The Far Planets - Voyager

In 1977 NASA began another bold mission to investigate Jupiter and Saturn. The program was called Voyager and included twin robotic spacecraft named Voyager 1 and Voyager 2. An illustration of a Voyager spacecraft is shown in Figure 8.3.

The various instruments on board were designed to detect and measure the solar wind and other charged particles, cosmic radiation, magnetic field intensities, and plasma waves. The original five-year Voyager mission was so successful that it was extended to include flybys of Uranus and Neptune. The total cost of Voyager's planetary explorations was $865 million.

The Missions

Both spacecraft were launched into space atop Titan rockets. Voyager 2 was the first to launch, on August 20, 1977. It was followed on September 5, 1977, by Voyager 1. Both spacecraft traveled for two years to fly by Jupiter. They made many scientific observations as they passed Jupiter and continued on to Saturn. Voyager 1 was on a faster trajectory than Voyager 2 and reached the planet first. Voyager 2 was directed on to fly by Uranus and Neptune. It was the first spacecraft to do so.

The Voyager spacecraft proved to be so hardy after completing their planetary journeys that they were sent on a new mission called the Voyager Interstellar Mission (VIM). The purpose of the VIM is to use the instruments on the spacecraft to explore the outermost edge of the heliosphere. This is the region of space dominated by energy effects from the Sun.

Planetary Achievements

The Voyager missions were two of the most successful in NASA's history. The spacecraft revealed a number of discoveries about the gas giants in the outer solar system:

• Jupiter, Uranus, and Neptune have faint ring systems.
• Jupiter has a complicated atmosphere in which lightning storms and aurora are common.
• Jupiter's moon Io has active volcanoes.
• Jupiter's moon Europa has a smooth surface composed of water ice.
• The radiation levels experienced during the Jupiter flyby were 1,000 times stronger than what is lethal to humans.
• Saturn's rings comprise thousands of strands (ringlets).
• Saturn's ringlets are not as uniform and separate as expected. Some are kinked or braided together. Additional gaps between rings were discovered.
• Saturn's weather is relatively tame compared to that on Jupiter.
• Saturn's largest moon, Titan, has a dense smoggy atmosphere that contains nitrogen and carbon-containing compounds.
• Saturn's moon Mimas has a massive impact crater.
• Neptune's moon Triton has a thin atmosphere.

The mission also uncovered twenty-three previously unknown moons (three around Jupiter, four around Saturn, ten around Uranus, and six around Neptune). The discovery of water ice on the surface of Jupiter's moon Europa was particularly exciting, because it raised the possibility that there was liquid water underneath.

Voyager Interstellar Mission

In February 1998 Voyager 1 became the most distant human-made object in space when it reached a distance of 6.5 billion miles from the Sun, surpassing the record of Pioneer 10. It continues to travel at a speed of nearly one million miles per day. Voyager 2 is a little slower than its sister ship.

In a September 2005 update NASA scientists announced that Voyager 1 had passed the termination shock. (See Figure 8.4.) The termination shock occurs where the solar wind slows considerably due to effects from interstellar wind. Solar wind emanates from the Sun and forms a long "wind sock" that moves with the Sun as it journeys through space. The heliosheath is the outer layer of the heliosphere. The heliopause is the boundary between the heliosphere and interstellar space. It lies more than thirteen billion miles from Earth.

As of October 2005 Voyager 1 was nearly 9.1 billion miles from Earth. Voyager 2 was roughly 7.2 billion miles away. Both have been traveling through space for nearly three decades. Some of the redundant instruments on board the spacecraft have been turned off to conserve power. According to NASA, both spacecraft should continue to function until at least 2020. They have enough propellant to last that long and should continue to generate adequate electrical power to run their scientific instruments. Voyager 1 is expected to pass through the heliopause by 2015. Voyager 2 should follow about five years later. NASA scientists hope that both spacecraft will be able to transmit data as they cross into the vast unknown of interstellar space.

Messages from Earth

The Voyager spacecraft carry written and recorded messages from Earth, in case they come across any intelligent life. Attached to each spacecraft is a twelve-inch gold-plated copper disk inside a protective aluminum case. The cover of the protective case has symbolic instructions for playing the disc and a diagram of Earth's location in the solar system carved into it. The disks contain recorded greetings in fifty-five different languages and various other sounds, including bits of music and natural and human-made sounds. There are 115 images encoded in analog form on the disks of various Earth scenes. These include pictures of people, objects, and places from around the world. The disks carried printed messages from President Jimmy Carter and the United Nations Secretary General.