CHAPTER 2. Pioneers to Jupiter

PIONEERS TO JUPITER

Reaching for the Outer Planets

Since the beginning of the Space Age, scientists had dreamed of sending probes to Jupiter and its family of satellites. Initially, robot spacecraft were limited to studying the Earth and its Moon. In 1962, however, the first true interplanetary explorer, Mariner 2, succeeded in escaping the Earth-Moon system and crossing 100 million kilometers of space to encounter Venus, studying Earth’s sister planet at close range using half a dozen scientific instruments. By the mid 1960s a U.S. planetary spacecraft had also flown to Mars, there had been a second flyby of Venus, and an ambitious program was under way for two more flybys of Mars in 1969, followed by a Mars orbiter in 1971. Based on this success with the inner planets, NASA scientists and engineers began to plan seriously to meet the challenge of the outer solar system.

Not only Jupiter, but Saturn and even Uranus and Neptune, were considered as possible targets. However, the distances between the outer planets are so vast that many years of flight would be required for a spacecraft to reach them, even using the most powerful rocket boosters then contemplated. If a cautious exploration program were followed, investigating one planet at a time before designing the next mission, it would be well into the twenty-first century before even a first reconnaissance of the solar system could be achieved. A way to bridge the space between planets in a more efficient, economical manner was needed.

In the late 1960s celestial mechanicians—scientists who study the motions of planets and spacecraft—began to solve problems posed by the immensity of the outer solar system. If a spacecraft is aimed to fly close to a planet in just the right way, it can be accelerated by the gravity of the planet to higher speeds than could ever be obtained by direct launch from Earth. If a second, more distant planet is in the correct alignment, the gravity boost given by the first encounter can speed the craft on to the second. Jupiter, with its huge size and strong gravitational pull, could be used as the fulcrum for a series of missions to Saturn, Uranus, Neptune, and even distant Pluto. In addition, the early 1980s would offer an exceptional opportunity, one repeated only about once every two centuries. At that time, all four giant planets would be in approximate alignment, so that gravity-assist maneuvers could be done sequentially. A single spacecraft, after being boosted from Jupiter to Saturn, could use the acceleration of Saturn to continue to Uranus, and in turn could be accelerated all the way out to Neptune. Such an ambitious, multiplanet mission was named the Grand Tour.

The first essential step in the Grand Tour was a flyby of Jupiter. However, this planet is ten times farther away from Earth than Venus or Mars. In addition, there were two potentially lethal hazards that had not been faced before in interplanetary flights: the asteroid belt and the Jovian magnetosphere.

The first danger was presented by the many thousands of asteroids that occupy a belt between the orbits of Mars and Jupiter. The largest asteroid, Ceres, was discovered in 1801 and was initially thought to be the “missing planet” sometimes hypothesized as lying between Jupiter and Mars. However, Ceres is only 1000 kilometers in diameter, too small to deserve the title of planet. Hundreds more of these minor planets were discovered during the nineteenth century, and by the 1960s more than 3000 had well-determined orbits. Most were only a few tens of kilometers in diameter, and astronomers estimated that 50 000 existed that were 1 kilometer or more in diameter. Any spacecraft to Jupiter would have to cross this congested region of space.

Even 50 000 minor bodies spread through the volume of space occupied by the asteroid belt would present little direct danger, although a chance collision with an uncatalogued object was always possible. Much more serious was the possibility that these larger objects were accompanied by large amounts of debris, from the size of boulders down to microscopic dust, that were undetectable from Earth. Collisions with pebble-sized stones could easily destroy a spacecraft. The only way to evaluate this danger was to go there and find out how much small debris was present.

A second danger was posed by Jupiter itself. In order to use the gravity boost of Jupiter to speed on to another planet, a spacecraft would have to fly rather close to the giant. But this would mean passing right through the regions of energetic charged particles surrounding the planet. Some estimates of the number and energy of these particles indicated that the delicate electronic brains of a spacecraft would be damaged before it could penetrate this region. Again, only by going there could the danger be evaluated properly.

The Pioneer Jupiter Mission

In 1969 the U.S. Congress approved the Pioneer Jupiter Mission to provide a reconnaissance of interplanetary space between Earth and Jupiter and a first close look at the giant planet itself. The Project was assigned by NASA to the Ames Research Center in Mountain View, California. The primary objectives were defined by NASA:

Explore the interplanetary medium beyond the orbit of Mars.

Investigate the nature of the asteroid belt, assessing possible hazards to missions to the outer planets.

Explore the environment of Jupiter, including its inner magnetosphere.

The Pioneer spacecraft was designed for economy and reliability, based on previous experience at Ames with Pioneers 6 through 9, all of which had proven themselves by years of successful measurement of the interplanetary medium near the Earth. Unlike the Mariner class of spacecraft being used to investigate Venus and Mars, the Pioneer craft rotated continuously around an axis pointed toward the Earth. This spinning design was extremely stable, like the wheels of a fast-moving bicycle, and required less elaborate guidance than a nonspinning craft. In addition, the spin provided an ideal base for measurements of energetic particles and magnetic fields in space, since the motion of the spacecraft itself swept the viewing direction around the sky and allowed data to be acquired rapidly from many different directions. The only major disadvantage of a spinning spacecraft is that it does not allow a stabilized platform on which to mount cameras or other instruments that require exact pointing. Thus the spacecraft design was optimized for measurements of particles and fields in interplanetary space and in the Jovian magnetosphere, but had limited capability for observations of the planet and its satellites.

As finally assembled, the Pioneer Jupiter spacecraft had a mass of 258 kilograms. One hundred forty watts of electrical power at Jupiter were supplied by four radioisotope thermoelectric generators (RTGs), which turned heat from the radioactive decay of plutonium into electricity. The launch vehicle for the flight to Jupiter was an Atlas-Centaur rocket, equipped with an additional solid-propellant third stage. This powerful rocket could accelerate the spacecraft to a speed of 51 500 kilometers per hour, sufficient to escape the Earth and make the billion-kilometer trip to Jupiter in just over two years. The specific scientific investigations to be carried out on Pioneer were selected competitively in 1969 from proposals submitted by scientists from U.S. universities, industry, and NASA laboratories, and also from abroad. Eleven separate instruments would be flown, in addition to two experiments that would make use of the spacecraft itself.

Three complete Pioneer spacecraft, with their payloads of 25 kilograms of scientific instruments, were built: one as a test vehicle and two for launch to Jupiter. One of these—the test vehicle—is now on display at the National Air and Space Museum in Washington. The first opportunity to launch—the opening of the “launch window”—was on February 27, 1972. However, it was not until shortly after dark on March 2 that all systems were ready, and Pioneer 10 began its historic trip to Jupiter.

Pioneer 10 was the first human artifact launched with sufficient energy to escape the solar system entirely. Fittingly, the craft carried a message designed for any possible alien astronauts who might, in the distant future, find the derelict Pioneer in the vastness of interstellar space. A small plaque fastened to the spacecraft told the time and planet from which it had been launched, and carried a symbolic greeting from humanity to the cosmos.

The second Pioneer was to wait more than a year before launch. By following far behind Pioneer 10, its trajectory—in particular how deeply it penetrated the radiation belts during Jupiter flyby—could be modified, depending on the fate of the first spacecraft. At dusk on April 5, 1973, Pioneer 11 blasted from the launch pad at Cape Canaveral and followed its predecessor on the long, lonely journey into the outer solar system.

Flight to Jupiter

Within a few hours of launch, each Pioneer spacecraft shed the shroud that had protected it and unfurled booms supporting the science instruments and the RTG power generators. After each craft had been carefully tracked and precise orbits calculated, small onboard rockets were commanded to fire to correct its trajectory for exactly the desired flyby at Jupiter. Pioneer 10 was targeted to fly by the planet at a minimum distance of 3 Jupiter radii (RJ) from the center, or 2 RJ (about 140 000 kilometers) above the clouds. This close passage, inside the orbit of Io, allowed the craft to pass behind both Io and Jupiter as seen from Earth, so that its radio beam could probe both the planet and its innermost large satellite. Pioneer 11 was intended to fly even closer to Jupiter, but the exact targeting options were held open until after the Pioneer 10 encounter.

On Pioneer 10, all instruments appeared to be working well as the craft passed the orbit of Mars in June 1972, just 97 days after launch. At this point, as it headed into unexplored space, it truly became a pioneer. In mid-July it began to enter the asteroid belt, and scientists and engineers anxiously watched for signs of increasing particulate matter.

Pioneer 10 carried two instruments designed to measure small particles in space. One, with an effective area of about 0.6 square meters, measured the direct impact of dust grains as small as one-billionth of a gram. The other looked for larger, more distant grains by measuring sunlight reflected from them. To the surprise of many, there was little increase in the rate of dust impacts recorded as the craft penetrated more and more deeply into the belt. At about 400 million kilometers from the Sun, near the middle of the belt, there appeared to be an increase in the number of larger particles detected optically, but not to a level that posed any hazard. In February 1973 the spacecraft emerged unscathed from the asteroid belt, having demonstrated that the much-feared concentration of small debris in the belt did not exist. The pathway was open to the outer solar system!

On November 26, 1973, the long-awaited encounter with Jupiter began. On that date, at a distance of 6.4 million kilometers from the planet, instruments on board Pioneer 10 detected a sudden change in the interplanetary medium as the spacecraft crossed the point—the bow shock—at which the magnetic presence of Jupiter first becomes evident. At the bow shock, the energetic particles of the solar wind are suddenly slowed as they approach Jupiter. At noon the next day, Pioneer 10 entered the Jovian magnetosphere at a distance of 96 R_J from the planet.

As the spacecraft hurtled inward toward regions of increasing magnetic field strength and charged plasma particles, the instruments designed to look at Jupiter began to play their role. A simple line-scan camera that could build up an image from many individual brightness scans (like a newspaper picture transmitted by wire) obtained its first pictures of the planet, and ultraviolet and infrared photometers prepared to observe it also. By December 2, when the spacecraft had crossed the orbit of Callisto, the outermost of the large Galilean satellites, the line-scan images were nearly equal in quality to the best telescopic photos taken previously, and as each hour passed they improved in resolution. Near closest approach, Pioneer 10 transmitted partial frames of Jupiter that represented a threefold improvement over any Earth-based pictures ever taken.

Earth at encounter Earth at launch Jupiter at encounter Jupiter at launch Asteroid belt

Tension increased as the spacecraft plunged deeper into the radiation belts of Jupiter. Would it survive the blast of x-rays and gamma-rays induced in every part of the craft by the electrons and ions trapped by the magnetic field of Jupiter? Several of the instruments measuring the charged particles climbed to full scale and saturated. Others neared their limits but, as anxious scientists watched the data being sent back, the levels flattened off. Meanwhile, the spacecraft itself began to feel the effects of the radiation, and occasional spurious commands were generated. Several planned high-resolution images of Jupiter and its satellites were lost because of these false signals. But again the system stabilized, and no more problems occurred as, just past noon on December 3, 1973, Pioneer 10 reached its closest point to Jupiter, 130 000 kilometers above the Jovian cloud tops. Pioneer had passed its most demanding test with flying colors, and at a news conference at Ames, NASA Planetary Program Director Robert Kraemer pronounced the mission “100 percent successful.” He added, “We sent Pioneer off to tweak a dragon’s tail, and it did that and more. It gave it a really good yank, and it managed to survive.” The Project Science Chief pronounced it “the most exciting day of my life,” and most of the hundreds of scientists and engineers who participated in the encounter probably agreed with him.

Pioneer 11 continued to follow steadily, emerging from the asteroid belt in March 1974. Based on the performance and findings of Pioneer 10, it was decided to send Pioneer 11 still closer to Jupiter, but on a more inclined trajectory. On April 19 thrusters on the spacecraft fired to move the Pioneer 11 aimpoint just 34 000 kilometers above the clouds of Jupiter.

In using the Pioneer 10 data to assess the hazard to Pioneer 11, scientists had to consider three aspects of the charged particle environment. First was the energy distribution of the particles: The most energetic presented the most danger. Second was the flux, the rate at which particles struck the craft. Third was the total radiation dose. One can make an analogy with a boxing match. The energy distribution tells you how hard the blows of your opponent are. The flux is a measure of how many times a minute he hits you, and the total dose measures how many blows land. The spacecraft reacts just like a boxer; the crucial question is how much total dose it absorbs. Enough radiation blows, and the system is knocked out. The trajectory chosen for Pioneer 11 resulted in higher flux, since the craft probed more deeply into Jupiter’s inner magnetosphere than had Pioneer 10. But by moving at a high angle across the equatorial regions where the flux is highest, the total dose could be kept below that experienced by Pioneer 10.

Pioneer 11 entered the Jovian magnetosphere on November 26, 1974, just a year after its predecessor. Closest approach took place on December 2. As with Pioneer 10, the radiation dose taxed the spacecraft to its limit. Again spurious commands were issued, this time affecting the infrared radiometer more than the imaging system. The craft flew at high latitude over the north polar region of Jupiter, an area never seen from Earth, and returned several excellent high-resolution pictures. Once more the little Pioneers had succeeded against the odds in opening the way to the giant planets.

Pioneer 11 Pioneer 10 Callisto Ganymede Europa Io Amalthea

Following their encounters with Jupiter, both Pioneer spacecraft returned to their normal routine of measuring the interplanetary medium. Pioneer 10 had gained speed from the gravity field of Jupiter and became the first craft to achieve the velocity needed to escape from the solar system. Pioneer 11, however, had used the pull of Jupiter to bend its trajectory inward, aiming it across the solar system toward Saturn. Following the successes at Jupiter, NASA announced that Pioneer 11 would be targeted for a close flyby of Saturn five years later, which was successfully carried out in September 1979. In early 1980, far beyond their design lifetimes, both spacecraft were still performing beautifully.

Jupiter Results

The scientific results of the Pioneer flybys of Jupiter were many and varied. As is always the case, some old questions were answered and new problems were raised by the spacecraft data. Highlights of these results are summarized below.

Photographs of Jupiter.

The line-scan imaging systems of Pioneer returned some remarkable pictures of the planet during the two encounters, showing individual features as small as 500 kilometers across. In addition, the Pioneers were able to look at Jupiter from angles never observable from Earth.

One of the discoveries made from these pictures was the great variety of cloud structures near the boundaries between the light zones and dark belts. Many individual cloud patterns suggested rising and falling air. The convoluted swirls evident in these regions appeared to be the result of dynamic motions; unfortunately, with only the few “snapshots” obtained during the hours of the flyby, the actual motions of these clouds could not be followed.

Thermal Emission.

In the year the Pioneer Project was begun, astronomers on Earth had measured that Jupiter emitted more heat than it absorbed from the Sun. From the Earth these measurements could be made only of the sunlit part of the planet; neither the night side nor the poles could be seen. One of the main objectives of the Pioneer flybys was to determine the heat budget accurately from temperature measurements at many points on both the sunlit and the night sides.

The Pioneer data confirmed the presence of a heat source in Jupiter and supplied a quantitative estimate of its magnitude. The global effective temperature was found to be -148° C, to a precision of ±3 degrees. This temperature implies that Jupiter radiates 1.9 times as much heat as it receives from the Sun. The corresponding internal heat source is 10¹⁷ watts. Surprisingly, the poles were as warm as the equator; apparently, the atmosphere is very efficient at transferring solar heat absorbed near the equator up to high latitudes, or perhaps the internal component of the heat comes preferentially from the polar regions.

Helium in the Atmosphere.

The Pioneer infrared experiment made the first measurement of the amount of helium on Jupiter. The ratio of the number of helium atoms to the number of hydrogen atoms was found to be He/H₂ = 0.14 ± 0.08. This is consistent with the known solar ratio of He/H₂ = 0.11. Measurements of helium in the upper atmosphere were also made by the ultraviolet experiment.

Distance (km) Visible clouds Hydrogen gas Cloud tops Ammonia crystals Ammonium hydrosulfide crystals Ice crystals Water droplets -123° C Transparent atmosphere Hydrogen/Helium gas 70 000 Transition zone 1980° C 60 000 Liquid hydrogen 50 000 11 000° C Transition zone 3 million atmospheres pressure 40 000 Liquid metallic hydrogen 30 000 20 000 Possible “sea” of helium 10 000 30 000 K Possible solid core 0

PIONEER SCIENCE INVESTIGATIONS Project Scientist: J. H. Wolfe, NASA Ames

Investigation Principal Investigator Primary Objectives

Magnetic fields E. J. Smith, JPL Measurement of the magnetic field of Jupiter and determination of the structure of the magnetosphere. Magnetic fields N. F. Ness, NASA Measurement of the magnetic (Pioneer 11 only) Goddard field of Jupiter and determination of the structure of the magnetosphere. Plasma analyzer J. H. Wolfe, NASA Ames Measurement of low-energy electrons and ions, determination of the structure of the magnetosphere. Charged particle J. A. Simpson, U. Determination of the composition Chicago number, energy, and composition of energetic charged particles in the Jovian magnetosphere. Cosmic ray energy F. B. McDonald, NASA Measurement of number and spectra Goddard energy of very high energy charged particles in space. Jovian charged J. A. Van Allen, U. Measurement of number and particles Iowa energy distribution of energetic charged particles and determination of magnetospheric structure. Jovian trapped R. Walker Fillius, UC Measurement of number and radiation San Diego energy distribution of energetic charged particles and determination of magnetospheric structure. Asteroid-meteoroid R. K. Soberman, Observation of solid astronomy General Electric particles (dust and larger) in the vicinity of the spacecraft. Meteoroid detection W. H. Kinard, NASA Detection of very small Langley solid particles that strike the spacecraft. Celestial mechanics J. D. Anderson, JPL Measurement of the masses of Jupiter and the Galilean satellites with high precision. Ultraviolet photometry D. L. Judge, U. Measurement of ultraviolet Southern California emissions of the Jovian atmosphere and from circumsatellite gas clouds. Imaging T. Gehrels, U. Arizona Reconnaissance imaging of photopolarimetry Jupiter and its satellites; study of atmospheric dynamics. Jovian infrared G. Münch, Caltech Measurement of Jovian thermal structure temperature and heat budget; determination of helium to hydrogen ratio. S-Band occultation A. J. Kliore, JPL Probes of structure of Jovian atmosphere and ionosphere.

Atmospheric Structure.

Several Pioneer investigations yielded information on the variation of atmospheric temperature and pressure in the regions above the ammonia clouds. Near the equator, at a level where the atmospheric pressure is the same as that on the surface of the Earth (1 bar), the temperature is -108° C. About 150 kilometers higher, where the pressure drops to 0.1 bar, is the minimum atmospheric temperature of about -165° C. Above this point the temperature rises again, reaching about -123° C near a pressure level of 0.03 bar. Presumably this temperature rise is due to absorption of sunlight by a thin haze of dust particles in the upper atmosphere of Jupiter.

Internal Structure.

The measurements of the amount of helium, of the gravitational field, and of the size of the internal heat source on Jupiter greatly clarified scientists’ understanding of the deep interior of the planet. Calculations showed that the core of Jupiter must be so hot that hydrogen cannot become solid, but must remain a fluid throughout the interior. Even at great depths, therefore, Jupiter does not have a solid surface. The theory that the Great Red Spot was the result of interactions with a surface feature below the clouds thus became untenable. Whatever its exact nature, the Red Spot must be a strictly atmospheric phenomenon.

Magnetic Field.

Pioneer data showed that the magnetic field of Jupiter has a dipolar nature, like that of the Earth, but 2000 times stronger. The calculated surface fields measured about 4 gauss, compared to a field of about 0.5 gauss on the Earth. The axis of the magnetic field was tilted 11 degrees with respect to the rotation axis, and it was offset by about 10 000 kilometers (0.1 R_J) from the center of the planet.

Satellite Atmospheres.

Two experiments yielded exciting new information on possible atmospheres of the Galilean satellites. First was an occultation, in which the Pioneer 10 spacecraft was targeted to pass behind Io as seen from Earth. At the moments just before the spacecraft disappeared, and just after reemergence from behind the satellite, the radio signal was influenced by a thin layer of ionized gas, in which the electrons had been stripped from the atoms by absorbed sunlight or by other processes. The ionosphere thus discovered had a peak density of about 60 000 electrons per cubic centimeter. In addition, a very extended far-ultraviolet glow, probably due to atomic hydrogen, was found near the orbit of Io by the ultraviolet photometer.

Masses of Jupiter and Its Satellites.

Precise radio tracking of the Pioneers as they coasted past Jupiter and its satellites revealed that Jupiter is about one percent heavier than had been anticipated, and several satellites were found to have masses that differ by more than ten percent from values determined previously. These improvements in knowledge of the masses were required to achieve the close satellite flybys being planned for later missions.

The Inner Magnetosphere.

A great deal of the scientific emphasis of the Pioneer missions was directed at characterizing the particles and fields in the inner magnetosphere, the region in which charged particles are trapped in stable orbits. The Pioneers found that it extended to about 25 RJ, well beyond the orbit of Callisto. Within this region, instruments on the spacecraft recorded the numbers and energy of electrons, protons, and ions. The electrons reached a maximum concentration near 3 RJ, and their numbers remained almost constant from there in toward the planet. The maximum concentration of protons observed by Pioneer 10 was at 3.4 RJ, a little inside the orbit of Io. Pioneer 11 penetrated deeper and found another maximum, about twenty times higher, at 1.9 RJ; at this distance, 10 million energetic protons hit each square centimeter of the spacecraft every second. It was believed that the gap between these two peaks was due to tiny Amalthea, the innermost satellite, which orbits at 2.5 RJ. Apparently this satellite sweeps up the particles as it circles Jupiter. Another large dip in the proton distribution was attributed to sweeping by Io, with smaller effects seen near the orbits of Europa and Ganymede. There was an additional small effect at 1.8 RJ, later found by Voyager to be due to Jupiter’s ring and its fourteenth satellite.

The Outer Magnetosphere.

From about 25 RJ outward to its boundary near 100 RJ, the Jovian magnetosphere is a complex and dynamic place. Beyond about 60 RJ, both Pioneers found the boundary to be highly unstable, apparently blown in and out by variations in the pressure of the solar wind, which consists of charged particles flowing outward from the Sun. In this region concentrations of Jovian particles are sometimes seen that rival the inner magnetosphere in intensity. Between 60 RJ and 25 RJ, a region sometimes called the middle magnetosphere, the particle motions are more ordered, and for the most part electrons and protons are carried along with the planet’s rotation by its magnetic field. Near the equatorial plane, the flow of these particles produces an electric current circling the planet, and this current in turn generates its own magnetic field, which approaches in strength that of Jupiter itself. Occasionally the outer magnetosphere collapses down to about 60 RJ, and energetic particles are squirted from the middle region into space; these bursts of Jovian particles can sometimes be detected as far away as Earth.

At the same time that Pioneer scientists were analyzing their results and developing new concepts of the Jupiter system, a new team of investigators had been selected for the next mission to Jupiter. From about 1975 on, attention shifted from Pioneer to its successor—Voyager.