Interstellar Journeys of the Pioneer and Spacecraft

The Pioneer 10 and 11 spacecraft, as their names imply, are true deep-space explorers—the first humanmade objects to navigate the main asteroid belt, the first spacecraft to encounter Jupiter and its fierce radiation belts, the first to encounter Saturn, and the first spacecraft to leave the solar system. These spacecraft also investigated magnetic fields, cosmic rays, the solar wind, and the interplanetary dust concentrations as they flew through interplanetary space.

The Pioneer 10 spacecraft was launched from Cape Canaveral Air Force Station, Florida, by an Atlas-Centaur rocket on March 2, 1972. It

Paths of the Pioneer 10 and 11 spacecraft, as well as the Voyager 1 and 2 spacecraft, through the heliosphere and into the interstellar medium.

Paths of the Pioneer 10 and 11 spacecraft, as well as the Voyager 1 and 2 spacecraft, through the heliosphere and into the interstellar medium.

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The Pioneer 10 and 11 spacecraft with their complement of scientific instruments. Each spacecraft's electric power was provided by a long-lived radioisotope thermoelectric generator (RTG). (NASA)

became the first spacecraft to cross the main asteroid belt and the first to make close-range observations of the Jovian system. Sweeping past Jupiter on December 3, 1973 (its closest approach to the giant planet), it discovered no solid surface under the thick layer of clouds enveloping the giant planet—an indication that Jupiter is a liquid hydrogen planet. Pioneer 10 also explored the giant Jovian magnetosphere, made close-up pictures of the intriguing Red Spot, and observed at relatively close range the Galilean satellites Io, Europa, Ganymede, and Callisto. When Pioneer 10 flew past Jupiter, it acquired sufficient kinetic energy to carry it completely out of the solar system.

Departing Jupiter, Pioneer 10 continued to map the heliosphere (the Sun's giant magnetic bubble, or field, drawn out from it by the action of the solar wind). Then, on June 13, 1983, Pioneer 10 crossed the orbit of Neptune, the major planet farthest out from the Sun. The historic date marked the first passage of a humanmade object beyond the major planet boundary of the solar system. Once across this solar system boundary, Pioneer 10 continued to measure the extent of the heliosphere as the spacecraft started its travels into interstellar space. Along with its sister ship (Pioneer 11), the Pioneer 10 spacecraft helped scientists investigate the deep space environment.

The Pioneer 10 spacecraft is heading generally toward the red star, Aldebaran. The robot spacecraft is more than 68 light-years away from Aldebaran, and the journey will require about 2 million years to complete. Budgetary constraints forced NASA to terminate routine tracking and project data-processing operations for Pioneer 10 on March 31, 1997. However, occasional tracking of Pioneer 10 continued beyond that date. The last successful data acquisitions from Pioneer 10 by NASA's Deep Space Network (DSN) occurred in 2002 on March 3 (30 years after launch) and again on April 27. The spacecraft signal was last detected on January 23, 2003, after an uplink message was transmitted to turn off the remaining operational experiment, the Geiger Tube Telescope. However, no downlink data signal was achieved, and by early February 2003 no signal at all was detected. NASA personnel concluded that the spacecraft's radioisotope thermoelectric generator (RTG) unit, which supplied electric power, had finally fallen below the level needed to operate the onboard transmitter. Consequently, no further attempts were made to communicate with Pioneer 10.

The Pioneer 11 spacecraft was launched on April 5, 1973, and swept by Jupiter at an encounter distance of only 26,725 miles (43,000 km) on December 2, 1974. The spacecraft provided additional detailed data and pictures of Jupiter and its moons, including the first views of Jupiter's polar regions. Then, on September 1, 1979, Pioneer 11 flew by Saturn, demonstrating a safe flight path through the rings for the more sophisticated Voyager 1 and 2 spacecraft to follow. Pioneer 11 (by then officially renamed Pioneer Saturn) provided the first close-up observations of Saturn, its rings, satellites, magnetic field, radiation belts, and atmosphere. The space robot found no solid surface on Saturn, but discovered at least one additional satellite and ring. After rushing past Saturn, Pioneer 11 also headed out of the solar system toward the distant stars.

The Pioneer 11 spacecraft operated on a backup transmitter since launch. Instrument power sharing began in February 1985 due to declining RTG power output. Science operations and daily telemetry ceased on September 30, 1995, when the RTG power level became insufficient to operate any of the spacecraft's instruments. All contact with Pioneer 11 ceased at the end of 1995. At that time, the spacecraft was 44.7 astronomical units (AU) away from the Sun and traveling through interstellar space at a speed of about 2.5 AU per year.

Both Pioneer spacecraft carry a special message (called the Pioneer plaque) for any intelligent alien civilization that might find them wandering through the interstellar void millions of years from now. This message is an illustration, engraved on an anodized aluminum plaque. The plaque depicts the location of Earth and the solar system, a man and a woman,

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plaque to facilitate discussion: (1) shows height of the woman compared to the Pioneer spacecraft; (2) is a schematic of the hyperfine transition of neutral atomic hydrogen; (3) represents the number 8 in binary form; the radial pattern (4) locates the solar system within the Milky Way galaxy; the solid bars indicate distance, with the long horizontal bar (5) with no binary notation on it representing the distance from the Sun to the galactic center, while the shorter solid bars denote directions and distances to 14 pulsars from the Sun; and (6) is a diagram of humans' solar system. (NASA)

plaque to facilitate discussion: (1) shows height of the woman compared to the Pioneer spacecraft; (2) is a schematic of the hyperfine transition of neutral atomic hydrogen; (3) represents the number 8 in binary form; the radial pattern (4) locates the solar system within the Milky Way galaxy; the solid bars indicate distance, with the long horizontal bar (5) with no binary notation on it representing the distance from the Sun to the galactic center, while the shorter solid bars denote directions and distances to 14 pulsars from the Sun; and (6) is a diagram of humans' solar system. (NASA)

and other points of science and astrophysics that should be decipherable by a technically intelligent civilization.

The plaque is intended to show any intelligent alien civilization that might detect and intercept either Pioneer spacecraft millions of years from now when the spacecraft was launched, from where it was launched, and by what type of intelligent beings it was built. The plaque's design is engraved into a gold-anodized aluminum plate, 6 inches (15.2 cm) by 9 inches (22.9 cm). The plate is approximately 0.05 inches (0.127 cm) thick. Engineers attached the plaque to the Pioneer spacecraft's antenna support struts in a position that helps shield it from erosion by interstellar dust.

The previous figure shows an annotated version of the Pioneer plaque. The numbers (1 to 6) have been intentionally superimposed on the plaque to assist in the discussion of its message. At the far right, the bracketing bars (1) show the height of the woman compared to the Pioneer spacecraft. The drawing at the top left of the plaque (2) is a schematic of the hyperfine transition of neutral atomic hydrogen used here as a universal "yardstick" that provides a basic unit of both time and space (length) throughout the Milky Way galaxy. This figure illustrates a reverse in the direction of the spin of the electron in a hydrogen atom. The transition depicted emits a characteristic radio wave with an approximately 8.3-inch (21-cm) wavelength. Therefore, by providing this drawing, the people of Earth are telling any technically knowledgeable alien civilization finding it that they have chosen 8.3 inches (21 cm) as a basic length in the message. While extraterrestrial civilizations will certainly have different names and defining dimensions for their basic system of physical units, the wavelength size that is associated with the hydrogen radio-wave emission will still be the same throughout the galaxy. Science and commonly observable physical phenomena represent a general galactic language—at least for starters.

The horizontal and vertical ticks (3) represent the number 8 in binary form. Hopefully, the alien beings pondering over this plaque will eventually realize that the hydrogen wavelength (8.3 inch [21 cm]) multiplied by the binary number representing 8 (indicated alongside the woman's silhouette) describes her overall height—namely, 8 x 8.3 inches = 66 inches (8 x 21 cm = 168 cm), or approximately five and one half-feet tall. Both human figures are intended to represent the intelligent beings that built the Pioneer spacecraft. The man's hand is raised as a gesture of goodwill. These human silhouettes were carefully selected and drawn to maintain ethnic neutrality. Furthermore, no attempt was made to explain terrestrial "sex" to an alien culture—that is, the plaque makes no specific effort to explain the potentially mysterious differences between the man and woman who are depicted.

The radial pattern (4) should help alien scientists locate the solar system within the Milky Way galaxy. The solid bars indicate distance, with the long horizontal bar (5) with no binary notation on it representing the distance from the Sun to the galactic center, while the shorter solid bars denote directions and distances to 14 pulsars from the Sun. The binary digits following these pulsar lines represent the periods of the pulsars. From the basic time unit that was established by the use of the hydrogen-atom transition, an intelligent alien civilization should be able to deduce that all times indicated are about 0.1 second—the typical period of pulsars. Since pulsar periods appear to be slowing down at well-defined rates, the pulsars serve as a form of galactic clock. Alien scientists should be able to search their astrophysical records and identify the star system from which the Pioneer spacecraft originated and approximately when it was launched, even if each spacecraft is not found for hundreds of millions of years. Consequently, through the use of this pulsar map, NASA's engineers and scientists have attempted to locate Earth, both in galactic space and in time.

As a further aid to identifying the Pioneer 10's (or Pioneer 11's) origin, a diagram of the solar system (6) is also included on the plaque. The binary digits accompanying each planet indicate the relative distance of that planet from the Sun. The spacecraft's trajectory is shown starting from the third planet (Earth), which has been offset slightly above the others. As a final clue to the terrestrial origin of the Pioneer spacecraft, its antenna is depicted pointing back to Earth.

This message was designed for NASA by Frank Drake (1930- ) and the late Carl Sagan (1934-96), and Linda Salzman Sagan (1940- ) prepared the artwork.

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