The Voyager 2 Neptune encounter

EDITOR'S NOTE:

Meet Ken Bartos and Bill Brundage, the project managers who have shared with us some special insights into their efforts to establish the communications link with Voyager 2 as it speeds past Neptune this summer.

This project is quite different than any we have published to date. Its purpose is to gain knowledge about space. To appreciate the project we must understand the nature of this NEW KNOWLEDGE FROM THE VOYAGER MISSION.

It involves technology and equipment unfamiliar to most people. There is a SCIENCE AND TECHNOLOGY which challenges the imagination. The elements of this science are discussed in a series of short articles on the various elements of the system --- The Deep Space Network, Voyager Spacecraft, Very Large Array, and the VLA-Goldstone Telemetry Array --- and The Data/Information Technology: Image Transmission.

Your attention is drawn to A MANAGEMENT CHALLENGE where Ken and Bill discuss some management problems they faced, their solutions, and some recommendations to all project managers.

Finally, for those who want to follow the VOYAGER/NEPTUNE ENCOUNTER more closely, Anita Sohus, technical writer for the project, provides a Schedule and Information Sources.

Our thanks to Ken Barto and Bill Brundage for sharing their project and experience with us and to Anita Sohus for her help and contributions.

Thanks also to Erika Jones and the Rio Grande Valley Chapter for bringing this very interesting project to our attention.

Ken Bartos' education is in civil engineering and applied mechanics, receiving a BSCE from the University of Nebraska and MSAM from California Institute of Technology. Ken joined the Jet Propulsion Laboratory (JPL), Pasadena, California, in 1963 where he was a team member for the design and construction of the first 64-meter Deep Space Network (DSN) antenna at the Goldstone Tracking Station near Barstow, California. In 1970 he transferred to Madrid, Spain, as the JPL resident for construction of the 64-meter antenna there. Ken has participated in various projects at JPL including energy conservation studies in the 1970's, major DSN upgrades in the 1980's and is presently the Implementation Manager for the Array Project. Ken actively supports his community entry in the Pasadena Tournament of Roses Parade.

Bill Brundage's education is in electrical engineering and radio astronomy, receiving a BSEE from the University of Toledo and MSEE from Ohio State University (OSU) where he continued post-graduate studies and work at the OSU Radio Observatory. In 1968, he joined the National Radio Astronomy Observatory (NRAO) at it's Green Bank, West Virginia radio telescope site where he was responsible for designing, building, and operating various radio receiver systems for the 300-foot and 140-foot radio telescopes. From 1983 to 1985 he also managed the Green Bank Electronics Division. In 1985, he transferred to Socorro, New Mexico and the VLA, as Project Engineer and VLA-Voyager Preparation Manager, Mr. Brundage is a member of the IEEE, American Management Association, and the Project Management Institute.

INTRODUCTION

For more than two decades the United States has been systematically exploring our solar system. The National Science Foundation (NSF), National Radio Astronomy Observatory (NRAO), and National Aeronautics and Space Administration (NASA) scientists have shown that the laws of science, as we have discovered them here on Earth, fit the way the universe behaves. We have landed on the Moon, studied its surface and returned samples of lunar material for laboratory study. Our spacecraft have visited each planet as far out from the Sun as Uranus. On August 25,1989, NASA's Voyager 2 will become the first spacecraft to fly past the giant gaseous planet Neptune, eighth planet from the Sun.

The primary goal of modern scientific exploration of our solar system is to understand its origin, evolution and present state. Current investigations are directed toward understanding the Earth by comparative study of the planets, and the relationship between evolution of the solar system and the appearance of life.

The exploratory missions to date have contributed greatly to the achievement of these goals. Unmanned exploration has gathered much information that has permitted a growing understanding of the processes that determined the nature of the planets and their atmospheres, moons and rings. Scientists have just begun to understand why the Earth is unique and how it happened that only here in our solar system have conditions developed conducive to the evolution of life.

Yet these marvelous voyages of scientific discovery have been only a reconnaissance of the major features of the varied and complex elements of the solar system. What surprises await in the future? The encounter with Uranus proved that the unexpected is the commonplace when exploring new worlds. The impending encounter with Neptune will provide its own surprises and unexpected contributions to scientific discovery.

Are there other planetary systems about other stars? The odds seem in favor of it. As in the past, our understanding will increase and perhaps someday we will be able to answer that fascinating question: “Are there really other worlds and intelligent life beyond our solar system?”

THE SCIENCE OF RADIO ASTRONOMY

In the radio sky, the brightest objects are not nearby stars; they are the Milky Way, our galaxy, and distant galaxies that are undergoing violent outbursts, ejecting great quantities of matter and energy far into space. If our eyes could see radio waves instead of light, the sky would look strangely different. A bright Sun would be visible, but the daytime sky would seem dark like night. The familiar stars would be gone. Instead the heavens would be full of glowing nebulous patterns of emission which arise from clouds of hot interstellar gas or from super high energy electrons that swarm along the magnetic pathways of our Galaxy.

Small star-like objects would appear to our radio eyes. These radio sources, called quasars, arise from extraordinarily energetic events that occurred in the inner cores of the most distant galaxies. The quasars are so distant that the radio waves we now receive from them started their journey to the Earth billions of years ago, giving us a look back in time as well as across space.

After traversing the vast distances of space, cosmic radio waves are so weak that large radio telescopes and highly sensitive receivers are needed to detect them. NRAO's Very Large Array (VLA) located near Socorro, New Mexico, is an astronomical instrument. With it, astronomers study cosmic objects ranging from the Sun and the planets of our solar system to distant galaxies and quasars at the edge of the universe. However, instead of analyzing light, these astronomers study the radio waves which are emitted by celestial objects. Just as an astronomer at an optical telescope can record a photographic image of the object he is studying, a scientist at the VLA can make an image using the radio waves that are emitted by that object. By studying and comparing both the radio sky and the optical sky, astronomers obtain completely new kinds of information on the physical conditions, past history, and the evolution of celestial objects.

CASSIOPEIA A - SUPERNOVA REMNANT

New clues to understanding the cataclysmic death of massive stars are captured in this supercomputer-processed radio image of the Cassiopeia A supernova remnant. This supernova event was probably witnessed on earth over 300 years ago when a powerful explosion blew off the star's outer layers. As the outermost layers continue to be slowed by sweeping up more interstellar matter, clumpy material from deeper with in the star punches through from inside. This action gives rise to the extensions from and voids within the nearly spherical shell, which has now expanded to twelve light years in diameter. Over 4 million picture elements make up this image, making it the most detailed radio image yet constructed. Only the speed and number-crunching power of a supercomputer allow the full potential of the VLA to be exploited, as the wealth of detail in this image illustrates.

NEW INFORMATION FROM JUPITER

Jupiter and one of its largest satellites, Io, are captured by Voyager's cameras. Io is the red satellite at the center left in the above picture. Many Jovian storm systems are visible, including the Great Red Spot, the white ovals below it, and alternating dark and light cloud bands called belts and zones. The strongest winds - up to 400 kilometers (250 miles) an hour - are found at boundaries between belts and zones. Between opposing jet streams the clouds are turbulent. The area immediately below the wide, bright, white zone is an example of the turbulence. Wind shear stretches convective features into long plumes; one can be seen near the center of the photo. Holes in the clouds permit observation of deeper layers of the atmosphere. One such area is a bluish region north of the equator and left of the white cloud plume. The blue regions are strong emitters of infrared radiation coming from higher temperatures deep in the atmosphere. The rim of the Great Red Spot circulates counter-clockwise in six days, while there is almost no circulation at the center. Almost all the white ovals exhibit the same counter-clockwise motion as the Great Red Spot, which has been observed continuously for over 300 years; formation of the white ovals was first observed about 40 years ago.

A dramatic view of Jupiter's Great Red Spot (below) and the surrounding area shows cloud details as small as 160 kilometers (100 miles). The turbulent cloud pattern to the left of the Great Red Spot is a region of extraordinarily complex and variable winds. It has the appearance of liquids, such as oil and water, that do not mix. As relatively slow winds above the Red Spot flow past it, the smooth flow pattern is severely disturbed, creating the tumbled and twisted appearance. Some material from the region is also drawn into the Great Red Spot. Scientists don't understand why the cloud colors remain un-mixed. As wind blows through the region between the Great Red Spot and the white oval, a turbulent pattern forms to the left. A lighter portion at the top of the Great Red Spot shows that some turbulent material is drawn into the Red Spot itself. Smaller amounts appear to interact similarly with the white oval.

The abundance of distinctive cloud patterns leads scientists to compare some aspects of Jupiter's winds with currents in the oceans of Earth.

THE SATURN ENCOUNTERS

The results of the two Jupiter encounters had a cautionary effect on planning for the Voyagers' encounters at Saturn. Astronomers had observed Jupiter extensively from Earth, but were nevertheless surprised by the Voyager discoveries. Their Earth-based observations of Saturn had been less complete, because of Saturn's greater distance and because less time had been spent at the telescopes. Therefore the scientists were a little wary of predicting what they would find when the Voyagers arrived at the ringed planet.

As the summer of 1980 continued, Voyager 1 began its encounter with Saturn. The planet appeared almost featureless in early photos. It was surrounded by the three classic rings that had always been studied from Earth: the outer A-ring, the middle and brightest B-ring, and the gossamer, innermost C-ring. (Other faint rings had been, and would be, discovered from Earth, from Pioneer Saturn, and the Voyagers. They would be called D-, E-, F- and G-rings.) But it appeared, in early photos, as if there were not just three rings, but scores, then hundreds, and finally thousands, of thin ringlets. It would turn out that they were not individual rings separated by gaps; some of the variations were caused by the gravitational attraction of nearby satellites, pulling millions of particles into motion, spiraling outward across the rings like waves in an ocean. The causes of other variations are still unknown; a very few may be due to tiny satellites embedded within the rings. Saturn's rings are dynamic, changing with every passing day.

The photos from Voyager 1 also revealed other baffling phenomena - dark features that resembled spokes in the bright B-ring. The spokes appeared to rotate around Saturn with the ring. Here again was something that defied quick explanation. The speed at which one object orbits another depends on its distance from the primary body. It moves rapidly if it is near; ever more slowly at successively greater distances. While some spokes appeared to follow that set of physical laws, others looked as if they kept their radial form as they circled Saturn.

Photopolarimeter results show that even the narrow F-ring consists of smaller strands.

Back on Earth, meanwhile, scientists pondered the same kinds of unexpected results they had encountered at Jupiter. So, in the few months that remained before Voyager 2's arrival at Saturn, the Voyager teams undertook to restructure a major portion of the encounter sequences for Voyager 2's eleven science instruments.

For this encounter, emphasis would shift from Saturn and Titan to the rings and to other satellites. The unexpected appearance of the rings dictated more time to their study, and detailed study of the satellites was a high-priority scientific objective.

The summer of 1981 began, and with it the encounter of Voyager 2 with Saturn. By now profiting from their earlier experience, the scientists were able to set camera exposures at more exact levels, to cope with the low light levels and the general blandness of Saturn. On this approach Saturn presented alternating dark and bright bands of clouds and high-speed jet streams. Swirling cloud patterns, which were smaller versions of the large and intense storms seen on Jupiter, were also visible through Saturn's haze layer.

Voyager 2's cameras zeroed in on the rings, and scientists searched for small satellites in the rings that might cause the multiringed appearance. Those moonlets, some scientists believed, might sweep up material in the rings, creating gaps. Voyager 2 would soar closer to the rings, and the improved resolution of the pictures should show structure as small as one kilometer (0.6 mile) in diameter.

One of Voyager 2's most important experiments involved an instrument called a photopolarimeter, which measures light intensity. As Voyager 2 passed above Saturn, a distant star named Delta Scorpii appeared to move behind the rings. By measuring the starlight as it passed through the rings, the photopolarimeter detected changes in the starlight's intensity as it was altered by changes in the thickness of the rings.

Quick analysis of the data showed that the rings' structure was far different from what it appeared to be in the photos. No region was totally empty of ring particles. The members of the photopolarimeter team have 800,000 samples, each one a 100-meter (330 foot) slice of the rings. It will take a decade to process and analyze the data.

Voyager 2 photographed and measured all the satellites that were then known - their number had swelled to 17. At the end of the encounter, scientists had detailed data on all of them.

THE URANUS ENCOUNTER

“If it weren't for the imaging experiment, we wouldn't even know the planet is there,” lamented one scientist less than 15 days before Voyager was to make its closest approach to Uranus. No radio noise, like that emitted from Jupiter and Saturn, could be heard. Nothing unusual appeared in data from the ultraviolet and infrared instruments. No evidence of a magnetic field had been observed. Even the pictures of Uranus returned by Voyager's cameras hadn't changed substantially. The planet, still a hazy blue sphere, only grew larger in the field of view of Voyager's cameras. Voyager scientists called it “the fuzzy blue tennis ball.” This was a planet that would not readily give up its secrets. Whatever it had to offer, as Voyager would find, it would hold until the last possible minute. But what the Uranian system did finally yield makes it one of the strangest collections of planet, moons, and rings in the solar system.

Voyager supplied the first big piece of the Uranian puzzle when it discovered the planet's substantial magnetic field, comparable in strength to the fields around Saturn and Earth. Planetary magnetic fields are thought to be generated by fluid motion in a planet's core (molten iron in Earth's core, for example). While Uranus is believed to have a partially molten core, the core is not big enough to generate by itself, the magnetic field Voyager observed. Voyager scientists now believe that the field is produced instead by electrically conducting water deep within the Uranian atmosphere. (At one time, scientists thought that Uranus' relatively abundant water was concentrated in a deep ocean.)

The picture above shows Uranus as human eyes would see it, a uniformly blue planet.

This atmospheric water is quite unlike anything on Earth: it is hot [3,000 degrees Celsius (5,400° Fahrenheit)] and under such great pressure that it becomes highly electrically conductive. It would require a pressure a few million times that at Earth's surface to produce the same effect on Earth.

Voyager scientists looked for, and finally found, the elusive Uranian magnetic field. What was unexpected was the orientation of that field, tilted from the planet's axis of rotation by 60 degrees and offset from it by one-third of Uranus' radius. As at Earth and other planets with magnetic fields, Uranus' field is swept back into a long tail by the wind of charged particles that streams from the sun. The odd tilt of the magnetic field, however, coupled with the fact that the planet's axis of rotation points toward the Sun, has the effect of twisting the tail into an unusual corkscrew shape that spirals in sync with Uranus' 17.24-hour rotation period.

Voyager found radiation belts at Uranus of an intensity similar to those at Saturn, although they differ in composition. The radiation belts at Uranus appear to be dominated by hydrogen ions, without any evidence of heavier ions (charged atoms) that might have been sputtered from the surfaces of the moons. Uranus' radiation belts are so intense that irradiation would quickly darken any methane trapped in the icy surfaces of the moons, possibly contributing to the dark appearance of their surfaces.

At the top of Uranus' atmosphere is a relatively uniform layer of hydrogen and helium. Beneath this layer, Voyager found clouds of methane ice. Scientist believe that farther down, clouds of ammonia and water may also exist.

The fact that Uranus contains a large proportion of melted methane ice reveals much about the neighborhood of the solar system in which it formed. The planets formed from the remnants of the solar nebula left over from the formation of the Sun. Silicon and iron aggregated into globes. In the inner solar system, the planets from Mercury to Mars were too small to attract and hold gases like hydrogen and helium. But in the outer solar system, the giant planets, Jupiter and Saturn, with their powerful gravitational fields, attracted and held thick atmospheres dominated by these two gases.

Some of the most abundant material existing two billion miles from the Sun was water ice. Uranus apparently drew upon this frozen material, building up a layer of melted ice which was then covered with an atmosphere of hydrogen and helium. Thus, Uranus is denser than Jupiter and Saturn, and has a thinner atmosphere and a different internal structure.

THE MOONS OF URANUS

Voyager photographed each of the five large moons of Uranus known before the encounter: from the innermost out these are Miranda, Ariel, Umbriel, Titania, and Oberon. Ten additional moons were discovered by Voyager. The largest of the new moons is about 170 kilometers (110 miles) in diameter.

Planetary scientists were not surprised to see the geologically altered surfaces on the Uranian moons. But they were amazed by the tortuously rendered face of Miranda, impressed that such a small, cold body had endured so much change. Here was a moon marked by crisscrossed grooves, with parallel fault systems encircling even more complex grooved terrain. Huge canyons - one 20 kilometers (12 miles) deep - slice this tiny moon's surface. Great plates of land appear to have been upthrust, leaving Miranda looking as though it was being spaded by a giant shovel when suddenly, all geologic activity ceased.

Some scientists think Miranda may have been frozen in the midst of a geologic process most terrestrial objects in the solar system underwent at an early age, a process in which the body almost literally turns inside out. Miranda may also be the reaggregated parts of one or more moons that were shattered in a collision or torn apart in a gravitational tug-of-war.

A FORECAST OF A FUTURE ENCOUNTER

The accomplishments of the Voyager mission have far exceeded all expectations, and now we look forward to one more rush of discovery the first planetary encounter of Neptune. Little has been learned about Neptune's atmosphere. Fine tuning of the flight path within day of the spacecraft's closest approach to Neptune will send Voyager over the planet's north pole. The path has been chosen to miss the vestigial rings that probably exist around the planet while allowing the spacecraft to fly as close to the top of the planet's atmosphere as safety will permit. Scientists expect that Neptune has a magnetic field, but penetration of the field is not likely until several hours before the spacecraft's closest approach to the planet.

Voyager's last close-up look at any body in our solar system will occur five hours after the closest approach to Neptune when the spacecraft will pass Neptune's moon Triton. This large moon is likely to be one of the most interesting objects Voyager will have studied in its long mission. Triton is thought to passess an atmosphere of methane and perhaps nitrogen, and may harbor pools of liquid nitrogen on its surface.

After Voyager's last encounter, the spacecraft will fly southward, out of the ecliptic plane. Eventually, its instruments may be the first of any spacecraft to sense the heliopause--the boundary between the end of the Sun's magnetic influence and the beginning of interstellar space. Voyager is expected to return valuable data well in the 21st century.

Nine images were combined to creat this mosiac of Miranda. The moon's surface consists of two strikingly different types of terrain: old, heavily cratered areas and young, complex regions characterized by scarps and ridges.

THE DEEP SPACE NETWORK

The NASA Deep Space Network is the largest and most sensitive scientific telecommunications and radio navigation network in the world. Its principal responsibilities are to support un-manned interplanetary spacecraft missions and to support radio and radar astronomy observations in the exploration of the solar system and the universe. The Network is a separate facility of the NASA Office of Space Operations and is managed, technically directed, and operated by the Jet Propulsion Laboratory (JPL) of the California Institute of Technology in Pasadena, California.

The Network currently consists of 12 deep space stations positioned at three Deep Space Communications Complexes, which are located on three continents: at Goldstone in Southern California's Mojave Desert; near Madrid, Spain; and near Canberra, Australia. Each complex consists of four deep space stations equipped with ultrasensitive receiving systems and large parabolic dish antennas. There are two 34-meter (111-foot) diameter antennas, one 26-meter (85-foot) antenna, and one 70-meter (230-foot) antenna at each complex.

The Network Operations Control Center, which controls and monitors operations at the three complexes, is located at JPL in Pasadena. The Network's Ground Communications Facility provides and manages the communications circuits that link the complexes, the control center in Pasadena, and the remote flight project operations centers together.

The unmanned space flight projects supported by the Network are managed and controlled by the NASA Office of Space Science and Applications or by foreign space agencies. The Network's responsibilities are to receive the telemetry signals from the spacecraft, to transmit commands that control the spacecraft operating modes, and to generate the radio navigation data that are used to locate and guide the spacecraft to its destination. The Network is also used for flight radio science, radio and radar astronomy, very long baseline interferometry, and geodynamics measurements, and will be a major participant in NASA's Search for Extraterrestrial Intelligence.(Set I)

The Very Large Array

Aerial view of the VI.A., The North arm extends toward the background the Southwest arm extends to the left and the Southeast arm to the right. The Array is shown in D-configuration with 26 of the maximum 27 antennas visible.

THE VERY LARGE ARRAY

Located on the Plains of St. Augustin in central New Mexico, the Very Large Array (VLA) consisting of 27 identical 25- meter diameter antennas takes radio photographs of distant objects in the sky. These radio photographs image a low frequency (long wavelength) form of light --- the radio wave end of the electromagnetic spectrum --- with the clarity of those obtained from the largest optical telescopes. The VLA collects radio waves in eight frequency bands as low as 74 MHz (near the FM radio broadcast frequencies) to as high as 24000 MHz (six times higher than a home satellite TV receiving dish). The eighth band, 8000 to 8800 MHz, was added just to be able to hear Voyager 2 at Neptune. Radio astronomers are already using this frequency band to explore the Sun's radio emissions, the planets through the use of radar reflections, and spectral emissions from interstellar clouds of ionized hydrogen, helium and carbon.

Each antenna of the array can be moved to different observing foundations along a Y-shaped layout. The antennas are moved by specially designed transporter vehicles riding on two pairs of railroad tracks. The arms of the wye extend approximately 13 miles over an area about 20 miles square. Every few months, the spacing of the antennas along the wye is changed to support a variety of astronomical research. When the array operates with antennas packed together, the radio photographs simulate those from a wide-angle lens on a camera. When operating with antennas as far apart as 22 miles, the VLA produces zoom lens photographs with detail as fine as 0.1 arc seconds. The VLA photographs all types of celestial objects - the Sun and the planets, galaxies, quasars, stars in formation, remnants of supernovas, and the faint all-sky radio glow from the “Big Bang” ten billion years ago.

The VLA is capable of collecting the very faint radio waves emitted by distant celestial objects throughout the Universe During an observation, all antennas are pointed at the source to be studied. The VLA control computer directs each antenna to follow that source in the sky for perhaps several hours. As the observation continues, each antenna focuses and collects the faint radio waves. The data signals are sent via underground wave guide to the data collection computer where they are combined to simulate a single radio telescope as large as the physical array. At the end of the observing run, a map or photograph of the radio source is produced by state-of-the-art computer computation.

Voyagers 1 and 2 are the most sophisticated robotic spacecraft ever flown. Unlike earlier spacecraft, they were pro grammed to make independent decisions that safeguard both the spacecraft and their ability to communicate with Earth.

The two spacecraft have been found to be ex-tremely adaptable since they were launched in 1977. This adaptability has allowed engineers to give Voyager 2, in particular, new capabilities as it flies from planet to planet.

Voyager 2 has been heavily reprogrammed during its flight and its six on-board computers have continually been giver newly developed and more expedient method* of processing and packaging data for return to Earth. Voyager 2 carries instruments to conduct 11 investigations. Among these are television cam eras, infrared and ultraviolet detectors, and a communications system that doubles as a radio experiment. Three sets of twin computers control the spacecraft's stability and govern its complex activities.

Voyager 2 has had a few mechanical problems on its journey of more than five billion miles.

The spacecraft's first major problem was the total failure of one of its two radio receivers. The remaining receiver -works, despite suffering a reduction in the range of frequencies that it can “hear” to only a thousandth of its original design capability. In the event that Voyager's remaining radio receiver might fail altogether, Voyager engineers have set aside a corner of the spacecraft's computer memory for what is called the “back-up mission load.” This is a load of computer commands that would instruct Voyager to carry out rudimentary investigations on its own if the radio receiver failed.

The second major problem affecting Voyager 2 occurred in 1981. The movable instrument platform jammed in one of its two axes, preventing pointing of the mounted instruments. Engineers later determined that the jamming was caused by a loss of lubricant and the consequent damage to a bearing in the high-speed section of the gear chain, precipitated by repeated high-speed movement of the platform during the busy Saturn encounter. The platform began moving again when commands were sent two days after it jammed. It performed flawlessly, but at intentionally low speeds, during the Uranus encounter

The Voyager Spacecraft

IMAGE TRANSMISSION

IMAGE AND BIT DISCUSSION

The arraying of the VLA and the Goldstone antennas cannot completely overcome the effect of the distance-squared law. Radio transmissions become weaker and light levels drop as the square of the distance travelled. For example, since Neptune is six times farther from Earth than Jupiter is, the signal strength from Neptune would normally fall by a factor of 36 below that at Jupiter.

DISCARDING UNNECESSARY PICTURE DATA

Voyager Project engineers developed a clever scheme to pre-process the imaging data to reduce the total number of data bits required to transmit a television picture to Earth. A special software routine, known as Image Data Compression (IDC), has been loaded into the onboard backup computer to pre-process the data bits.

Uncompressed Voyager television images contain 800 lines, 800 dots or pixels (picture elements) per line, and 8 bits per pixel (a bit is either “0” or “1”). The 8 bits express the pixel's shade of gray on a scale from 0 to 256. This means that over five million bits are required to represent every uncompressed television image. However, much of the information in atypical planetary television image is frequently dark space or low-contrast cloud features. Therefore, by counting only the difference in gray level between adjacent pixels (rather than the full 8-bit values), IDC can reduce the number of bits that characterize each image by 60% or more. The end result is to reduce the time needed to transmit a complete television image from Neptune to Earth.

In general, the reconstructed compressed image will be indistinguishable from the uncompressed image, as the IDC scheme loses no information for low-contrast scenes. Even for scenes with greatly contrasting pixel intensities, only minor clipping occurs near the edges of the frame.

MORE ACCURACY FOR FEWER BITS

Another trick devised to offset the distance-squared penalty is the use of an onboard “experimental” Reed-Solomon data encoder. For those of you who know about secret codes used to hide information in the context of spy thrillers, it may be reassuring to learn that there are also codes designed to preserve the “truth” of information. Data sent to Earth passes through an interstellar plasma that may phase modulate the signals with noise; i.e., turn a “correct” 0-bit into a “wrong” 1-bit, or vice versa.

Encoding data has a price, and the price paid for the old encoding algorithm was an overhead of 100%—one bit of code was required for each bit of data. The new Reed-Solomon encoding scheme reduces this overhead to about 20%. In addition, it reduces the number of bit errors from five in 100,000 to only one in a million.

TAKING GOOD PICTURES IN FEEBLE LIGHT LEVELS

A third penalty is imposed on the Neptune encounter by the distance-squared law. Visual reflected solar radiation from the Neptunian system is received by the spacecraft instruments at very reduced light levels, some 900 times fainter than at Earth. Under this light condition, very long exposure times are required to gather the light which makes pictures of moving targets appear blurry or smeared. This will be a problem for the images of the Neptune ring-arcs and the moon Triton.

The problem facing Voyager engineers is somewhat analogous to a situation confronting a photographer in a dimly lit room without a flash. To offset the required long exposure times, the photographer must steady the camera on a tripod, use very sensitive film, or open the camera aperture. If the subject is moving, the photographer must also compensate for the subject's motion by smoothly panning his camera to “track” the target. To track several targets during the near-encounter phase, the spacecraft's gyroscopes and thrusters will be used to smoothly turn the entire spacecraft during long exposures. This technique is called Image Motion Compensation.

A new capability, Nodding Image Motion Compensation, has been developed for the Neptune encounter. The technique will be to briefly “nod” the spacecraft toward the target during single exposures without taking the antenna completely away from Earth-point. This will allow the pictures to be transmitted directly to Earth in real-time without intermediate storage on the digital tape recorder as must be done with conventional image motion compensation. The nodding motion of the spacecraft is accurately controlled by precisely calibrating the small attitude control thrusters, and then programming the onboard computer to fire the thrusters a predetermined number of times as needed to turn the spacecraft at the desired rate. Another compensation capability, Manueverless Image Motion Compensation, will slowly move the instrument platform on which the cameras are mounted during an exposure.

Voyager is also being steadied as an observing platform. As the spacecraft flies along in a zero gravity environment, even the start/stop motion of its tape recorder can add jiggle to the spacecraft's natural oscillatory motion. To reduce these types of disturbances, new software was devised that fires the thrusters to offset the tape recorder speed change whenever the tape recorder starts or stops. This tiny attitude control has been created to provide a steadier spacecraft and thus decrease the number of blurred television images.

Implementing the VLA-Goldstone Telemetry Array required that NRAO, the VLA, and JPL provide, install, and operate equipment at the VLA and that JPL provide, install and operate equipment at Goldstone. For the installation of 8.4 GHz receiver systems on the VLA antennas, NRAO provided cooled front ends and cryogenic compressors while VLA made additions to and upgrades of electronics and built the feed support towers. JPL provided ultra-low noise high electron mobility transistor (HEMT) devices to NRAO and built the feed horns. VLA also provided and installed signal summing devices and on-line control system computers and software.

For the electric power generation system, VLA provided architect and engineering design and structures. JPL provided and installed two 1.4 megawatt diesel generators and their control system. The power facility is adequate to power the VLA site during thunderstorms which occur almost daily in August at Voyager Encounter. JPL provided and installed the telemetry receiver (RCVR), real time signal processor (RTSP), and symbol stream recording capability (SSR). NASA Communications provided a satellite earth transmitter for real time transmission of telemetry data to Goldstone. VLA personnel operate the VLA, feeding the signal collected from the 27 VLA antennas to the JPL telemetry receiver. JPL DSN Operations personnel operate the JPL equipment.

Similarly, JPL provided and installed complementary equipment at Goldstone, including a satellite earth receiver for the telemetry data transmitted from the VLA, real time signal processor (RTSP), symbol stream recorder and combiner (SSRC), and voice communications between the VLA and Goldstone and JPL Pasadena. JPL DSN Operations personnel operate all equipment and antennas at Goldstone.

The telemetry receiver at the VLA is a phase tracking receiver based on designs existing in the DSN but with an improved telemetry phase detector. It is designed to take the Voyager signal from the VLA signal summing devices and demodulate the Voyager downlink telemetry sidebands to baseband. The baseband signal is sent to Goldstone via satellite link in real time where it is added to the Goldstone baseband signal before it is sent to the Voyager Project computers for processing.

The real time signal processor is the computer link between the operator and the various electronic subassemblies. It also contains the most important and complicated subassembly, the combiner, that aligns and adds the VLA and Goldstone signals.

Should the satellite link fail, real time combining would necessarily cease. To mitigate against the loss of the data, the baseband telemetry is demodulated in the symbol stream recorder and recorded as soft quanitzed symbols on magnetic tape. The recorded symbols from both VLA and Goldstone can be played back and combined at a later date in the symbol stream recorder and combiner located at Goldstone.

A MANAGEMENT CHALLENGE:

SPEEDING TOWARD YOUR DEADLINE AT 42,000 MILES PER HOUR.… WITHOUT BRAKES!

THE VOYAGER MISSION

Once every 175 years the giant outer planets - Jupiter, Saturn, Uranus, and Neptune - are in an arrangement allowing a spacecraft launched from Earth to Jupiter to fly by all four planets using “gravity assist” techniques. Each planet's gravity bends the spacecraft flight path toward the next planet and increases the spacecraft's velocity. Such a mission would have required the development of expensive new technology. As tempting as the opportunity may have been, budget constraints prevailed and the four-planet “Grand Tour” mission idea was shelved.

In 1972 the Congress approved funding for a spacecraft mission to Jupiter and Saturn. To achieve the scientific goals of the Jupiter-Saturn mission, yet not preclude the concept of the more ambitious mission to all four planets, NASA's Jet Propulsion Laboratory (JPL) engineers designed and built two Voyager spacecraft. The spacecraft were built to the specifications of the mission they were funded to fly; they were designed to last four years and outfitted to study just Jupiter and Saturn and their moons. No special equipment was put on board for a possible encounter with Uranus or Neptune.

Both spacecraft were launched in 1977 and reached Jupiter in 1979. The leading spacecraft, Voyager 1, arrived at Saturn late in 1980 and then set out on a course into interstellar space in search of the boundary of the solar system, where the solar wind fades away and the wind from the stars replaces it. Voyager 2 encountered Saturn in the summer of 1981. With Saturn's gravity assist, Voyager 2 was placed on a trajectory to Uranus and Neptune.

Despite the lack of funding for the spacecraft to conduct Uranus and Neptune encounters, Voyager officials kept those options open. Their hope was that funding would catch up with Voyager 2 as it flew across the solar system. And indeed it did. Funding for the Uranus leg of the mission was authorized in 1981, and for the Voyager Neptune encounter in 1985.

The Neptune encounter date was predetermined by celestial mechanics: August 25, 1989. Speeding toward Neptune at an average of 42,000 miles per hour, Voyager 2 will be the first spacecraft to visit Neptune. But as Voyager 2 flies farther away from Earth, its signal received on Earth becomes weaker, due to what is sometimes referred to as the distance-squared law. The radio signal from Voyager's transmitter is about the same wattage as the light bulb in a refrigerator. By the time the signal reaches Earth from Neptune, it will be billions of times weaker than the wattage used to run a digital watch. Part of the effect of distance from Earth could be offset by lowering the rate at which the spacecraft transmits data. This normal trade-off for getting high-quality data from Voyager at Neptune would reduce the amount of data transmitted. However, Voyager scientists want more pictures, not fewer.

The picture data, “imaging”, is the bulkiest of the data. If the imaging data could be compressed, more pictures could be sent. For Uranus and Neptune, engineers enhanced the Voyager 2 data transmission capability by radioing new software to the spacecraft to program a back-up computer to compress all imaging data before it is transmitted to Earth, thereby compensating for some of the effect of the spacecraft distance from Earth.

The Earth-based communications link with the spacecraft is provided by an extensive system consisting primarily of three radio telescope complexes located around the world, NASA's Deep Space Network (DSN). These complexes are strategically located so that at all times, as the Earth rotates, at least one radio telescope can communicate with the spacecraft. On Earth, engineers have designed and built equipment that adds (arrays) the weak signals from radio telescopes or groups of radio telescopes that are located hundreds or even thousands of miles apart to compensate for an additional portion of the distance effect of the spacecraft from Earth. To this end the VLA, arrayed with the DSN Goldstone facility near Barstow, California, will be employed to increase the data return.

THE VLA-GOLDSTONE ARRAY PROJECT

The VLA-Goldstone Array Project is only one of many data return enhancement tasks being implemented in support of the Voyager Project. Long-range planning to maintain the option of a Neptune encounter prompted JPL management to pursue VLA support beacuse use of the VLA would add the equivalent of about one and one-half 70-meter antennas to the DSN. The first overture was made in 1982, followed by high-level management plans being completed in 1983 calling for “good early planning, as well as continuous monitoring of performance by the responsible managers appointed by each organization.” An interagency Memorandum of Understanding between NASA and the NSF was signed in 1984. When the Neptune encounter was authorized in 1985, the Management Plan for the Array Project was nearly ready for approval.

The Array Project Management Plan specified the organization and management, schedule, funding, system engineering and testing, hardware and software responsibilities (deliverables), and operational support of the encounter. Within the organization and management area, the next level of planning detail was outlined and assigned to the responsible organization. Detail plans were required for equipment design and fabrication, technical interfaces, implementation, and operation of the completed facilities. The detail plans, specifications, and requirements were contained in a series of eight project documents that have been used to monitor performance and to assure that the deliverables met the specifications and could be operated by the DSN and VLA personnel to furnish data to the Voyager Project.

The paths of Voyager 1 and Voyager 2 through the Solar System and beyond.

The technology, schedule, and the participating organizations themselves provided the management challenge to the Array Project team. Briefly, the task involved validation that the technical concept of arraying was possible using the VLA (primarily an astronomy tool), developing the required new equipment embodying the latest technology, implementing operational equipment to meet the rigid deadline (the spacecraft encounter with Neptune won't wait), and accomplishing the task by drawing from the JPL and NRAO organizations whose management structures, styles, objectives and operating philosophies were different. For example, JPL and the Array Project operate in the matrix format, while NRAO and VLA are functional organizations. From the point of view of project management, the splitting or partitioning of parts of the Work Breakdown Structure (WBS) between the two organizations was nothing more than a matrix application at the project level. What was quietly thought about, however, was how the background of the two would come into play, if at all. The appointment of managers leading their respective organizations precluded any problems from the differences in organizational structure and separate bureaucracies.

Traditional management tools, tailored to the organizations and to the individuals selected for the team, were used. Communication management played a strong role, both for reason of the team structure and the personalities of the individual team members. Risk management was extremely important because of the rigid constraint upon the operational use date. Critical Path Methodology (CPM) was used at the project level to plan and monitor the progress. Of equal importance, CPM provided insight with respect to the logical dependency of an activity in one organization on the activities in the other organization. Lower level WBS scheduling responsibility was delegated to the supervisors in charge of the various cost centers. Technical design reviews, both formal and informal peer level reviews, were used with great success for early identification of problems.

The management strength of several key individuals in the day-to-day communication environment “made it happen”. Electronic mail and data transfer, fax, and generous amounts of personal contact between the two organizations promoted the essential interactive communications. Some personal tools that proved particularly effective include:

1. Regularly published action lists stating the issue, author, actionee and due date. This tool served as a management reminder, assignment stimulant, low-key score card, and progress report of actions that were closed.

2. Use of day-by-day schedules covering an interval of 10 weeks or less for unplanned problems. This created a focus for the few individuals involved in a specific problem and in all cases stimulated a real team effort.

3. Working meetings in the company cafeteria. This meeting format promoted team participation and provided the added benefit of removing the threat and confrontational atmosphere that many times is resident in conference rooms.

4. Informal communication between the managers and the team members. Providing the manager listened actively, early warning of a potential problem was almost always revealed in this communication environment.

PROBLEM:…AN ISSUE THAT AFFECTS PROJECT ACCOMPLISHMENT.

Managers are problem solvers, problem recognizers, and in the ultimate, problem preventers. From the list of problems the Array Project team solved, there emerged a management issue that problem-preventing managers may wish to consider in their current and future projects: SOFTWARE MANAGEMENT. Managing software development will soon become a basic PM function in every organization, if it isn't already. At JPL a software management standard is in place, while at the NRAO few formal software controls are utilized because of the scientific research climate.

Software developers, by nature, become involved in the detail and the elegance of their program, easily loosing sight of any schedule constraint and the need for adequate documentation. Today's project manager will need to become trained in software management and the language of the metrics of progress measurement to maintain PM success in this rapidly expanding area. In the Array Project, too little time was planned to accomplish software testing. Another lesson was sent home when a key programmer left the project before software testing was complete, leaving behind less than complete documentation. Valuable contingency time was consumed while the new programmers learned the code sufficiently to correct anomalies. In all software applications, structured software design documentation is imperative to support software anomaly correction and maintenance, and as a basis for future software change.

The manager must fight the urge to “wait and see” when a solution to a software problem is elusive. All the arguments against adding resources (“it is too late in development for them to help”) or bringing in expert consultants (“they will be bad for my programmer's morale”) should be considered, yet the problem should be attacked head on. The reluctance of supervisors to accept management resource help is likely buried in fear that they are being criticized or that they are being perceived as having failed. In this situation the manager needs to adjust his communication style to assure that human emotion does not drive the decision-making process.

PEOPLE MAKE THE DIFFERENCE

The authors are of the opinion that the people involved make the difference between success and failure of any endeavor: they solve your problems, but they also make your problems. There is as much people management as there is management of the basic PM functions. The manager must create a project team wherein the people are competent in their area of expertise and each is enthusiastic about the goal which causes them to be committed. Interpersonal communication between the manager and the team members is a key ingredient of success.

VOYAGER NEPTUNE ENCOUNTER SCHEDULE AND INFORMATION SOURCES

Anita Sohus, Administrative Specialist, Jet Propulsion Laboratory

GENERAL SCHEDULE:

Neptune Encounter period begins: Monday, June 5, 1989

Closest approach to Neptune: Friday, August 25, 0400 GMT

(Thursday, August 24, 9 p.m. PDT)

Closest approach to Triton: Friday, August 25, 0914 GMT

(Friday, August 25, 2:14 a.m. PDT)

Note: Traveling across 2.75 billion miles at the speed of light (about 186,000 miles/second), data from the spacecraft will reach Earth 4 hours 6 minutes after Voyager 2 transmits it. Much of the data obtained near closest approach will be recorded on board the spacecraft and played back to Earth over the next several days.

Neptune Encounter period ends: Monday, October 2, 1989

Voyager Interstellar Mission Begins: Monday, October 2, 1989

For more information, contact:

Jet Propulsion Laboratory
4800 Oak Grove Drive Pasadena, CA 91109
Attn:Public Information Office
Mail Stop 180-200
(818) 354-5011

Media Requests
Public Education Office
Mail Stop
180-205 (818) 354-8594

Educational Requests
Educational Outreach
Mail Stop CS 530
(818) 354-6916

Slides and video tapes of results of the Voyager missions are available for purchase from Holiday Film Corporation, 12607 E. Philadelphia, Whittier, CA 90608.

FOR MORE INFORMATION:

NASA Spacelink: An electronic bulletin board for teachers and historians is operated for NASA out of Marshall Space Flight Center, Huntsville, Alabama. Anyone with a computer and modem can access this data base of NASA facts and information. There is no fee to users apart from normal telephone charges while online. Spacelink supports 300, 1200, and 2400 baud, as well as XMODEM transfers of both text and graphics. To access Spacelink, dial (205) 895-0028. Computer settings should be 8 bits, 1 stop bit, no parity.

NASA Select TV: On the evening of August 25, three to six hours of Neptune Encounter programming will be broadcast from JPL, directed at science centers, planetaria, and universities in the continental U.S. The satellite is the GE Satcom, F2R, Transponder 13. The receiver information is 72 degrees west, vertical polarity, 3960 megahertz, C-band. The program will include data displays, specially processed video and science data, pre-recorded Voyager Project summaries, interviews, computer animations, review of future JPL missions, etc. At JPL, contact the Public Education Office. For NASA Select T.V. information, contact William W. Robbins, NASA/Johnson Space Center, Houston, TX (phone: 713-483-5111).

BOOKS ON VOYAGER:

Note: Listings here are for information only and do not necessarily imply endorsement

The following are sold by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402:

Morrison, David C. and Jane Samz, Voyage to Jupiter, NASA SP-439, NASA, 1980.

Morrison, David C., Voyagers to Saturn, NASA SP-451, NASA, 1982.

GENERAL BOOKS ON THE SOLAR SYSTEM:

Beatty, J. Kelly, Brian O'Leary, and Andrew Chaikin, eds., The New Solar System, Cambridge University Press and Sky Publishing Corporation, 1981.

Frazier, Kendrick and The Editors of Time-Life Books, Solar System, Planet Earth series, Time-Life Books, Inc., 1985.

Gallant, Roy A., National Geographic Picture Atlas of Our Universe, National Geographic Society, 1980.

The Editors of Time-Life Books, The Far Planets, Voyage Through the Universe series, Time-Life Books, 1989.

Trefil, James S., SpaceTimelnfinity, Pantheon Books and Smithsonian Books, 1985.

MAGAZINE ARTICLES:

Instrument and Investigation Summaries:

Space Science Reviews, Vol. 21, No. 2 (November 1977) and No. 3 (December 1977)

Official results - of the first analyses of the Voyager encounter data are traditionally published in Science, the magazine of the American Association for the Advancement of Science:

Vol. 204, pp. 913-924 and pp. 945-1008, 1 June 1979 (Voyager 1 at Jupiter)

Vol. 206, pp. 925-996, 23 November 1979 (Voyager 2 at Jupiter)

Vol. 212, pp. 159-243, 10 April 1981 (Voyager 1 at Saturn)

Vol. 215, pp. 459 and 499-594, 29 January 1982 (Voyager 2 at Saturn)

Vol. 233, pp. 1-132,4July 1986 (Voyager 2 at Uranus)

Voyager science results have been published in professional magazines such as

Icarus, Journal of Geophysical Research, Nature, Science, Scientific American, and Space Science Reviews.

POPULAR ARTICLES:

National Geographic Articles:

Gore, Rick, “Voyager Views Jupiter's Dazzling Realm,” Vol. 157, No. 1, January, 1980.

Gore, Rick, “Saturn, Riddles of the Rings,” Vol. 160, No. 1, July 1981.

Gore, Rick, “Uranus, Visit to a Dark Planet,” Vol. 170, No. 2, August 1986.

Neptune Encounter:

Beatty, Kelly, “Voyage to a Far Moon,” Omni, December 1988.

Berry, Richard, “Triton”, Astronomy, February 1989.

Kohlhase, Charles, “Aiming for Neptune,” Astronomy, November 1987.

Miner, Ellis D., “On to Neptune”, The Planetary Report, November/December 1986.

Sohus, Anita, and Ellis Miner, “The Voyager Mission to Neptune,” Mercury, (magazine of the Astronomical Society of the Pacific), September/October 1988.

Articles on Voyager often appear in popular magazines such as:

Astronomy, Discovery, Mercury, National Geographic, New Frontier, Omni, Science News, Sky and Telescope, and The Planetary Report.

Anita Sohus has been associated with the Voyager Project since 1976, primarily as a technical writer and editor. She has also worked for the Solar System Exploration Division at NASA Headquarters, Washington, D.C., and JPL's Space Station Office in Reston, Va. She received an English degree from UCLA in 1973. In her spare time she rides horses, backpacks, skis, reads and travels.

1. California Institite of Technology operates the Jet Propulsion laboratory under contract to the National Aeronautics and Space Administration.

2. Associated University, Inc., operates the National Radio Astronomy Observatory under National Science Foundation Sooperative Agreement No. AST-8814515.