SP-349/396 PIONEER ODYSSEY

5

First into the Outer Solar System

[ 61 ] THE FIRST PUBLIC discussion of the experiments for the Jupiter mission was announced in a press release addressed to Bay Area Editors: On Thursday, May 7 [1970], the 13 experimenters participating in the first mission to Jupiter will be at NASA's Ames Research Center, Mountain View, for a project coordination meeting. The Pioneer F and G scientific spacecraft now are scheduled for launch to the vicinity of Jupiter in 1972 and 1973. At 11:30 a.m. we will present a review and question and answer session for news media on the mission and experiments.

Several hundred scientists and engineers crowded the main auditorium of the Ames Administration Building . . . a planning session for trajectories to Jupiter aimed at a launch date in early 1972. Questions raised by scientists in connection with their experiments were pondered by celestial dynamicists and spacecraft engineers.

'The ultraviolet experiment needs to look at sunlit side of the planet."

"It would be important to look at the solar wind outside the ecliptic plane. Could the trajectory to Jupiter provide for this?"

"Can Pioneer reach the region of space, perhaps 30 to 100 times the Earth's distance from the Sun, where the solar wind stops blowing?"

"Does 15 times the Earth's distance from the Sun represent the limit of communication with Pioneer?"

These and other questions sparked this first of a series of coordination meetings to plan the mission to Jupiter. It was stressed that one of the prime purposes of the Pioneer Jupiter mission was to provide information on the environment near the major planet of the Solar System so as to be able to plan more sophisticated spacecraft for later missions to the outer planets.

Among matters debated by scientists was whether or not a passage over a pole of Jupiter would be fruitful since the atmosphere seemed to be more transparent over the poles of the planet. Hence, Pioneer might be able to look down into the 137,000 km (86,000 miles) diameter ball of hydrogen and see much deeper than by a passage over equatorial regions. This, too, was abandoned for the first Pioneer because the spacecraft mission was directed at the radiation belts which were believed to be concentrated about the equatorial regions.

Considerable discussion took place at this and other early meetings about the effects the nuclear powered Radioisotope Thermoelectric Generators (RTGs) might have on the scientific instruments. Pioneer Jupiter was designed as an electromagnetically clean spacecraft so as to be able to measure very weak fields in deep space. The radiation [ 62 ] from the RTGs could "dirty" the spacecraft with neutrons and subatomic particles. Representatives of the Atomic Energy Commission explained how elimination of impurities from the radioactive fuel elements could reduce the radiation hazard from the RTGs and how this was being planned.

At a press conference following the technical meetings' the exploratory and ambitious nature of the Jupiter mission was emphasized. The duration of flight would be greater than that for any other space project to date. There were tremendous unknown factors in the environmental hazards between Mars and Jupiter and in close approach to Jupiter itself. Moreover, as pointed out by Glenn A. Reiff' Pioneer Program Manager from NASA Headquarters: "The telecommunications network will be stretched to the limits of its capabilities."

Charles F. Hall, Project Manager at NASA-Ames, confirmed that all the spacecraft design had been completed and that construction of the first of the two spacecraft was under way at TRW Systems, Redondo Beach, California. "Experimenters and staffs are here to discuss the design of the scientific equipment to integrate with the spacecraft and its mission profile." Everything was on schedule, said Mr. Hall, as he pointed out how important it was to keep to the tight schedule, since the launch window would be open for only 18 days in 1972.

Almost a year later in March 1971, at a similar meeting held in the main auditorium at TRW Systems scientists stated that the RTGs were generating higher radiation than expected at the May 1970 meeting, but instruments had been adapted to this radiative environment. Details of the science experiments showed the wealth of data expected from the mission in space and at Jupiter. Scientific equipment, like the spacecraft, were being readied on schedule, even though the scientists had to make many guesses at what conditions would be like in the asteroid belt and at Jupiter. Thus' instruments had to be designed with a wide range of capabilities because of extremely sparse information about the outer Solar System.

Meanwhile, trajectory analysts continued their work and evaluated various approaches to the planet. Targeting to have the spacecraft occulted by a satellite of Jupiter would allow the radio signals to probe through any atmosphere possessed by the satellite and determine its composition. Questions revolved on which satellite. Io was a prime objective for such an experiment because somehow it modulates the radio waves from Jupiter itself. But the other satellites were also of interest to scientists as potentially possessing atmospheres.

On November 16, 1971, Ralph W. Holtzclaw, Spacecraft Systems Manager at NASA-Ames, discussed the needs to make the spacecraft reliable for its long mission. "No single component failure can be catastrophic to the mission," he said.

Mr. Holtzclaw pointed out that the Pioneer mission is quite different from earlier space missions. Scientists have to spend several years planning the experiments. Then the spacecraft operates for seven or eight years more in space, possibly for even longer. Scientists are thus being asked to dedicate 10 years or more of their lives to a single experiment.

All the preparations went well. From contract award to the scheduled launch date, early March 1972, was I month less than 2-1/2 years. But the thousands of people involved in the Pioneer Jupiter program performed their tasks on schedule to meet the critical time of launch window (Figure 5-1).

Charles F. Hall praised those who had made the program possible: "It is most appropriate to compliment the many dedicated people who have worked so hard to reach this first goal of the Pioneer F mission to Jupiter and to congratulate all for a job well done. I estimate that at the time of the Pioneer F launch, more than 15 million manhours will have been expended to make this goal possible. I am sure that you all feel as I do that a successful mission wherein we will be exploring new frontiers in space will be a just compensation....

[ 63 ]

Figure 5-1. Many thousands of people contributed to the development and fabrication of the Pioneer spacecraft destined for the Jupiter mission. Their dedication made it possible to meet the tight schedule for the first available launch window.

[ 64 ] ....for this large effort and that we are, indeed, a fortunate, select group which has been given the opportunity to participate in and contribute to the Pioneer F Mission."

By December 22, 1971, the Atlas Centaur launch vehicle stood ready on Launch Complex 36A at the John F. Kennedy Space Center, Florida. The spacecraft was airlifted with its full complement of scientific instruments, but without its RTGs, from TRW Systems, California, to Florida on January 15, 1972.

To ensure operational readiness of the spacecraft and its science instruments for launch, it was tested through a simulated countdown in Building AO (Figure 5-2). The RTGs were installed and the spacecraft loaded with propellant and, finally, mated to the third stage of the launch vehicle and encapsulated. The whole assembly was then transferred to the launch pad and mated to the Atlas-Centaur.

Yet when the spacecraft was ready on its launch pad, all was not smooth sailing. Upper atmosphere winds delayed the launch after the window opened on February 27, 1972.

Within 59 minutes of the planned 8:52 p.m. liftoff, blockhouse electrical power failed. Then, high winds made it too hazardous to launch the spacecraft. These winds prevented launching on February 28 and March 1, and it was not until March 2, 1972, at 8:49 p.m. EST, that the Atlas Centaur lifted from the launch pad (Figure 5-3) carrying Earth's first space probe to the outer planets. The beauty of a night launching from Cape Kennedy was punctuated and enhanced by distant thunder and by lightning flashing on the cloud tops as the brilliant light of the Atlas engines' exhaust jet rose through the clouds.

Events happened fast and forced quick decisions. Mission controllers had to know if the spacecraft was affected in any way by the stresses upon it. Telemetered data poured into the control centers by radio from the launch vehicle and the spacecraft. If anything looked awry, corrective action had to be taken immediately to try to.....

Figure 5-2. A simulated countdown at Kennedy Space Center thoroughly checked the spacecraft prior to launching.

Figure 5-3. Finally, at 8:49 p.m., 2 March, 1972, Pioneer 10 started its journey to Jupiter and thence to the stars.

[ 65 ] .....save the spacecraft and its scientific payload. But atop the powerful Atlas-Centaur booster, Pioneer 10 withstood the pounding thrust of the rocket engines while it absorbed enough kinetic energy to break free from Earth's gravitational fetters.

Powered flight lasted some 17 minutes, accelerating the spacecraft to 51,682km (32,114 miles) an hour, almost 11,300 km (7,000 miles) per hour faster than any previous man-made object. The Atlas exhausted its propellants and dropped behind as the white brilliance of the Centaur took over the thrusting into space. But the tiny ball of brilliant white fire began to disappear into the black void of the night sky. Soon it was lost to view of the people on the beaches, lost to view of Earthmen as a tangible thing. The spacecraft was on its way to Jupiter, and onwards to infinity, for it will ultimately escape from our Solar System and journey among the distant stars. Successfully on its path to the giant planet, Pioneer F became Pioneer 10.

Shortly after separation from the upper stages, the spacecraft deployed its RTG power units at the end of two arms, thereby slowing its rate of rotation. Then the third boom, tipped with the sensitive magnetometer, crept slowly out to its full 5.2 meter (17 foot) length.

Launching achieved unprecedented accuracy, requiring a correction of only 50.4 km (31.3 miles) per hour to the launch velocity. This correction was commanded and took place on March 7. Actually, Pioneer could have reached Jupiter, but the correction was needed to obtain a time of arrival that would better suit some of the experiments.

After Pioneer 10 had separated from its launch vehicle, a sequencer activated the attitude control system to turn the spacecraft around and orient it for its long voyage so that the big dish antenna pointed toward Earth. Actually, while near to Earth, Pioneer 10's orientation was such that sunlight illuminated it from the side, thus causing heating problems for several weeks after launch. To reduce these temperature problems, the spacecraft....

Figure 54. Only 11 hours after launch Pioneer 10 passed the orbit of the Moon, shown conceptually in this artist's sketch. Actually the Moon was at a more distant part of its orbit.

....was, in fact, commanded to point slightly away from Earth so that a shadow of the dish antenna would shield vulnerable parts, such as the battery, from solar heat.

Pioneer 10 passed the orbit of the Moon in less than 11 hours after launch (Figure 5-4).

The plasma analyzer was the only instrument carried by Pioneer 10 designed to look directly at the Sun. Most others could not point sunward without risking serious damage. So, in the early stages of the flight, when the Sun illuminated the spacecraft from the side, some of the instruments were left unenergized while others were protected in various ways, such as by sunlight screens.

Instruments such as the magnetometer and charged particle detectors are not affected by sunlight. They were turned on quickly to provide an in-flight calibration in the well-explored magnetic and radiation environment of space surrounding the Earth.

Two days after liftoff, the cosmic ray telescope was turned on, and then in the next few following days, the ultraviolet photometer. the asteroid-meteoroid detector, imaging photopolarimeter, and the plasma analyzer, in that order. All scientific instruments had been turned on by 10 days after launch.

Scientists, located at the Pioneer Mission Operations Center, watched each instrument closely, as....

[ 66 ]

Figure 5 5. A first task was to investigate the Zodiacal Light, a faint glow seen in dark Earth skies after sunset and before sunrise.

....commands were sent to turn on each of the scientific instruments onboard the spacecraft. The operations science chief for Pioneer, the cognizant instrument engineer, and the principal investigator for the respective science instruments, inspected the returned data, critically seeking any information that might reveal an instrument malfunction that required corrective action. All went well with Pioneer 10; all the science experiments survived the stress of the launch.

After the first month of its journey, the performance of Pioneer 10 continued to be excellent. As Ralph W. Holtzclaw said at the time: "Now that we have had a chance to recover from the emotional trauma of getting Pioneer 10 launched, it is time to sit down and perform a factual engineering examination of this machine 'we' have wrought. As Pioneer 10 settles into the 'cruise' phase of its voyage to Jupiter, many analyses must be made of the live operation of this vehicle in a space environment to ensure specified performance during the crucial Jupiter encounter. Preliminary indications are that Pioneer 10 is a good spacecraft and a good mission. "

Ninety-seven days after launch, Pioneer 10 had covered one seventh of the time of the mission to Jupiter and one quarter of the distance. There had been a few anomalies within the spacecraft, but nothing serious. Experimenters were enthusiastic [ 67 ] about the way equipment behaved. The characteristics of charged particles, the interplanetary plasma, the Zodiacal Light, had all been observed and studied in detail.

Project scientists announced some preliminary findings of Pioneer 10 as it crossed the orbit of Mars and headed out into unexplored space. Said Charles F. Hall: "This meeting today is to cover the passage of the Pioneer spacecraft into a new area. By tomorrow we will have crossed a point farther from the Sun than the farthest distance of Mars. From now on, we will be in a new area of space. "

So far the explorer had been "East of the Mississippi," but on crossing the most distant point of the orbit of Mars. it became a true pioneer into the unknown. Here the solar wind was expected to change dramatically. Here, the flux of small particles might build up to damaging proportions in the asteroid belt. Here, the spacecraft signals became weaker and the time of command and action began to increase. The controllers and the scientists were gaining experience in controlling very distant spacecraft, a learning process of anticipating and doing things in advance.

The imaging photopolarimeter, the instrument that would later in the mission provide data to build up photographic quality images of Jupiter and perhaps one of its satellites, had been busy investigating the Zodiacal Light. If one looks at the sky from Earth on a moonless night (Figure 5-5). away from the glare of city lights, a very faint glow is seen over the whole of the sky, but concentrated near to the path of the Sun through the celestial sphere, along the constellations of the Zodiac. In spring, in the northern hemisphere, this Zodiacal Light is most easily seen as a cone of light in the western sky, after the sky has darkened at sunset; and in autumn, this cone precedes the rising of the Sun in the East.

Pioneer 10 measured the intensity of the Zodiacal Light in interplanetary space, and for the first time investigated, away from Earth, a concentration of the Zodiacal Light in a direction away from the Sun. This anti-solar concentration is called the Gegenschein or counterglow. Although the prevailing scientific opinion was that this glow results from sunlight scattered by interplanetary particles there was some speculation that the Gegenschein might somehow be connected with the Earth, possibly a reflection from a tail of particles streaming out away from the Sun. But Pioneer 10 showed that the Gegenschein still shines as far as Mars and confirmed that it is not an Earth-related phenomenon.

Now a further correction was needed to get the spacecraft precisely to Jupiter for an observation of a satellite. Said Charles F. Hall, "If we don't make any further correction to the trajectory, we [the spacecraft] will not be occulted by lo. We are in error by 14 minutes at the arrival. Our plan is to correct at the end of June or by mid-July. We have waited this long because there is a pressure on the spacecraft due to light energy coming from the Sun. We wanted to allow the spacecraft to coast for a long period of time so we can accurately determine just what this solar pressure is."

On July 15, 1972, Pioneer 10 became the first spacecraft to enter the asteroid belt. Since the belt is too thick to fly over without prohibitively expensive launch vehicles, all missions to the outer planets must fly through it.

Based on a variety of analyses, project officials expected a safe passage, but the risk was always present that analyses from Earth could be wrong. Pioneer's closest approach to any of the known asteroids, visible by telescope, was 8.8 million km (5.5 million miles). One was a 1 km (1/2 mile) diameter asteroid on August 2, and the other was Nike 24 km (15 miles) in diameter on December 2, 1 972.

But any particle over 0.05 cm (1/50 inch) in diameter could seriously damage the Pioneer 10 spacecraft since it could impact with 15 times the speed of a high powered rifle bullet. An estimate of such an impact was one in ten, or a 90 percent chance of passing through the belt undamaged.

Pioneer Jupiter reached 322 million km (200 million miles) from Earth on September 1,....

[ 68 ]

Figure 5-6. Pioneer investigated the concentration of small meteoroids in space, finding that this varies somewhat with distance from the Sun but not entirely as had been expected. Pioneer 10 penetrated the asteroid belt and remained undamaged.

.....1972, and was then deep in the asteroid belt and still undamaged (Figure 5-6). Now it took controllers 43 minutes to send a signal to the spacecraft and get a reply from it. Ten of the spacecraft's experiments were operating. The remaining experiment was not needed until the spacecraft reached Jupiter, but this infrared radiometer had been turned on monthly to check its operational status.

In August 1972, several unprecedented storms on the Sun (Figure 5-7) provided Pioneer 10 with a unique opportunity to measure the behavior of the solar wind at much greater interplanetary distances than ever before.

Known as region 331, a huge area of the Sun unexpectedly erupted to produce three enormous storms on August 2 and another on August 7. This latter event produced in one hour enough energy to satisfy the present rate of electrical power consumption of the United States for 100 million years.

Effects of the storm in Canada, the northern United States, Sweden, and Alaska were severe [ 69 ] since the solar wind warped the Earth's magnetic field and caused power and communications blackouts and other magnetic disturbances.

Pioneer 10's measurements were correlated with those from a series of earlier Pioneers in orbit around the Sun. These four spacecraft, Pioneers 6, 7, 8, and 9, are located at different azimuthal positions from Earth, but at solar distances only slightly different from that of Earth. The Pioneers measured the solar wind and its magnetic fields as it swept through space (Figure 5-8). Pioneer 9, close to Earth, saw the highest solar wind speeds ever recorded: 3,597,000 km (2,235,000 miles) per hour. But in crossing the 214 million km (133 million miles) to Pioneer 10 in just 76 hours, the wind slowed to about half this velocity.

Pioneer 10 measured the enormous equivalent temperature of 2 million degrees Kelvin, similar to that of the solar corona itself.

Dr. John Wolfe, Pioneer Project Scientist, explained: "The velocity of the solar wind in the interplanetary medium is dependent on the temperature of the solar corona, and from a temperature point of view, the solar corona is quite inhomogeneous. Thus, the Sun emits both fast and slow flowing plasma. The energy density of the solar wind is 100 times that of the interplanetary magnetic field, so the solar wind drags along and carries the magnetic field with it. This magnetic field not only screens incoming cosmic rays and prevents the low energy ones from outside the Solar System from entering into the inner Solar System, but also stops the fast flowing plasmas from penetrating the slow flowing plasmas.

"Because the Sun rotates, a fast solar wind can catch up with a slow solar wind. When the fast and slow winds interact, they produce a snowplow effect and steep gradients are produced at the interface. These gradients scatter incoming cosmic rays. One of the missions of Pioneer 10 is to check where these cosmic ray scattering regions might fade away, perhaps at 10 to 15 times the distance of Earth from the Sun; way beyond the orbit of Jupiter."

Figure 5-7. In August 1972 an unprecedented event on the Sun provided Pioneer 10 with a unique opportunity to observe the solar wind. A series of immense flares erupted on the Sun and sprayed interplanetary space with highly energetic particles. (Photo: NOAA)

Figure 5-8. Pioneer 10, operating in conjunction with other Pioneer spacecraft closer to Earth, observed the distribution of the solar wind from these immense flares.

[ 70 ] Since the only star that man can study at close range is the Sun, the data from Pioneer 10 is expected to help in man's understanding of all Sun-like stars.

Up to the point half way through the asteroid belt, Pioneer 10 still proceeded undamaged. It passed through the two regions of greatest asteroid density - at 400 million km (250 million miles) and 480 million km (300 million miles) from the Sun unscathed, although sightings of larger asteroid particles appeared to increase in number for about a week near to the 400 million km (250 million miles) region.

In February 1973, Pioneer 10 at last officially emerged from the asteroid belt, at a distance of 550 million km (340 million miles) from the Sun, having completed the 435 million km (270 million miles), seven-month passage through the belt without incident. Pioneer showed that the belt appeared to contain much less material in the small particle sizes than had been anticipated. The way was open to exploration of the outer Solar System.

As Pioneer 10 emerged from the asteroid belt, Pioneer G, its follow-on spacecraft, was readied for launch at Kennedy Space Center. Should Pioneer 10 in any way fail during the rest of the mission, Pioneer G would repeat the failed part of the mission. Otherwise, Pioneer G would be retargeted to fly a different course through the Jovian environment and obtain another set of samples of that environment.

The first launch window opened at 9:00 p.m. on April 5, 1973. A few seconds later, Pioneer G was successfully launched to become Pioneer 11 (Figure 5-9). Subsequent hours proved tense when one of the RTG booms failed to extend properly. However, the trouble was corrected, and Pioneer 11 followed its sister ship toward Jupiter: all systems working properly, all scientific instruments performing well, and additionally, with an improved star sensor.

Pioneer 11 repeated the experiments of Pioneer 10 in the interplanetary mission and like Pioneer 10, it too passed safely through the asteroid belt. Pioneer 11 was targeted to be closest to Jupiter at 9:21 p.m. PST on December 2, 1974, and then to fly on to the planet Saturn, for a rendezvous with the ringed planet in September 1979 to seek information about that mysterious planet before a more sophisticated spacecraft, Mariner Jupiter-Saturn, arrives there several months later.

At the beginning of November 1973, controllers entered the busiest activity period connected with Pioneer 10. They readied the spacecraft for its time of closest approach with Jupiter. early in December. By November 6, long range imaging tests commenced on the planet at a distance of 25 million km (15.5 million miles). And Pioneer 10 crossed the orbit of Jupiter's outermost satellite Hades, on November 8. Controllers were now in the process of starting the sequence of sending some 16,000 commands to the spacecraft to direct all the various scientific experiments and the spacecraft for the 60-day encounter period during which Pioneer 10 made its close passage to within 130,354 km (81,000 miles) of Jupiter's cloud tops on December 3, 1973.

Figure 5-9. Pioneer 11 followed Pioneer 10 toward Jupiter on April 5, 1973. Launching took place within a few seconds of the opening of the launch window.

[ 71 ] The mission had by now set an array of records:

Pioneer 10, the first NASA spacecraft to use all nuclear electrical power, had traveled farther and faster than any other manmade object. Its launch speed of 51,682 km (32,114 miles) per hour, a record in itself, was, however, soon to be surpassed by its speed at closest approach to Jupiter of 132,000 km (82,000 miles) per hour.

Pioneer 10 had communicated many times farther than ever before, and held promise of communicating to the immense distance of Uranus' orbit, nearly 3.22 billion km (2 billion miles) from Earth.

Pioneer 10 was the first spacecraft to fly beyond Mars' orbit; the first to cross the asteroid belt; and would become the first man-made object to escape our Solar System.

Moreover, the accuracy of the control was such that Pioneer 10 was expected to reach its closest approach to Jupiter within one minute of the planned time: one minute in almost two years of flight.

Shifts of people maintained watch on Pioneer 10 around the clock for the several weeks of close approach and passage through the Jovian system (Figure 5-10). They included experts on the spacecraft subsystems and on the scientific instruments, not only project personnel but also experimenter personnel and even a trained volunteer (H. H. Dodeck) from Germany's HELIOS project.

By November 29, 1973, with all systems aboard the spacecraft functioning perfectly, Pioneer 10 had crossed the orbits of all seven outermost satellites of the Jovian system and was readying for its plunge toward the radiation belts and its close encounter with the giant planet. Dr. Hans Mark, Director of the Ames Research Center, told newsmen gathered in the auditorium at Ames for the....

Figure 5-10. Shifts of people maintained watch on Pioneer 10 around the clock for the several weeks of encounter with Jupiter.

[ 72 ]

Figure 5-11. NASA Administrator Dr. James C. Fletcher and Ames Research Center Director, Dr. Hans Mark, brief newsmen and visiting scientists gathered in the auditorium at NASA-Ames Research Center to hear updated accounts of the events from experimenters and project personnel.

....historic Jupiter encounter (Figure 5-11): "This is an unusual event. The planet Jupiter, as you know is an object that has been the subject of fairly extensive investigation for almost 400 years. Galileo, who looked at the planet through his primitive telescope in 1610, discovered . . . the four brilliant moons that surround the planet. This observation provided, I think, the first really visceral proof, as it were. that the Copernican model of the Solar System wasn't exactly the way it looks. Jupiter, therefore, served, perhaps, the function of quite profoundly changing the way we think about ourselves and the way we think about the universe."

Charles F. Hall added: "We are really only twelve generations away from Galileo and his first crude look at the planet. Twelve generations later. we are actually there measuring many of the characteristics of the planet itself" (Figure 5-12).

Pioneer 10 had, by now, already passed through the bow shock where Jupiter's magnetic field affects the solar wind. This took place about noon on November 26, at a distance of about 108.9 Jupiter radii or about 6.4 million km (4 million miles), a little farther out than had been anticipated. A first indication that the spacecraft had crossed the shock wave was instruments sensing approximately a 50 percent drop in solar wind speed. This information, of course, arrived at Earth 45 minutes after the spacecraft measured the drop in velocity. Prior to the shock the solar wind blew on the spacecraft at 451 km (280 miles) per second. Immediately after crossing the shock, this wind dropped to about 225 km (140 miles) per second, while its temperature rose from about 50,000 degrees Kelvin to a half million degrees. Of course the spacecraft, itself, did not experience this temperature which was that of a highly rarefied plasma unable to transfer significant quantities of heat to the spacecraft.

A day later November 27, 1973, at noon - Pioneer 10 crossed the boundary between the shocked solar wind and the magnetic field of Jupiter, called the magnetopause. The distance was [ 73 ] now 96.4 Jupiter radii. Explained Dr. John Wolfe (Figure 5-13): "The observation is that this is the point at which the pressure of gas coming from the Sun, after it has gone through the shock wave, becomes equal to the pressure of Jupiter's magnetic field, and the gas which is contained within that field. So the solar wind stopped at this point."

While similar to Earth, this environment of Jupiter was in some ways quite different. Near the boundary of Earth's magnetic field, all the strength that holds off the solar wind is due to the Earth's magnetic field. But for Jupiter, this turns out not to be true. Much plasma is contained within Jupiter's magnetic field near the boundary, and helps to hold off the solar wind. This additional barrier is about equal to the magnetic field itself. Another point discovered is that the magnetopause is close to the bow shock.

Pioneer 10's instruments confirmed what radio astronomers had postulated about the magnetic field of Jupiter, that it is inverted, compared to that of the Earth. The magnetic north pole is to the south. The science experiments showed, too, that the magnetosphere of Jupiter is somewhat different from that of the Earth, being flattened in its outer regions. This was inferred by the way Jupiter's magnetic field lines were stretched out from the planet in the outer regions. The magnetic field of Jupiter is offset from the axis of rotation so that at any point in space, around the planet, the field appears to move up and down; the disc of the outer magnetosphere wobbles relative to the surrounding space.

As for the spacecraft, none of the redundant circuits had yet been needed. So optimism was high that even if the Jupiter passage did damage some equipment, there would still be backup equipment available for the post encounter period of flight beyond Jupiter.

The spacecraft had, however, speeded up slightly over its anticipated course and was to arrive at Jupiter one minute earlier than previously calculated. This arose because Jupiter turned out to be slightly heavier than calculated from Earth-based observations.

Figure 5-12. Twelve generations after Galileo's first telescopic look at Jupiter, we are actually there making measurements (Charles F. Hall).

Figure 5-13. Dr. John Wolfe explains observations made by Pioneer 10 as it traverses the Jovian system.

[ 74 ] In the previous 24 days, thousands of commands had been transmitted to the spacecraft and during that time, only one ground data system failure occurred, when one of the computers became overloaded. Recovery was within two minutes. The numbers of commands were, by this time, building up daily from 400 to 2,000 per day as the closest approach to Jupiter neared. A special command sequence was developed to reconfigure the imaging photopolarimeter regardless of what spurious command functions were activated by the radiation. Also, a sequence of contingency commands was periodically transmitted so that the spacecraft could be corrected even before the signals telling of the spurious commands could be received at Earth.

During the encounter, only one scientific objective was missed because of false commands generated by Jupiter's intense radiation. This was the close-in imaging of the satellite Io. The imaging photopolarimeter responded to spurious commands 10 times, but the reconfiguration countered these commands so that only a few partial closeups of Jupiter were lost, in addition to the Io image.

Pictures of Jupiter had now been coming back for several days, each one built up from a number of scans as the rotation of the spacecraft swept the imaging photopolarimeter's narrow-angle telescope system across the disc of Jupiter. Twelve pictures of Jupiter were received at Earth on November 26 and many more in subsequent days. Images were returned in two colors, red and blue, from which a detailed color picture of the planet could be produced later.

Quick-look pictures from the spacecraft were displayed on television screens in the auditorium at NASA Ames. As the spacecraft data arrived they were placed vertically on the screen as a series of bars until the complete disc of the planet was assembled.

The Pioneer Image Converter System (PICS) was developed by L. Ralph Baker of the University of Arizona. The system was designed to present real-time display of Pioneer spin-scan images to allow scientists to monitor operation of the imaging photopolarimeter during encounter, and also to provide a video signal so that the images could be displayed to the press and made available to television networks. Thus, the public was able to view the results of the flyby of Jupiter as it took place.

The imaging photopolarimeter on the spacecraft scanned the planet in two colors, red and blue. But these colors, although chosen to get best scientific results from Jupiter, are not sufficient to produce a visually satisfactory image. If the red and blue images from Pioneer 10 had been simply mixed together, they would have produced a magenta image, unlike Jupiter and unnatural to the human eye. Instead, the red and blue signals were mixed to make a synthetic green signal, and a normal three-color combination was then obtained (Figure 5-14).

To begin with, the displayed pictures were pictures of Jupiter similar to those taken from Earth. This was because, with the spacecraft far from the planet, there was little distortion. But as the spacecraft sped towards the Jovian cloud tops, the rapidly changing geometry made the disc of Jupiter look like a painting of the planet on a rubber sheet which had then been stretched out of shape. Even so, the visible details (Figure 5-15) held tremendous promise for the time when these close-in pictures could later be processed and corrected by the computers, as described in later chapters. The PICS system has been improved to remove some of this distortion for the Pioneer 11 encounter. For this real-time display of images of Jupiter, the Pioneer Program received an EMMY award (Figure 5-16).

By December 2, 1973, the imaging of Jupiter began to exceed the best pictures obtained previously from Earth. When Pioneer 10 approached to six times the radius of Jupiter and still functioned well, it had cleared the way for the 1977 Mariner Jupiter-Saturn mission, as this would be the planned closest approach for that mission.

But Pioneer 10 explored farther down into the hostile environment of Jupiter, to two Jupiter radii....

[ 75 ]

Figure 5-14. A Pioneer Image Converter System allowed scientists and the public to see images of Jupiter almost as quickly as they were received at Earth. Color images of Jupiter were built up for display as they were received at Pioneer Mission Operations Center. Not only could scientists monitor the quality of the experiment during encounter, but also the images provided the public with a first-hand view of the large planet as Pioneer flew by Jupiter. The Pioneer Image Converter System (PICS) produced a synthetic green image of Jupiter to mix with the blue and red images returned from Pioneer and provide a reasonable color rendition that was also suitable for public television.

Figure 5-16. For the real-time display and release to the media of Jupiter images, the Pioneer Program received an EMMY award.

Figure 5-15. Excitement rose as the PICS system displayed images of Jupiter of ever increasing size as Pioneer 10 plunged in at high speed toward its closest approach to the planet. But perhaps the most dramatic moment was after encounter and the spacecraft had been hidden behind Jupiter. PICS started to show a few spots which gradually built up into a very distorted crescent Jupiter. "Sunrise on Jupiter," an experimenter exclaimed excitedly. "We've made it safely through periapsis." Subsequent PICS images showed a crescent Jupiter which gradually decreased in size. (See following pages.)

[ 76 - 77 ] Figure 5-15.

[ 78 ] ....above the cloud tops. In the words of Robert Kraemer of NASA Headquarters: "We can say that we sent Pioneer 10 off to tweak a dragon's tail, and it did that and more. It gave it a really good yank and . . . it [Pioneer 10] managed to survive."

Some spacecraft systems began to feel the effect of the Jovian radiation environment. One of the investigators used up 99 percent of the range on his instrument to obtain data despite the radiation, but the instrument made it through the encounter with just the remaining one percent to spare.

As expected, some of the instruments had saturated in the intense radiation field close to Jupiter. Two of the interplanetary cosmic ray detectors saturated a day before the closest encounter. Scientists had planned for this and provided a special Jovian radiation belt detector which worked well. Protons were measured for the first time in the radiation environment of the giant planet.

The interplanetary electron measuring instrument also saturated on the way in, but again, the experimenters were ready with a pair of special detectors for the Jovian environment to measure successfully the close-in electrons and protons.

All experimenters experienced anxious hours as telemetered data showed their detectors climbing towards the limits. They breathed sighs of relief as peaks were reached, and the intensities measured began to drop.

Up until noontime of December 3, as the Pioneer 10 approached periapsis, all went well with the imaging photopolarimeter. Many thousand commands had been sent successfully. Then about 10:00 a.m., at a distance of nine Jupiter radii, the instrument started to act as though it had received spurious commands which upset the imaging sequence. The problem was quickly overcome, but it occurred again on the way out from Jupiter at about the same distance.

Nevertheless, the equipment obtained close-in images of the terminator and the Great Red Spot. But then Pioneer 10 went behind Jupiter and communication with Earth ceased. Anxiously, the experimenters waited for the return of the signals. Would all the scientific instruments continue working...

[ 79 ]

Picture- no caption.

[ 80 ] ....after the the radiation bath at closest approach? Lyn R. Doose, one of the imaging experimenter staff at the University of Arizona, describes the drama of the emergence: "We watched the PICS image displayed in real time as the signals came back from the distant planet. A single bright spot appeared, and then another, until a line gradually built up. We knew we were seeing sunrise on Jupiter as the PICS image showed a crescent-like shape (Figure 5-16). We had survived passage through periapsis: the IPP was still working." The following hours produced more unique crescent images of Jupiter as Pioneer 10 headed away after periapsis.

All other equipment performed as expected. Ultraviolet and infrared scans and meteoroid dust sampling went according to plan.

Commented Robert Kraemer immediately after the close encounter: "The Mission, by all standards, is written down right now as 100 percent successful. It is very hard to see how it [Pioneer 10] could have done its job any better. All elements went beyond the project teams' expectations getting off to a good launch a couple of years ago, tracking the spacecraft, getting all the data back, has been just a beautiful effort."

The spacecraft contractor's Project Manager from TRW Systems, B. J. O'Brien, commented: "We did see the radiation effects at about the points we predicted ... the small indications of what failures we had were precisely in those areas we would have predicted, namely the power.

"We feel a little bit like Professor Higgins in Pygmalion who said, 'We did it'."

Project Science Chief, Richard O. Fimmel, commented: "This has been the most exciting day of my life! " Many of the principal investigators agreed wholeheartedly.

Pioneer 10 did what it was supposed to do . . . find out if spacecraft could explore Jupiter despite the hazards of the Jovian environment. Pioneer 10 found out what the environment of Jupiter really is and provided enough new data in itself to whet our appetites for more exploration of the giant planet.

Afterwards, Pioneer 10 headed for the outer reaches of the Solar System to cross Saturn's orbit in 1976 and the orbit of Uranus in 1980, where communication will probably soon be lost. By 1983, it crosses the orbit of Pluto and then continues at 40,000 km (25,000 miles) per hour into interstellar space, man's first emissary to the stars.

[ 81 ]

Picture- no caption.

[ 82 ] Meanwhile, Pioneer 11 had been following Pioneer 10 for a rendezvous with Jupiter. Severe thunderstorms at the Florida launch site had delayed preparations with a spell of bad weather. But crews at the site made up for the pre-launch delays and had the spacecraft ready for the launch window. The lift-off took place as scheduled at 6:11 p.m. PST on 5 April 1973. The spacecraft separated from the launch vehicle at 6:26 p.m., but as the RTG booms were deployed to slow the spin of the spacecraft and place the radioactive material as far as possible from the body of the spacecraft, trouble arose. One of the booms failed to extend to its full extent. The spacecraft continued to spin at too fast a rate.

Thrusters were fired to cause vibrations and extended the boom slightly. Then the spacecraft was reoriented to prevent excessive solar heating. The boom extended fully and the spin rate adjusted to its correct value of 4.8 rpm. Now all systems were operating correctly on the spacecraft.

On April 11,1973, the course of the spacecraft was changed slightly by an Earth-line velocity change maneuver. This correction aimed Pioneer 11 to a passage to the right of Jupiter as seen from Earth and about 20,000 km (12,400 mi.) above the cloud tops of the giant planet. This aiming point was chosen to provide several flyby options, including a continued journey beyond Jupiter to Saturn, and allowed the choice of option to be made later in the mission by another maneuver.

With the spacecraft 11 million km (7 million mi.) on its way the solar wind and cosmic ray instruments were sending good data about the interplanetary medium.

Trajectories of Pioneer 10 and Pioneer 11 as seen from the celestial North Pole projected onto the ecliptic plane are shown in Figure 5-16. During encounter with Jupiter the flyby trajectory is such that the speed of Pioneer 11 is almost doubled. The transfer of energy from Jupiter to the spacecraft boosts the speed of the spacecraft as effectively as a rocket engine and flings Pioneer 11 ....

Figure 5-16. Pioneer 10 and 11 heliocentric trajectories. The time ticks shown along the trajectories and planetary orbits represent the distance traveled during each year.

....toward the ringed planet Saturn. Pioneer 11's trajectory from Jupiter to Saturn is about three times as long as its path from the Earth to Jupiter. Along this trajectory Pioneer 11 flies high above the plane of the ecliptic, reaching a maximum height of 164 million km (102 million mi.) in the later part of 1976. This flight is the first spacecraft to probe deep space far from the ecliptic plane.

Pioneer 11 completed a safe passage through the asteroid belt on 20 March 1974, emerging unscathed as its predecessor. The experiments on Pioneer 11 confirmed the findings of Pioneer 10. As the spacecraft traveled outward in the Solar System from Earth's orbit, the smallest particles (0.001 mm) detected by the spacecraft's instruments appeared to decline in number. Somewhat larger particles (0.01 to 0.1 mm) seemed to be evenly distributed all the way from Earth's orbit through the asteroid belt itself, with no increase in the belt. Still larger particles (0.1 to 1.0 mm) were found to be three times as frequent in the center of the belt as near Earth.

The modified gas-cell instrument aboard Pioneer 11 found some results different from those of Pioneer 10. Walls of the gas cells on the Pioneer 11 detectors were thicker, so that only particles from 0.02 to 0.1 mm diameter (1 /100 millionth to one millionth of a gram) were recorded.

[ 83 ] For these particles, about half as many more gas-cell penetrations were found near the Earth by Pioneer 11 than by Pioneer 10. This implies about the same number of small and large particles were present. However, between 180 and 344 million km (112 and 214 million mi.) from the Sun Pioneer 11 found a virtual absence of larger particles. Its detector recorded only one penetration. In the asteroid belt, the larger particles appeared again, but only about one sixth as many as in the total range measured by Pioneer 10. This appears to mean that in the asteroid belt smaller particles of 0.01 to 0.1 mm size are three times as common as larger particles.

Between the Earth and the outer edge of the asteroid belt, Pioneer 11 counted 20 penetrations, 7 of them taking place while the spacecraft was within the belt.

The larger asteroidal particles, measured by the asteroid-meteoroid telescope, are mostly in the range of size 0.1 to 1.0 mm diameter, one millionth to one thousandth of a gram. A few of the particles seen by Pioneer 10 were as large as 10 to 20 cm in diameter. Analysis of the Pioneer 10 data suggests that there are almost three times as many of these larger particles inside the asteroid belt as there are between the Earth and the belt. The data from Pioneer 11 confirm this finding.

Thus, the findings of the two Pioneer spacecraft indicate that the asteroid belt does not contain high-velocity projectiles which might penetrate spacecraft and damage them. Particles in the center of the belt which orbit the Sun at about 61,200 km (38,000 mi.) per hour would penetrate one centimeter thick aluminum even if the particle weighed only 0.001 gram. But most of the particles seen by Pioneer 10 and Pioneer 11 were smaller than this' and the total number of such particles was found to be far lower than had been predicted prior to the Pioneer mission. Though the belt contains quite large bodies as well as dust, dangerous concentrations of high-velocity dust particles that would be hazards to spacecraft do not seem to exist in the belt.

Just after Pioneer 11 emerged from the asteroid belt its trajectory was modified by command from Earth. On 19 April, 1974, thrusters on the spacecraft were commanded to add another 63.7 meters per sec (210.2ft/sec) to the spacecraft's velocity thereby correcting the aiming point at Jupiter to 43,000 km (26,725 mi.) above the cloud tops. The main mission of Pioneer 11 at Jupiter was to penetrate deeper into the radiation belts. The inner radiation belt of the planet could easily destroy electronics of a spacecraft if the intensity of particles continued to increase beyond the maximum measured by Pioneer 10 at its closest approach to 2.86 Jovian radii; i.e., 132,252 km (82,000 mi.) above the cloud tops. So Pioneer 11 was directed to approach three times closer than Pioneer 10 and thereby obtain unique scientific observations of both Jupiter and its environment. The close approach also allowed the spacecraft to be accelerated by Jupiter to a velocity of 55 times that of the muzzle velocity of a high speed rifle bullet to 173,000 km (108,000 mi.) per hour so that it would be carried across the Solar System some 2.4 billion km (1.5 billion mi.) to Saturn.

Meanwhile, as Pioneer 11 cruised toward Jupiter, the interest in the giant planet sparked by the findings of Pioneer 10 was producing new discoveries by Earth-based observations.

Charles Kowal, a research assistant in astronomy at Caltech, developed a new technique to search for small objects near the giant planet. Using the Palomar Schmidt telescope's enormous light gathering power, a special screen to mask the glare of Jupiter, and photographic plates baked in nitrogen gas to increase sensitivity, Kowal discovered a thirteenth satellite of Jupiter in September 1974. (A fourteenth satellite was discovered a year later while Pioneer 11 was on its way to Saturn.) These satellites were too small to be seen by the imaging system of Pioneer.

The trajectory of the Pioneer 11 spacecraft had been selected to approach Jupiter from below the South Pole so that it would hurtle almost straight up through the intense radiation belt and thereby reduce the time of exposure to the radiation (Figure 5-17). B. J. O'Brien, Pioneer project manager at...

[ 84 ]

Figure 5-17. Pioneer 10 and 11 encounters with Jupiter, as viewed from Earth. The hour time marks show spacecraft locations relative to the periapsis target point. TRW where the spacecraft were built, told the press that the total radiation dose Pioneer 11 would be expected to receive will be less than Pioneer 10's, "and we're betting that's what counts."

The problem is that the billions of electrons and protons that are trapped in Jupiter's magnetic field bombard the spacecraft. Some of these particles are traveling fast enough to dislodge electrons from atoms and even dislodge whole atoms in the spacecraft. When this happens to a critical part of the spacecraft's electronics spurious information can be generated or the electronics fail completely. Explained O'Brien: "Pioneer 10 passed along the magnetic equator, where most energetic particles appear to concentrate. The spacecraft took lots of hits over a long time. But Pioneer 11 will go in slow, slip through the area of maximum radiation fast, and come out in the clear pretty quickly. The radiation counts will probably soar at a pace that will scare us half to death just before closest approach, but the total dose Pioneer 11 receives won't be as great as Pioneer 10 took because the time will be much shorter."

"Pioneer 11 will be out of communication with Earth at the time of closest approach," continued O'Brien. "It will have gone behind Jupiter at 9:01 p.m., 21 minutes before closest approach. Its onboard magnetic memory will be recording data for later transmission to Earth. Then we sit and wait .... and fidget. At 9:44 p.m., 22 minutes after closest approach, Pioneer 11 will come out from behind Jupiter. But we have another 40 minutes wait before we hear anything because of the signal's travel time from the distance of Jupiter to the Earth. So, at 10.24 p.m., if Pioneer is still working, we'll hear that we made it."

The close path by Jupiter would also provide a bonus in that it permitted images of the polar regions of the giant planet to be obtained by the spin-scan system, thereby providing views of Jupiter that can never be obtained from Earth observation. And unlike Pioneer 10, Pioneer 11 was directed to fly by Jupiter against the direction of the planet's rotation (Figure 5-18). After passing in front of Jupiter as the planet moves along its orbit round the Sun, the spacecraft then goes around the dark side of Jupiter and completes a circuit of the planet by crossing the spacecraft's own incoming trajectory and heading for Saturn.

Figure 5-18. Pioneer 10 and 11 encounters with Jupiter as viewed from the celestial North Pole. Pioneer 10 swings around the giant in the counterclockwise direction, but Pioneer 11 attacks the planet in a clockwise course. Jupiter rotates counterclockwise in this view. The time in hours before and after closest approach is given by the numbers alongside each trajectory.

[ 85 ] Views of Jupiter as seen from the spacecraft are shown in Figure 5-19. These PICS images returned from the spacecraft were displayed live to many thousands of interested people over cable TV in the San Francisco Bay area. During the encounter, several of the public halls to which the TV images and a running commentary by astronaut Colonel A. Worden were relayed were jammed to capacity for many hours.

Before closest approach to Jupiter, the spacecraft's view of the planet showed the terminator boundary between day and night on the planet - near the let's hand edge of the disc. After closest approach the terminator was near the upper right-hand edge of the disc. The south polar regions were seen prior to closest approach and the north polar regions afterwards.

As with Pioneer 10 there were five phases to the encounter. The first occupied about three weeks starting 3 November 1974, when the spacecraft passed from interplanetary space into the Jovian system, moving from about 24 million to 10 million km (15 million to 6 million mi.) from the planet.

Phase two covered entry of Pioneer 11 into the inner system, following penetration of the bow shock wave in the solar wind created by the interaction of Jupiter's magnetic field with that wind. This occurred on 25 November 1974 at 10:00 p.m. PST. Almost a day later 9:00 p.m. PST on 26 November the spacecraft entered the magnetosphere where the magnetic field of Jupiter prevents the wind from approaching closer to the surface of the planet. Pioneer 11 was now 7 million km (4.3 million mi.) from Jupiter. But at 10:20 am. PST on 27 November at a distance of 6.6 million km (4.1 million mi.), Pioneer 11 popped out of the magnetosphere for five and a half hours before crossing the bow shock again and resuming into the magnetosphere at 6.44 million km (4.0 million mi.) from Jupiter. These repeated bow shock crossings, first experienced by Pioneer 10, confirmed the model of the Jovian magnetosphere that likens it to an unstable soft balloon that is buffeted by the solar wind and often squeezed in towards Jupiter on the side facing the Sun.

The third phase of the encounter was when Pioneer 11 continued flying through the outer magnetosphere from about 4.8 to 3.2 million km (3 to 2 million mi.) from the planet.

Phase tour, the period around closest approach, covering the day and a half before and after periapsis, was where the spacecraft made most of its measurements and obtained the better spin-scan images of Jupiter and the large satellites.

During stage five Pioneer 11 left Jupiter behind and repeated many of the earlier experiments but in reverse sequence.

November 7, Pioneer 11 crossed the orbit of Hades, outermost satellite of Jupiter. In the next few days the spacecraft successively crossed the orbits of Pan and Andrastea. By 21 November, Pioneer had crossed the orbit of Hera at just over 11.25 million km (7 million mi.) from Jupiter; then the orbits of Demeter and Hestia. But the Jovian system is so large that despite Pioneer l l's enormous speed it was not until 1 December, the day before closest approach, that the spacecraft began to cross the orbits of the large Galilean satellites. There were no really close approaches to these satellites. Pioneer 11 came within 786,000 km (488,700 mi.) of Callisto, within 692,200 km (430,200 mi.) of Ganymede, and within 265,500 km (195,000 mi.) of lo. Finally Pioneer passed within 587,000 km (365,000 mi.) of Europa and within 127,000 km (79,000 mi.) of the innermost satellite, Amalthea.

Spin scan images were obtained of several of these satellites (see Chapter 9 ).

By this time TV screens at NASA's Ames Research Center were displaying good-sized images of Jupiter showing an orange and gray-white striped sphere with detailed cloud features and a prominent Great Red Spot (see Figure 5-19).

The excellent image of the Great Red Spot, shown in Chapter 9 , was only obtained as a result of quick revision to the command sequences for the spacecraft. Pioneer 11 was flying by Jupiter at high speed in the opposite direction to the rotation....

[ 86 - 87 ]

Figure 5-19. This series of PICS images shows views of Jupiter as viewed from Pioneer 11's it approached the planet from the south, flew by, and then rose high above the north polar region.

[ 88 ] ....of Jupiter on its axis. The combination of these two rapid motions made it mandatory for the timing of a close-in image of the spot to be extremely precise.

Months before the encounter Lyn Doose of University of Arizona, working with the imaging photopolarimeter team, contacted ground-based observatories to determine the spot's position and drift rate in order to estimate where it would be precisely at the time of Pioneer l l's flyby. When a final position had been established, a series of computer drawings was prepared, simulating how the planet would appear to the spacecraft at one-hour intervals during the close encounter. From these drawings the best observing opportunity was selected and reserved for imaging of the Red Spot; other activity of the imaging photopolarimeter was worked around the timing for the Red Spot image.

Just before encounter Doose explained: "A somewhat different approach from that used on Pioneer 10 would be employed. The Red Spot would be scanned nearer to the center of the planet's illuminated hemisphere so that it would not be foreshortened and would be well away from the terminator and evenly illuminated."

"The commands to the imaging photopolarimeter were written, rewritten, checked and rechecked," continued Doose. "Only ten days before the flyby we discovered an error. The time for obtaining the best image of the spot as derived from the computer-generated drawings was referenced to when the telescope should execute the commands, but they had been interpreted as being when the commands should be transmitted."

"The rotation of Jupiter, coupled with the motion of the spacecraft would have put the Red Spot outside the field of view of the image."

For two days the imaging command sequence for the several hours before closest approach was revised. New commands were written and command files were prepared for transmission to the spacecraft in the tight command sequence. With these last minute changes the Red Spot sequence worked perfectly and a unique image of the Great Red Spot was obtained (see Figure 9-11).

As well as presenting higher latitudes to the spinscan imaging system, the flyby trajectory chosen allowed the magnetic field and radiation environment to be explored to higher latitudes of the magnetosphere. Also, while Pioneer 10 maintained an almost constant view relative to Jupiter for several hours during closest approach because its direction of travel was the same as the direction of rotation of Jupiter, Pioneer 11 traveled oppositely to Jupiter's rotation and traversed a full circle of longitude of the planet during its close observations in the 4 hours around periapsis.

Approximately 1300 commands were transmitted to Pioneer 11 on each of two days at closest approach. Many of these commands were intended to make sure that the equipment carried by the spacecraft would continue to operate in a correct configuration in spite of the effects of the radiation environment. Thus the spacecraft was repeatedly commanded to the correct data format, to the correct data bit rate, to keep the transmitter switched on, and to keep the scientific experiments operating. Also the spin-scan imaging photometer, which lost several important images during Pioneer 10 encounter because of false commands generated by radiation, was periodically reset (indexed) to a basic position from which it was directed to the correct aspect angle for planetary imaging. This command technique had proved invaluable during the encounter of Pioneer 10 and was now expanded in scope for the more rigorous encounter of Pioneer 11.

The more serious problem than the radiation environment of Jupiter was a threatened strike of diesel operators in Australia which endangered the mission in the last few hours before close encounter. The strikers permitted technical personnel to operate the ground station for the encounter. Had this not been permitted the mission would have lost 6 to 8 hours of scientific data each day. Flight operations could not be certain that the Deep Space Network station at Canberra would, indeed? be available for the encounter- the strike situation looked so bad. In the less than 30 minutes available they reprogrammed the encounter [ 89 ] sequence to enable the Goldstone Deep Space Station to maintain communications with the spacecraft for a longer period than normally, almost until the spacecraft set at Goldstone. This also required the bit rate to be dropped from 2048 to 1024 bits per second. So the mission was seriously endangered by social problems on Earth more so than the harsh environment half a billion miles from Earth.

The spacecraft went behind Jupiter at 9:02 p.m. PST on 2 December. The telemetered signals continued until 9:42 p.m. PST because of the time delay in transmission over the millions of miles to Earth. Now everyone waited anxiously. It was during this occultation that the spacecraft would hurtle through its closest approach to Jupiter, skimming 43,000 km (26,725 mi.) above the cloud tops as it passed through the greatest intensity of the radiation belts. Would it survive?

The scheduled emergence from behind Jupiter was 9:44 p.m. PST, 22 minutes after closest approach. But the signals, if they were still coming from the spacecraft would not arrive at Earth until 10:24 p.m. PST.

Everyone waited.

Eleven seconds after 10:24:05 p.m. PST, the Deep Space Network station at Canberra, Australia, picked up the signal and relayed it to the Pioneer Mission Control at Ames Research Center. Engineers, scientists, and newsmen covering the event at Ames Research Center cheered. Ten seconds later the big antenna at Goldstone in the Mojave Desert of California picked up the whisper-faint signals from the distant spacecraft. All was well. Pioneer 11 had survived its encounter with the giant of the Solar System.

Anomalies occurred during flyby in the plasma analyzer, the infrared radiometer, the meteoroid detector and the spin-scan imaging photopolarimeter. Also there was a small decrease in output current of the spacecraft power system; but this was less than that experienced by Pioneer 10 and caused no difficulties.

The most serious problem was spurious commands that caused the infrared radiometer to miss observing the northern hemisphere of Jupiter. As soon as the signals reached Earth and the problem was detected the project science chief and the science advisor immediately prepared a 108-command sequence to correct the infrared radiometer register settings thereby saving 50 percent of the planned observations from the northern hemisphere.

In the next few hours scientists were able to analyze the details of the radiation belts and fantastic spin-scan images were returned to Earth of the northern hemisphere of Jupiter. Predictions of the high energy electron intensity during the close passage proved to be correct, but, surprisingly, the actual proton flux measured was about ten times less than that predicted in advance of the flyby and based on extrapolations from the Pioneer 10 data. Pioneer 11 showed that near the planet the radiation belt is intense but occupies a smaller volume than expected. Though Pioneer 10 and 11 found shells of dangerous, extremely-high-energy protons near Jupiter's magnetic equator, the shells were only at low latitudes and posed a relatively minor hazard to spacecraft flying through them at highly inclined trajectories. Also, the intensity of high energy electrons turned out to be only slightly higher than that found by Pioneer 10 even though Pioneer 11 went three times closer to the planet. But Pioneer also found that there is a flux at higher latitudes that is greater than expected from the Pioneer 10 results.

Again the picture emerged of an enormous spinning magnetic field buffeted by the solar wind and stirred by Amalthea and the Galilean satellites.

During this encounter, Pioneer 11 determined very accurately the mass of Callisto as 1.5 lunar masses. Also the close approach provided more accurate determinations of the gravitational field of Jupiter itself.

In making the first observations of Jupiter's immense polar regions (Figure 5-20) Pioneer 11 found that the planet's cloud tops are substantially lower at the poles than at the equator, and are covered by a thicker but transparent atmosphere. Though there is much less evidence of rapid atmospheric circulation at the poles than at the equator,....

[ 90 ]

Figure 5-20. Pioneer 11 obtained the first photographs of Jupiter's polar regions.

....the polar areas unexpectedly showed many small convective cells, dwarfing similar Earth thunderstorms. Blue sky was also visible at the poles and is attributed to the same cause as Earth's blue skies - multiple molecule scattering of light by gases of the transparent atmosphere at Jupiter's poles.

Many more flow features were also revealed in the clouds around the Great Red Spot than were seen a year previously by Pioneer 10. And within the spot new details were revealed that suggested convection and circulation patterns. The center of the spot appeared brighter than its edges.

Immediately after the encounter with Jupiter, Pioneer 11 was renamed Pioneer Saturn as it headed to an exploration of the next outer planet of our Solar System. Several weeks after the encounter, problems arose in the spacecraft with spurious commands which could not be attributed to radiation. These problems continued for several months as Pioneer 11 headed toward Saturn. Analysis of special test results indicated that the asteroid-meteoroid detector had been damaged by radiation during the encounter and was now the source of the signals responsible for the spurious commands. The instrument was turned off and the spurious commands ended. But in the series of tests to isolate the cause of the spurious commands by turning off each piece of equipment in turn, the plasma analyzer was switched off. When commanded on again, the power for the instrument turned on but the instrument did not produce data. This instrument, important for the flyby of Saturn is being subjected to special command sequences to try to reestablish data output for the encounter with the ringed planet.

The unique ringed planet Saturn (Figure 5-21) has so low a density it would float in an ocean big enough to contain it. It is a large planet, 1 20,800 km (75,000 mi.) in diameter. which revolves around our Sun in 29.46 years at a mean distance of 1,426,000,000 km (886,000,000 mi.). Like Jupiter it rotates very rapidly on its axis in 10 hours and 14 minutes-and is flattened considerably at the poles and bulged at the equator by this rapid spin.

[ 91 ]

Figure 5-21. The planet Saturn with its unique ring system is the second target for Pioneer 11 after its encounter with giant Jupiter. Pioneer 11 was renamed Pioneer-Saturn. (Photo. Catalina Observatory)

Its 278,600 km (174,200 mi.) diameter ring system is unique among planets of the Solar System.

The objectives of Pioneer 11 at the ringed planet are very similar to the science objectives at Jupiter:

Map the magnetic field of Saturn and determine its intensity, direction, and structure. Although such a field has not been detected from Earth, several experts in planetary magnetism believe that Saturn does have a magnetic field. Saturn is also believed to have a massive core, possibly extending to 30 percent of its radius. Such a core is inferred from the motions of natural satellites and would support the probability of Saturn having a magnetic field.

Determine how many electrons and protons of various energies are distributed along the trajectory of the spacecraft through the Saturn system.

Map the interaction of the Saturn system with the solar wind.

Measure the temperature of Saturn's atmosphere and that of Titan, the large satellite of Saturn.

Determine the structure of the upper atmosphere of Saturn where molecules are expected to be electrically charged and form an ionosphere.

[ 92 ] Map the thermal structure of Saturn's atmosphere by infrared observations coupled with radio occultation data.

Obtain spin-scan images of Saturn in two colors during the encounter sequence, and close-up images of special planetary features, including the ring system, and make polarimetry measurements of the planet, the rings, and some of the satellites.

Probe the ring system and the atmosphere of Saturn with S-band radio waves at occultation.

Determine more precisely the masses of Saturn and its larger satellites by accurate observations of the effects of their gravitational fields on the motion of the spacecraft.

Obtain information to calculate with greater accuracy the orbits and ephemerides of Saturn and its larger satellites.

As a precursor to the Mariner Jupiter/Saturn mission, verify the environment of the ring plane to find out where it may be safely crossed by the Mariner spacecraft without serious damage.

Two target locations have been tentatively suggested for the flyby of Saturn; an outside target results in penetrating the ring plane just outside the outer visible (A) ring with a closest approach to Saturn of 1.4 planetary radii and an escape from the system past Titan. An inside target results in penetration within the visible rings -penetrating at 1.15 radii of Saturn and approaching to 1.06 radii -but directly through the D-ring which was discovered photographically only recently. The inside passage may damage the spacecraft so that its subsequent close passage by Titan would not provide any data on that intriguing satellite. On the other hand, close passage outside the A ring is not certain to be safer.

Either of these two trajectories can be chosen as late as 1977 when the spacecraft will be aligned in space so that it can be most accurately maneuvered to the selected course. During this maneuver the time of arrival at Saturn would be changed to insure dual coverage by stations of the Deep Space Network at the hour of closest approach to Saturn.

The celestial mechanics of the Pioneer Saturn encounter might best be exploited through the flyby that carries Pioneer inside the innermost visible ring and about 9000 km (5000 mi.) above the cloud tops of the planet (Figure 5-22). Such a close approach to the planet would not only define most accurately Saturn's gravitational field, but also provide data which, when combined with corresponding observations from a Mariner Jupiter/ Saturn flyby in 1981, would allow a good estimate of the mass of the ring system to be made.

The close approach also provides information about the internal structure and shape of Saturn. Even if the spacecraft is damaged or put out of action by its passage through the ring plane, valuable gravitational data would still be obtained prior to penetration of the ring plane.

If the spacecraft arrives at Saturn during the few hours from late evening on 1 September 1979 to early morning on 2 September, a close approach can also permit the spacecraft to leave the planet in a direction that would carry it close to Titan about 26 hours after its periapsis with Saturn.

The large satellite Titan is known to have an atmosphere. The diameter of Titan is about 4800 km (3000 mi.) and this satellite is regarded as a prime candidate among bodies of our Solar System that might support life. The constituents of its atmosphere are believed to be of the type that supported primitive life forms on Earth before the terrestrial atmosphere became oxygen enriched. Titan orbits Saturn in just under 16 days at a mean distance of 1,222,000 km (759,300 mi.).

If Pioneer Saturn does survive its passage through the plane of Saturn's ring system and any radiation environment of Saturn, its subsequent...

[ 93 ]

Figure 5-22. One of several options to fly by Saturn would carry the Pioneer spacecraft behind the rings as well as the planet and provide occultations to probe the rings and the planetary atmosphere with the radio signals from the spacecraft to Earth. The marks on the trajectory signify periods of one hour. The trajectory after encounter carries the spacecraft alongside Titan for an inspection of that large satellites.

[ 94 ] ....close approach to Titan, will measure the satellite's mass more accurately. It will also obtain spin-scan images of the surface of the satellite which might show large surface markings, measure the temperature of the surface, and improve the ephemeris to aid future missions. However, much depends on the choice of the path for Pioneer past Saturn.

A complication in selecting the time of arrival of Pioneer 11 at Saturn is the motion of the Earth around the Sun. As Saturn moves relative to the Earth to pass behind the Sun as seen from Earth, radio noise from the Sun will interfere with the faint radio signals from the spacecraft. So arrival at Saturn has to be timed in advance of Saturn moving into superior conjunction which takes place 11 September 1979. The Sun-Earth-spacecraft angle is closing at about 1 degree per day around the time of the encounter, so moving the encounter forward in time provides an opportunity to have a subsequent encounter with Titan and still receive radio signals back from the spacecraft. Unfortunately there may be about I week of observations of the magnetosphere lost on the outward leg of the flight beyond Saturn because the spacecraft will pass behind the Sun as viewed from Earth and its signals will be lost in solar noise.

At the distance of Saturn it will also be necessary to accept a reduced bit rate for data transmission; only 256 bits per second compared with 1024 from Jupiter. This lower bit rate will primarily affect the data from imaging and will reduce the image scans to smaller sectors of about one half to one quarter those possible at Jupiter.