Voyager

I

Introduction

Voyager, name of two United States spacecraft, launched in 1977, designed to explore the giant outer planets of the solar system and to venture beyond the solar system. The Voyagers took advantage of a rare configuration of the outer planets. This alignment enabled the spacecraft to use the gravitational pull of each planet like a slingshot. As the spacecraft flew by each planet, its gravitational force hurled the spacecraft on to the next planet. As a result the Voyagers never had to rely on large on-board propulsion systems. Voyager 1 visited Jupiter and Saturn, while Voyager 2 flew by Jupiter, Saturn, Uranus, and Neptune, the solar system’s four giant outer planets. The dwarf planet Pluto was not included in the Voyagers’ tour.

Both Voyager spacecraft were launched from the National Aeronautics and Space Administration (NASA) Kennedy Space Center at Cape Canaveral, Florida, by a Titan Centaur rocket. The two-stage rocket boosted the spacecraft into a low orbit around Earth and catapulted the craft toward Jupiter. Voyager 2 was launched first, on August 20, 1977. Voyager 1 was launched on a faster, shorter path on September 5, 1977.

Voyager 1 made its closest approach to Jupiter on March 5, 1979, and Voyager 2 followed on July 9, 1979. Voyager 1 flew past Saturn on November 12, 1980, and Voyager 2 flew past on August 25, 1981. After its Saturn flyby, Voyager 1 continued northward out of the ecliptic plane (the plane in which most of the planets’ orbits lie). Voyager 2 passed Uranus on January 24, 1986, and Neptune on August 25, 1989, and then flew southward on its way to interstellar space.

II

Spacecraft

The Voyager spacecraft were identical. Each weighed 815 kg (1,797 lb), and each was built from 65,000 individual parts. The spacecraft were designed with radiation-hardened parts, and the more sensitive parts were shielded to withstand radiation.

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A set of 16 thrusters allowed each spacecraft to control attitude and correct trajectory. Each of these tiny assemblies produced a thrust of only 0.85 newton (3 oz) to allow the craft to perform delicate maneuvers. Because the spacecraft were to travel far from the Sun, they could not use solar panels to collect sunlight for power. Instead they used radioisotope thermoelectric generators, which produce electricity from the heat resulting from the radioactive decay of plutonium. The generators power the instruments, computer, radio, and other systems. The spacecraft are controlled and their data returned to Earth through the Deep Space Network, a global spacecraft-tracking system.

The Voyager spacecraft were equipped with instruments to conduct ten different scientific experiments. The instruments included television cameras, infrared and ultraviolet sensors, magnetometers, plasma detectors, and cosmic-ray and charged-particle sensors. In addition, the spacecraft radios were used to gather information about planetary atmospheres and ring systems.

A message was placed aboard the Voyager spacecraft to communicate with any extraterrestrial beings that might come into contact with the probes. The message was carried on a 30.5-cm (12-in) gold-plated copper phonograph record containing 115 images and a variety of natural sounds selected to portray the diversity of life forms and cultures on Earth. Musical selections from different cultures and eras; spoken greetings in 55 languages; and printed messages from Jimmy Carter, former president of the United States, and from Kurt Waldheim, former United Nations secretary general, were also included.

III

Scientific Highlights of the Voyager Missions

The Voyager spacecraft provided much new information about the outer planets. The probes revealed that the Great Red Spot of Jupiter is actually a complex swirling storm. They also found an array of smaller storms and eddies throughout the banded clouds of the planet. Active volcanoes were discovered on the large Jovian moon Io—the first erupting volcanoes found on another body in the solar system. The spacecraft also found a faint, dusty ring of material around Jupiter beginning at about 99,000 km (about 61,500 mi) and extending outward about 30,000 km (about 18,600 mi).

Data transmitted from the spacecraft as they flew by Saturn revealed atmospheric features similar to those of Jupiter, although smaller. The large Saturnian moon Enceladus was found to be marked by huge faults and valleys. The spacecraft revealed a crater on the moon Mimas so large that the impact that caused it must have nearly broken the moon apart. In Saturn’s rings unexpected structures such as kinks and radial spokes were discovered, along with both very thin rings and broad, diffuse rings that had not been observed from Earth.

During its Uranus flyby, Voyager 2 found a previously undetected magnetic field and a high haze layer over the planet’s sunlit pole. The craft also found ten moons that had not been detected before. On the innermost of the planet’s large moons, Miranda, Voyager 2 discovered fault canyons as deep as 20 km (12 mi), terraced layers, and a mixture of old and new surfaces that indicated great geologic activity. The spacecraft provided information about the planet’s rings, which were found to be distinctly younger and made of larger particles than the rings of Jupiter and Saturn.

Voyager 2 revealed Neptune to be an unexpectedly dynamic planet that possesses several large dark spots similar to those on Jupiter. Voyager 2 also found that Neptune had the strongest winds of any planet, reaching speeds of up to 2,000 km/h (1,240 mph). Voyager 2 images of Triton, the largest of Neptune’s moons, showed geyserlike eruptions spewing nitrogen gas and dark dust particles far into the moon’s tenuous atmosphere. Before continuing on its journey to interstellar space, Voyager 2 transmitted a final image of the entire expanse of the solar system.

To accomplish their original missions, the Voyager spacecraft were initially built to last five years. However, as a result of software improvements, scientists were able to reprogram the Voyagers’ onboard computers to greatly extend the usefulness of the two probes. Scientists estimate that both spacecraft may have enough remaining power to continue sending data back to Earth until about 2025.

As the Voyager probes have headed toward interstellar space, they have continued to send data about particles and magnetic fields on the edge of the solar system. These data have allowed scientists to study the shape and structure of the heliosphere, the region of space dominated by the Sun’s magnetic field and the charged particles streaming from the Sun called the solar wind. The heliosphere surrounds the solar system in a kind of bubble that is buffeted by the gases, particles, and magnetic fields that lie between the stars as the Sun orbits within our Milky Way Galaxy.

Scientists think that both Voyager probes have passed beyond a zone called the solar wind termination shock and into the outermost layer of the heliosphere called the heliosheath. The solar wind termination shock occurs where pressure from the gas and magnetic field in interstellar space abruptly slows the solar wind. The stream of particles from the Sun then becomes denser and hotter as it presses against interstellar gas.

Voyager 1 passed the termination shock in December 2004 and Voyager 2 in August 2007. Voyager 2 made more detailed measurements with its still-functioning plasma detector and actually passed through the termination shock a number of times as the shock wave moved back and forth like surf on a beach. Data from the Voyager probes indicate that the interstellar magnetosphere pushes the heliosphere into a squashed shape.

The Voyager probes will enter interstellar space when they pass beyond the outer edge of the heliosphere called the heliopause. Scientists estimate Voyager 1 may pass beyond the heliopause by 2015, with Voyager 2 entering interstellar space as well about five years later.

In their joint mission, Voyager 1 and Voyager 2 explored all the giant outer planets of the solar system, many of the planets’ moons, and each of the planets’ systems of rings and magnetic fields. The information that the Voyager spacecraft relayed to Earth helped resolve certain key questions in astronomy while raising intriguing new questions about the origin and evolution of the planets in the solar system. The Galileo orbiter mission to Jupiter and the Cassini-Huygens orbiter-lander mission to Saturn and Titan both drew on discoveries made by Voyager, as does the Interstellar Boundary Explorer space probe designed to study the outer edge of the heliosphere.

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