Pioneer 10 was the first space probe to travel to the outer planets, the first to provide a close-up look at Jupiter, the first to cross the asteroid belt between Mars and Jupiter, and the first to escape the solar system while still returning data to researchers on Earth from as far as 7.6 billion miles away.
In the opening days of space exploration, the search for an understanding of the evolution of the solar system began with studies of the inner planets (Mercury, Venus, Earth, and Mars), followed by studies of the outer planets (Jupiter, Saturn, Uranus, Neptune, and Pluto). Enormous differences exist between the outer and inner planets. For example, Jupiter is large enough to hold fourteen hundred Earths, and its volume is almost two and one-half times more than that of all the other planets together. It is a huge, rapidly spinning sphere of cold gases, hydrogen, helium, methane, water, and ammonia, and more complex, but as yet unknown, chemicals. Jupiter has a diameter eleven times that of Earth and spins on its axis twice as fast as Earth. In the 1960’s, Congress authorized the first mission to explore the outer planets. The mission was designated as Pioneer 10. The previous Pioneer missions, 1 through 9, were designed to explore space in the vicinity of Earth.
The Ames Research Center was assigned responsibility for project management and for mission and spacecraft design, with the understanding that the spacecraft would be built under a system contract with U.S. industry. Responsibility for deep-space communications, Earth to spacecraft and spacecraft to Earth, was assigned to the Deep Space Network
The Pioneer 10 spacecraft was launched from Cape Canaveral on March 3, 1972, aboard an Atlas-Centaur launch vehicle that incorporated a solid-propellant third stage. At the time, Pioneer 10 attained the highest injection energy ever achieved, as is attested by the fact that the spacecraft required only eleven hours to cross the lunar orbit. After a twenty-one-month flight, the spacecraft arrived at its radius of closest approach to Jupiter at a distance of approximately 2.8 Rj (Jovicentric Jupiter radii) on December 4, 1973.
Spacecraft had already traveled to Venus and Mars, but this Pioneer spacecraft had to face traversing the asteroid belt that lies between Mars and Jupiter. The asteroid belt was known to contain many thousands of small, rocky bodies, possibly including untold numbers of very small particles. There were doubts that spacecraft could safely cross this region, yet it was important to study the outer giants, because to reach the more distant planets requires full use of Earth’s orbital velocity around the Sun and a spacecraft trajectory directly through the heart of the asteroid belt. The gravity and orbital motion of Jupiter are then used to urge a spacecraft flying by that planet to the high velocities needed to reach other distant planets—Saturn, Uranus, and Neptune—within a reasonable time and with reasonable scientific payloads. Pioneer 10 then had to face the unknown radiation environment of Jupiter.
The spacecraft was spin-stabilized, having a spin rate of 4.8 revolutions per minute. The spacecraft spin axis was parallel to the axis of the 2.74-meter-diameter high-gain antenna reflector and was kept pointed toward Earth in order to maximize the communication bit rate. The maximum bit rate used during the Jupiter encounter was 1,024 bits per second. Spacecraft spin axis precession maneuvers were required periodically in order to keep the bit stream pointed at Earth. These maneuvers were performed approximately six days prior to and two days after the Jupiter flyby. Electrical power for the experiments and spacecraft subsystems was supplied by four radioisotope thermoelectric generators. The generators were located at the ends of two long booms. Inspection of in-flight data indicated that these batteries produced negligible interference with the Pioneer 10 experiments. During the encounter, the spacecraft approached Jupiter in the midmorning sector of the sunlit hemisphere and exited near Jupiter’s dawn meridian.
Pioneer 10 carried these images etched on a 6-by-9-inch gold anodized aluminum plate. The images denote the spacecraft’s origin from the third planet from the Sun (bottom), a man, a woman, and other symbology designed to convey information about Earth and its inhabitants to any intelligent life that might intercept the satellite in the future.
Pioneer 10 was not only the first spacecraft to fly by Jupiter; it also achieved many other firsts: returning the first spacecraft images of the large Galilean satellites; going beyond the last known planet of the solar system for the first time (June 13, 1983); and carrying a message to announce Earth and its inhabitants to potential civilizations. By July, 1990, the spacecraft was 7.5 billion kilometers from the Sun. At that point, the radio signal required seven hours to reach Earth from the spacecraft.
Pioneer’s science instruments were designed to gather data on interplanetary space between Earth and Jupiter, and beyond; to characterize the atmospheres of Jupiter and its satellites; to record data on the gravitational fields of these bodies; to detect gravitational waves; and to test Albert Einstein’s theory of relativity. Discoveries made by Pioneer 10 had far-reaching consequences for future space exploration and for understanding the solar system. Pioneer studied the vast magnetospheres and energetic plasmas of the outer solar system, which cannot be duplicated in laboratories on Earth. The myth of a hazardous asteroid belt was dispelled; the anticipated concentration of small particles did not exist. Spacecraft could reach Jupiter safely and use the Jovian gravitational “slingshot” to hurtle them to more distant planets.
The Pioneer 10 and (later) 11 missions revealed more about the distribution of particles causing the zodiacal light and the Gegenschein, glows in the night sky that can be observed from Earth. They also mapped the background of starlight. At Jupiter, the Pioneers explored the giant planet’s magnetosphere and found that it is disk-shaped and bigger than the Sun. The magnetic field of Jupiter, ten thousand times stronger than Earth’s field, is opposite in direction to Earth’s field; its dipole moment is offset and tilted so as to cause a wobbling of the huge magnetosphere as the planet spins on its axis.
Also discovered was a ring current within the magnetosphere and radiation belts whose trapped electrons have an intensity ten thousand times that of Earth’s trapped electrons, and its protons, one thousand times that of Earth’s. Jupiter’s magnetosphere was revealed as the source of high-energy electrons observed everywhere that spacecraft have traveled in the heliosphere.
The ratio of helium to hydrogen in the Jovian atmosphere was found to be 0.14, fairly close to the 0.11 ratio of the Sun. The Pioneers returned close-up images of Jupiter’s belts and zones, of its Great Red Spot, and of its polar regions. A new understanding was obtained of the weather patterns on a giant, rapidly rotating planet with no solid surface. The heat balance was measured and showed that Jupiter emits 1.7 times the heat the planet receives from the Sun, indicating that heat is still being generated in its vast interior.
Prior to the Pioneer 10 mission, the only information about Jupiter came from Earth-based astronomy and theoretical investigations. Scientific interest in Jupiter is centered on the facts that it is the largest planet in the solar system and that its bulk density, like those of the other outer planets—Saturn, Uranus, and Neptune—indicates that it is primarily composed of hydrogen and helium gas, unlike the inner planets. Theory suggests that a large region of the interior of Jupiter is composed of metallic hydrogen. The measured behavior of the magnetic field conformed to this expectation by showing the generation of an immense magnetic field by a dynamo at the center of the planet with a complicated structure and gravitational measurements of the planet that confirmed that it is almost entirely fluid. Measurements of the radiation belts and electrical flow revealed the details of an electromagnetic environment that has, for many years, made Jupiter known as the largest source of radio emissions in the sky.
Jupiter’s atmosphere is mostly molecular hydrogen, but it also contains helium. The flow of heat from the interior, in excess of the energy received from the Sun, is consistent with the slow cooling off of such a large body from the time of its formation. Preliminary infrared measurements of Jupiter by aircraft had indicated that even more heat was emitted by the planet. Because of the flow of heat from the interior and the planet’s rapid rotation (it has a ten-hour period), weather patterns appear quite different from those on Earth. The familiar pressure systems with clockwise and counterclockwise flow are stretched over many degrees of longitude; the major features are stretched all the way around the planet. Visible and infrared results are consistent with a picture in which the bright and cool bands are regions of upwelling atmosphere and the dark and warm areas are regions of subsidence. Rapid convection of the atmosphere causes incrementally higher temperatures toward the interior; the warm stratosphere is consistent with absorption of sunlight by methane and other hydrocarbon gases present in the atmosphere and by haze particles.
Pioneer 10 was the first spacecraft to visit Jupiter, and its scientific results are best viewed together with its “twin,” Pioneer 11, which differed by the addition of another magnetometer experiment and a trajectory which took it closer to the planet and allowed the imaging and infrared experiments to view the polar regions. Pioneer results are often overlooked in the wealth of high-resolution imaging and detailed remote-sensing spectral data obtained less than a decade later by the missions of Voyagers 1 and 2.
In addition to being the first spacecraft to reach Jupiter, Pioneer 10 was the first spacecraft to penetrate the asteroid belt. It traversed and communicated from greater distances than ever before. It was also the first spacecraft to use all-nuclear electrical power. It demonstrated the viability of spin-scan imaging, and its probing of the asteroid belt and the radiation environment of Jupiter paved the way for the later sophistication of the Voyager spacecraft. It also showed that a probe could survive penetration into the Jovian atmosphere and an orbiter could survive many months in the neighborhood of the planet.
Pioneer 10’s final transmitted signal was received through the Deep Space Network on January 22, 2003. At this point, signals from the spacecraft, 12.23 billion kilometers (7.6 billion miles) distant from Earth, took eleven hours and twenty minutes to arrive. It had continued to send data concerning the nature of the far reaches of the outer solar system until its radioisotope nuclear power supply dwindled. The spacecraft would continue to head out of the solar system as a silent ambassador of humanity.
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Beatty, J. Kelly, Carolyn Collins Petersen, and Andrew Chaikin, eds. The New Solar System. 4th ed. New York: Sky Publishing, 1999. General description of solar-system bodies that contains detailed articles pertinent to Jupiter and Pioneer discoveries. The book is readable at high school or college levels. -
Dyal, P., and R. O. Fimmel. “Exploring Beyond the Planets: The Pioneers 10 and 11 Missions.” Journal of the British Interplanetary Society 37 (October, 1984): 468-479. Describes the main goals of the mission for the 1990’s. -
Fimmel, Richard O., William Swindel, and Eric Burgess. Pioneer Odyssey. NASA SP-396. Washington, D.C.: National Aeronautics and Space Administration, 1977. Authorized history of the Pioneer 10 and 11 missions to Jupiter. This work is a revision of the original NASA Special Publication with more Pioneer 11 results added, including several images of the Galilean satellites. It is suitable for high school and college levels. Appendixes include a list of all images taken of Jupiter by Pioneers 10 and 11. -
Fimmel, Richard O., James Van Allen, and Eric Burgess. Pioneer: First to Jupiter, Saturn, and Beyond. NASA SP-446. Washington, D.C.: National Aeronautics and Space Administration, 1980. The continued mission of Pioneer 10 into the outer solar system necessitated an update of the earlier publication by Fimmel, Swindell, and Burgess (cited above). Results from further analyses of data from the encounter with Jupiter and interplanetary results are included. -
French, Bevan M., and Stephen P. Maran. A Meeting with the Universe. NASA EP-177. Washington, D.C.: U.S. Government Printing Office, 1981. Describes what was learned about the universe by going into space. It is a history of space exploration by NASA, universities, other government agencies, and industries. A novel experiment in writing about science for nontechnical readers. -
Irwin, Patrick G. J. Giant Planets of Our Solar System: Atmospheres, Composition, and Structure. London: Springer-Praxis, 2003. Provides an in-depth comparison of Jupiter, Saturn, Uranus, and Neptune, incorporating data obtained from astronomical observations and planetary spacecraft encounters. -
Mead, Gilbert D. “Pioneer 10 Mission: Jupiter Encounter.” Journal of Geophysical Research 79 (September 1, 1974): 25. Contains comprehensive reports of almost all the Pioneer 10 experiments. Interesting reading. -
Montoya, Earl J., and Richard O. Fimmel. Space Pioneers and Where They Are Now. NASA EP-264. Washington, D.C.: U.S. Government Printing Office, 1987. As part of the modern exploration of space frontiers, this volume describes the accomplishments of the Pioneer missions, including Pioneer 10, and the scientific objectives of that mission in the future as it heads out of the solar system.
Voyagers 1 and 2 Explore the Outer Planets
First Ring Around Jupiter Is Discovered
Comet Shoemaker-Levy 9 Collides with Jupiter
Galileo Achieves Orbit Around Jupiter
Cassini-Huygens Probe Is Launched