Direct and Remote Measurements
The diverse measurements made by Voyager of a planet and its environment can be divided into two broad categories, usually called direct or in situ measurements and remote sensing measurements. A direct measurement involves the analysis of the immediate environment of the spacecraft; remote measurements can be made by analyzing radiation from distant objects.
The direct measurement instruments on Voyager measure cosmic ray particles, low energy charged particles, magnetic fields, plasma particles, and plasma waves. Their activity began immediately after launch, monitoring the Earth environment and then interplanetary space until the magnetosphere of Jupiter was reached a few days before the actual flyby. Long after the flybys of Jupiter and Saturn are completed, particles and fields data can continue to be acquired by the Voyagers as they speed outward into previously unexplored regions of space.
The remote sensing investigations are essentially astronomical in nature, measuring the light reflected from or emitted by the planet and its satellites. On Voyager, however, these instruments far outstrip their terrestrial counterparts in capability. Primarily, they derive their advantage from their proximity to what they observe—at its closest, Voyager was more than a thousand times nearer to Jupiter than are Earth-based telescopes; and for the still closer satellite encounters, Voyager was nearly ten thousand times nearer than astronomers on Earth. In addition, Voyager provided perspectives, such as views of the night side of Jupiter, that are impossible from Earth. Finally, these instruments could exploit the full spectrum of electromagnetic radiation without concern for the opacity of the terrestrial atmosphere, which restricts ground-based astronomers to certain spectral windows and blocks all observation at other wavelengths.
Five of the remote sensing instruments—the two TV cameras, the infrared spectrometer, the ultraviolet spectrometer, and the photopolarimeter—are mounted together on a scan platform. This platform can be pointed to almost any direction in space, allowing exact targeting of the observations.
One remote sensing instrument, the planetary radio astronomy receiver, is not on the scan platform. It measures long-wave radio emission without requiring special pointing.
The fully deployed Voyager spacecraft is capable of a wide variety of direct and remote sensing measurements. The instruments and their objectives were selected many years before the first Jupiter encounter. Because of the exploratory nature of the Voyager mission, every effort was made to fly versatile instruments that could yield valuable results no matter what the nature of the Jovian system. [P-18811AC]
Narrow angle TV Wide angle TV Steerable platform Science instrument boom Plasma detector Cosmic ray detector High-gain antenna Low-gain antenna Extendable magnetometer boom Photopolarimeter Low energy charged particles detector Infrared interferometer spectrometer and radiometer Ultraviolet spectrometer Thrusters Star trackers Radioisotope thermoelectric generators Science instrument calibration panel and shunt radiator Thrusters Planetary radio astronomy and plasma wave antenna
The science instrument boom supports the plasma particle detector, the cosmic ray detector, and the low energy charged particle detector. These instruments began collecting data immediately after launch, monitoring the Earth environment and then interplanetary space until the magnetosphere of Jupiter was reached a few days before the actual flyby. [353-2992BC]
A final Voyager investigation did not fit into this pattern of direct versus remote sensing instruments. In fact, it required no special instrument at all. This investigation deals with radio science, and it utilizes the regular communications link between the spacecraft and Earth to derive the masses of Jupiter and its satellites, to probe the atmosphere of Jupiter, and to study properties of the interplanetary medium.
The Voyager Imaging Science Team