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.

Imaging

The eyes of Voyager are in its imaging system. Two television cameras, each with a set of color filters, look at the planets and their satellites and transmit thousands of detailed pictures to Earth. The imaging system is probably the most versatile and therefore the most truly exploratory of the Voyager instruments. No matter what is out there, the imaging system will let us see it and hence, we hope, begin to understand its nature.

Unlike other Voyager instruments, the imaging system is not the result of a competition among proposals submitted by groups of scientists. NASA assigned the development of the cameras directly to JPL, to be integrated from the beginning with the design of the Voyager spacecraft and its subsystems. The members of the Voyager Imaging Science Team were selected, as individuals, on the basis of the scientific studies they proposed to carry out. Initially, ten members of the Imaging Science Team were selected, but by the time of the Jupiter encounters, the team had been expanded to 22 scientists.

The Team Leader is Bradford A. Smith, a professor in the Department of Planetary Science at the University of Arizona. Smith was involved in imaging science on several previous missions, including Mariners 6 and 7 and Viking. He was also active in ground-based photography of Jupiter at both New Mexico State University and University of Arizona, and he is a member of the team developing a planetary camera for the Space Telescope, scheduled for operation in Earth orbit in the mid-1980s.

Originally, the Deputy Team Leader was Geoffrey A. Briggs, a young British-born physicist from JPL. However, in 1977 Briggs took a position at NASA Headquarters in Washington and was replaced by geologist Laurence A. Soderblom of the U.S. Geological Survey in Flagstaff, Arizona. An energetic and articulate scientist, Soderblom, with his interest in satellite geology, complemented Smith, whose personal scientific interests are directed more toward the atmosphere of Jupiter.

The objectives of imaging involved multicolor photography of Jupiter and its satellites. Both wide- and narrow-angle cameras were needed to obtain the highest possible resolution while retaining the capability to study global-scale features on Jupiter and the satellites. In normal photographic terms, both cameras used telephoto lenses. For the wide-angle camera, a focal length of 200 millimeters was selected, giving a field of view of about 3 degrees. This field is similar to that obtained with a 400-millimeter telephoto lens on a 35-millimeter camera. The narrow-angle Voyager camera has a focal length of 1500 millimeters and field of view of 0.4 degrees. The camera optics are combinations of mirrors and lenses, designed for extreme stability of focus and for freedom from distortion.

Each camera has a rotating filter wheel that can be used to select the color of the light that reaches the camera. For the wide-angle camera, these colors are clear, violet, blue, green, orange, and three special bands for selective observation in sodium light (589-nanometer wavelength), and in methane spectral lines at 541 nanometers and 618 nanometers. For the narrow-angle camera, the filters are clear, ultraviolet, violet, blue, green, and orange. To create a color picture, the cameras are commanded to take, in rapid succession, pictures of the same area in blue, green, and orange light. These three pictures can then be reconstructed on Earth into a “true” color image. Other combinations of colors are used to investigate particular scientific problems and to determine the spectrum of sunlight reflected from features on Jupiter and its satellites.

The detector in the cameras is not photographic film but the surface of a selenium-sulfur vidicon television tube, 11 millimeters square. Unlike most commercial TV cameras, these tubes are designed for slow-scan readout, providing 48 seconds to acquire each picture. The shutter speed can be varied from a fraction of a second (for Jupiter and Io) to many minutes (for searches for faint features, such as aurorae on the night side of Jupiter).

Each picture consists of many numbers, each of which represents the brightness of a single picture element, or pixel, on the image. There are 640000 pixels in each Voyager image, representing a square image of 800 × 800 points. This information content is more than twice that of an ordinary television picture, which has only 520 lines. For each pixel, eight binary numbers are required to specify the brightness; the total information in a single image is thus 8 × 640000 = 5120000 bits. Even at a transmission of one frame per 48 seconds, the “bit rate” is more than 100000 bits per second. For comparison, the bit rate from the first spacecraft to Mars (Mariner 4) was about 10 bits per second, requiring a week to transmit 21 pictures, with a total information content equivalent to a single Voyager picture. Altogether, Voyager took nearly 20000 pictures at each Jupiter encounter, representing 10¹¹—a hundred billion—bits of information.

Infrared Spectrometer

The infrared investigation on Voyager is based on one of the most sophisticated instruments ever flown to another planet. In the past, most infrared instruments on planetary spacecraft measured at only a few wavelengths, but Voyager carries a true spectrometer, capable of measuring at nearly 2000 separate wavelengths, covering the spectrum from 4 to 50 micrometers.

Twelve scientists, led by Principal Investigator Rudolph Hanel of the NASA Goddard Space Flight Center at Greenbelt, Maryland, proposed this infrared instrument. Hanel is an acknowledged world leader in infrared spectroscopy from space. With his co-workers at Goddard, he has pioneered in adapting the extremely complex art of interferometric spectroscopy to the rigors of space flight. His spectrometers have made many studies of the Earth’s atmosphere from meteorological satellites, and a Hanel interferometer flew successfully to Mars on Mariner 9.

The primary goals of the infrared spectrometer investigation are directed toward analysis of the composition and structure of the atmosphere of Jupiter. Among the molecules to be searched for on Jupiter were hydrogen (H2), helium (He), methane (CH4), ammonia (NH3), phosphine (PH3), water (H2O), carbon monoxide (CO), simple compounds of silicon and sulfur, and a variety of organic compounds consisting of atoms of carbon and hydrogen (e.g., C2H2, C2H4, C2H6). In addition to indicating the abundance of these constituents of the atmosphere, the infrared spectra also contain information on atmospheric structure, that is, on the variation of temperature and pressure with altitude. The presence of clouds or dust layers can also be inferred from the shapes of spectral lines.

In addition to its spectroscopy of Jupiter, the Voyager instrument could be used as a heat-measuring device to map the temperatures of both the satellites and the atmosphere of Jupiter. Particularly interesting for the satellites are measurements of the surface cooling and heating rates, since these rates reveal the physical compactness of the surface, easily distinguishing between rock and sand or dust.

The infrared instrument is a Michelson interferometer at the focus of a gold-plated telescope of 51-centimeter aperture. The spectrum is not obtained directly, by moving a prism or grating, but indirectly through the interference effects of light of different wavelengths: hence its name, an interferometer. The complex interference pattern generated by the motion of one of the mirrors in the light path is transmitted to Earth, where computer analysis is required to transform it into a recognizable spectrum. The entire instrument, which has a mass of 20 kilograms, is called IRIS, for infrared interferometer spectrometer.

The development of the infrared system for Voyager posed many problems. IRIS was designed to cover the optimum spectral region for studies of the atmospheres of Jupiter and Saturn. However, when it was decided in 1974 that an extended Voyager mission might also allow a visit to Uranus, Hanel and his colleagues realized that their instrument had serious deficiencies for investigation of that colder and more distant planet, and they proposed that a modified IRIS (MIRIS) be substituted for the original design. A crash program was authorized to develop MIRIS in parallel with IRIS, and in early 1977, as launch approached, it appeared that the improved instrument would be ready. Problems occurred during testing, however, and for several weeks in June and July the decision hung in the balance. Unfortunately, there simply was not enough time to solve all the problems; both Voyagers were launched carrying the original IRIS, and the final flight qualification of MIRIS came in October, about six weeks after launch. With no other missions planned beyond Saturn, no alternate use for MIRIS has been found; it remains “on the shelf,” one of the rare cases where a technological gamble by NASA did not pay off.

Ultraviolet Spectrometer

A second spectroscopic instrument on the Voyager scan platform examines short-wave, or ultraviolet, radiation. The Principal Investigator for the ultraviolet spectrometer (UVS) is A. Lyle Broadfoot of Kitt Peak National Observatory in Tucson, Arizona. Broadfoot’s expertise lies in instrumentation for atmospheric research, and he was previously the Principal Investigator for a similar ultraviolet instrument on the Mariner 10 mission to Venus and Mercury. Associated with Broadfoot in the Voyager investigation are fourteen other scientists from the United States, Canada, and France.

In the upper atmosphere of a planet, the lighter gases tend to diffuse, rising above their heavier neighbors, and unusual chemical reactions take place as the action of sunlight breaks some chemical bonds and stimulates the formation of others. The study of these tenuous regions of planetary atmospheres is called aeronomy. Many of the exotic chemical processes that occur can best be studied by examining radiation of very short wavelength, and it is to these problems that much of the work of the UVS team is directed.

The observations are also sensitive to special processes in the Jovian atmosphere resulting from the magnetosphere. At the very top of the atmosphere, energetic charged particles interacting with the atmospheric molecules produce ultraviolet aurorae that indicate the location and nature of the bombarding electrons and ions. Emissions from atoms in the extended gas clouds around Io and possibly around other satellites were also expected to be observable.

The ultraviolet instrument is a relatively straightforward optical spectrometer, adapted for use in space. A diffraction grating disperses the light into a spectrum, and the other optical elements are gold-coated mirrors. The field of view is rectangular, about 0.1 degrees × 0.9 degrees. An array of sensitive electronic detectors provides simultaneous measurements in 128 wavelength channels over a wavelength range from 50 to 170 nanometers.

Photopolarimeter

The fourth scan-platform instrument is designed to measure the brightness and polarization of light with high precision. Unlike the imaging system, however, it can look at only a single point (one pixel) at a time; thus it sacrifices spatial capability in favor of precision of measurement. The prime objectives of this photopolarimeter are related to study of the clouds of Jupiter.

During the development stage for this instrument, the Principal Investigator was Charles F. Lillie of the Laboratory for Atmospheric and Space Physics of the University of Colorado at Boulder. Later, the Principal Investigator’s role was assumed by Lillie’s colleague, Charles W. Hord, a physicist with considerable experience in space missions, primarily with ultraviolet instruments. Four other scientists are Co-Investigators on the photopolarimeter.

The instrument itself is essentially a small Cassegrain reflecting telescope of 15-centimeter aperture. Two consecutive filter wheels select the color and polarization of the light, which is then detected and measured by a photomultiplier tube. The entire photopolarimeter weighs just 2.5 kilograms.

In spite of their apparent simplicity, however, the Voyager photopolarimeters were plagued with troubles almost from the moment of launch. A succession of mechanical failures on Voyager 1 led to the sticking first of the polarization wheel and then of the filter wheel. Even when repeated commands from Earth managed to free the wheels, their behavior was erratic. Ultimately an electronic failure also took place, leading to the reluctant decision to shut down the Voyager 1 photopolarimeter. On Voyager 2, similar mechanical problems greatly restricted the ability of the instrument to carry out its observations. The photopolarimeter thus was unable to contribute its share to unraveling the mysteries of Jupiter and its satellites.

Planetary Radio Astronomy

The final Voyager remote sensing instrument is designed to measure radio emission from Jupiter and Saturn over a wide range of frequencies. These emissions, which sound like hiss or static if played through an audio receiver, result from interactions of charged particles in the magnetospheres and ionospheres of the giant planets. The planetary radio astronomy (PRA) Principal Investigator is James W. Warwick, an astronomer in the Department of Astro-Geophysics of the University of Colorado at Boulder. Eleven colleagues from the United States and France participate as Co-Investigators. Warwick has been studying Jupiter longer than any other Voyager Principal Investigator. He has monitored its radio emissions since the 1960s, and he played a central role in the discovery that Io influenced these emissions.

Jupiter emits many kinds of radio radiation, ranging from bland thermal emission at short (centimeter) wavelengths, to synchrotron emission from energetic electrons at intermediate (decimeter) wavelengths, to erratic, extremely intense bursts at long (meter and decameter) wavelengths. The origin of these latter, nonthermal emissions constitutes one of the major unsolved problems of the Jovian system, and, more generally, of the physics of plasmas. One of the most important advantages of the Voyager PRA for studying Jovian radio emission lies in its proximity to Jupiter and hence its ability to locate the sources of different kinds of radio bursts.

The PRA instrument consists of an antenna and a radio receiver. The antenna is made up of two thin metal poles, each 10 meters long, extended from the spacecraft after launch at an angle of 90 degrees to each other. These are electrically connected to two receivers of extremely high sensitivity and broad frequency response: from 1.2 kilohertz to 40.5 megahertz. The PRA can operate in a number of modes, depending on the measurements desired. At its lowest level of activity, it monitors intensity in all 198 bands and transmits the data at 266 bits per second. In its highest mode, where searches are made for variations with very short time scales, the data rate goes up to 108000 bits per second, essentially the same as that required by imaging. In fact, the high-rate PRA data actually use an imaging frame as a display form, and occasionally throughout the mission an unfamiliar looking “image” was transmitted that was actually a block of PRA data—a portrait of electrical events in the Jovian atmosphere and magnetosphere that only Warwick and his colleagues could interpret.

Magnetometer

The first of the direct sensing instruments to be discussed is the magnetometer, designed to measure the magnetic fields surrounding the spacecraft. Such measurements can be interpreted to yield the intrinsic fields of Jupiter and its satellites and to characterize, in conjunction with data from particle and plasma instruments, the processes taking place in the magnetosphere of the planet. The Principal Investigator for this instrument is Norman F. Ness of the NASA Goddard Space Flight Center. Ness is an intense, competitive scientist with a great deal of previous experience in spacecraft magnetometers, primarily on board Earth satellites. Ness is also the only Voyager Principal Investigator who has previous experience at Jupiter; he was Principal Investigator on one of the two magnetometer instruments flown on Pioneer 11. For the Voyager investigation, Ness is joined by four colleagues from Goddard and one from Germany.

The magnetometer instrument consists of two systems: a high-field magnetometer and a low-field magnetometer. Each system contains two identical three-axis magnetometers that measure the intensity and direction of the magnetic field. The low-field system requires isolation from magnetic fields induced by electric circuits in the spacecraft itself. To achieve this isolation, it is mounted on the largest component of Voyager—a 13-meter boom, about as long as the width of a typical city house lot. This boom was coiled tightly in a canister during launch; later, when the package was opened, it uncurled and extended automatically.

By using two magnetometers, Ness and his colleagues are able to correct for the residual artificial magnetic fields that reach even 13 meters from the main spacecraft. The dynamic range extends from a maximum field of 20 gauss down to 2 × 10?8 gauss—a factor of one billion. The fields can be measured as frequently as 17 times per second. The total mass, exclusive of the 13-meter boom, is 5.6 kilograms.

Plasma Particles

Plasma is the term given to a “gas” of charged particles; the electrons and protons are separate, yet there are equal numbers of each, producing a zero net charge. If the velocities of the electrons and protons are less than about 0.1 percent of the speed of light, they can be measured by the Voyager plasma instrument; if their energies are higher, they are measured by one of the other two particle instruments—the low energy charged particle (LECP) detector or the cosmic ray detector. The plasma instrument, like the magnetometer, was designed to provide basic data on the particles and fields environment of Voyager.

The Principal Investigator for the plasma instrument is Herbert S. Bridge of the Massachusetts Institute of Technology in Cambridge, Massachusetts. An experienced space physicist, Bridge has flown similar instruments on many Earth satellites and planetary probes. He also holds the position of Director of the Laboratory for Space Experiments at MIT. On the Voyager experiment, he is joined by one German and ten U.S. Co-Investigators.

The objectives of the plasma investigation are directed toward study of both the interplanetary medium and the Jovian magnetosphere. At Jupiter, Bridge expected to determine the plasma populations and processes in the inner and outer regions of the magnetosphere and in the plasma tail that extends beyond Jupiter, much as a comet tail is blown outward by the solar wind. The densities and temperature of electrons were measured, and their origins determined: Some originate near Jupiter and diffuse outward through the magnetosphere; others derive from the solar wind.

The plasma instrument, with a mass of 9.9 kilograms, was designed to view in two directions: one toward the Earth and Sun, primarily to study the solar wind, and the other sideways, looking toward the direction plasma would flow if it were caught up in the rotating Jovian magnetic field. If it is desired to look in other directions, the entire spacecraft must be tipped, a maneuver that was carried out several times near Jupiter. The detectors directly sense the flow of electrons, protons, and alpha particles (helium nucleii, made up of two protons and two neutrons each). Analysis of the energy spectra can also yield data on positive ions of higher mass.