Pioneer (spacecraft)

I

Introduction

Pioneer (spacecraft), first United States space probe designed to travel beyond Earth's orbit. The initial series was created in early 1958 by the Department of Defense Advanced Research Projects Agency (ARPA). The National Aeronautics and Space Administration (NASA) inherited the program when NASA officially began operating in October 1958. Under NASA, nine Pioneer spacecraft explored interplanetary space from the region of Earth's orbit to beyond Pluto and made important observations of Venus, Jupiter, and Saturn. Four other Pioneer spacecraft explored the space between Earth and the Moon.

II

Early Series

Pioneers 1 through 4 weighed 38.3 kg (84.4 lb), 39.2 kg (86.4 lb), 5.9 kg (13.0 lb), and 6.1 kg (13.4 lb), respectively, and were launched between October 1958 and March 1959 on military Thor-Able (Pioneer 1 and 2) and Juno II (Pioneer 3 and 4) booster rockets. They were designed to measure radiation, temperature, and meteorites in the vicinity of the Moon; however, none achieved its objectives fully, and a variety of other spacecraft in this series that never received official Pioneer names were also largely unsuccessful. Nonetheless, these craft did provide an early picture of the magnetic fields and radiation surrounding Earth.

In 1960 Pioneer 5, the last of the early series, weighing 43.0 kg (94.8 lb), was launched by an Air Force Thor-Able booster rocket and carried experiments from the new NASA Goddard Space Flight Center into solar orbit, successfully exploring the space between Earth and Venus. Pioneer 5 also transmitted the first data on solar flares (explosions of energy on the surface of the Sun) from space.

III

Solar Studies

In May 1960 NASA began studying probes for use in exploring the influence of the Sun on inner solar-system space. A new Pioneer spacecraft series was designed to explore interplanetary space using radiation and particle detectors but no photographic or imaging sensors. These craft, like the earlier series, were equipped with solar panels to generate electrical power. This series was complementary to the early Mariner series of probes launched in the 1960s that concentrated on observations and images of individual planets. Planning and design of this new Pioneer series continued through 1965.

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Between 1965 and 1968, Pioneers 6 through 9, weighing 62.1 kg (137 lb), 62.8 kg (138 lb), 65.4 kg (144 lb), and 65.4 kg, respectively, were all launched by Thor-Delta rockets into orbits around the Sun slightly inside or outside of the orbit of Earth, allowing the spacecraft to explore the interaction of the interplanetary environment with that of Earth. It was known that the Sun shed from its surface a plasma (its own hydrogen broken down into charged components) that traveled rapidly through the solar system, carrying with it the solar magnetic field. As the Sun slowly rotated, this plasma and the magnetic field were twisted into an expanding spiral. Pioneers 6 through 9 carried their magnetic-field and particle detectors through this spiral, mapping the gross characteristics, exploring the fine structure, and observing the dynamic changes as solar flares sent more energetic bursts of material toward Earth. In addition, these spacecraft provided advance warning of material and radiation ejected from solar flares, which would affect radio communications and electric utility lines and were potentially hazardous to astronauts traveling beyond the shielding of the radiation belts, regions of radiation surrounding Earth. Pioneers 6 through 9 all continued to operate into the 1980s; Pioneer 6, 7, and 8 are still functioning, though NASA only makes contact with the spacecraft occasionally.

IV

Outer Planetary Missions

In 1968 twin probes designed to study the region of the outer planets were made part of the series and designated Pioneers 10 and 11. These hexagonally shaped spacecraft, weighing 258 kg (569 lb) and 259 kg (571 lb), respectively, and carrying mainly particle and field detectors, would serve as pathfinders for later, more complex spacecraft in the Mariner series. Pioneers 10 and 11 generated their electrical power from radioisotope thermoelectric generators (which produce electricity using the heat generated by the natural decay of a plutonium isotope), since the solar panels used on craft in the inner solar system would be ineffective so far from the Sun. Pioneer 10 also carried a plaque with pictorial information about Earth's location and inhabitants, in case the spacecraft made contact with other intelligent life forms.

Pioneer 10, launched in March 1972 by an Atlas-Centaur rocket, became the first spacecraft to traverse the asteroid belt, a region outside the orbit of Mars that was found to pose far less danger of collision with an asteroid than previously believed. During its Jupiter flyby in December 1973, Pioneer 10 returned the first closeup images of the planet and its larger moons. It also returned surprising data on the giant planet's magnetic field, a whirl of turbulent radiation far more complex and intense than expected. Accelerated by Jupiter's gravity, Pioneer 10 continued on a path out of the solar system to explore the outer regions of the Sun's influence.

Pioneer 11, launched in April 1973 by Atlas-Centaur rocket, traveled three times closer to Jupiter in December 1974 than had its predecessor a year earlier. It returned detailed color views of Jupiter's bands, zones, and red spot and the boundaries between these dynamic features. Pioneer 11's observations led to a more precise measurement of the radiation of the planet and the first determination of its atmospheric ratio of hydrogen to helium, found to be about that of the Sun.

Using the slingshot effect of Jupiter's gravity on such a close approach, Pioneer 11 continued on its course to fly by Saturn in September 1979. The Saturn flyby resulted in the discovery of new rings, new moons, and important characteristics of the planet's magnetic field. Observations of both Jupiter and Saturn were continued through the 1980s by descendants of the Mariner series, Voyagers 1 and 2.

Pioneers 10 and 11 continued making observations in the outer reaches of the solar system through the mid-1990s. In late 1995 Pioneer 11 ran out of power and moved out of Earth's range of communication. Pioneer 10 transmitted information about the Sun's influence in space until the end of its mission in 1997. The spacecraft still had power, however, and astronomers were able to track it for several more years. In 1998 scientists found that Pioneer 10 was slowing faster than they had predicted it would based on the known forces acting on it, such as the gravitational forces of the Sun and the planets. Some scientists believe that the increased deceleration was caused by the way Pioneer 10 released heat, or by a tiny fuel leak. Other scientists believe that the spacecraft’s motion may be evidence of a new cosmological force. The last contact with Pioneer occurred in 2003.

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