAF Academy
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