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THE VOYAGERS - THE FIRST SPACE ROBOTS

The Voyager mission was launched to take advantage of the unique arrangement of the outer planets in our solar system during the late 1970's and the 1980's. This layout of the planets, namely Jupiter, Saturn, Uranus and Neptune, occurs about every 175 years and allows a spacecraft on a particular flight path to "sling shot" from one planet to the next with minimal use of any onboard thrusters. The flyby of each planet bends Voyager's flight path and increases its velocity enough to push the spacecraft along to the next planet. The major benefit of using this "gravity assist" technique is that the flight time to Neptune can be reduced from 30 years to a mere 12 years. It was supposed to swoop within 51,000 miles of the planets' surface and complete many scientific observations all of which had to be done without any order or commands sent from earth. Since it would take the radio signals approximately three hours to travel in space, the signals have to cover 1.8 billion miles of space to reach Voyager II and then another three hours for the signal response to return. Therefore, Voyager II had to be programmed to detect or anticipate a problem, to determine what should be done and finally to react independently.

While scientists knew the four-planet mission was possible, most thought it would be far too expensive. Thus the Voyagers were developed specifically to carry out intensive flyby studies of Jupiter and Saturn only. Over 10,000 trajectories were thoroughly studied before choosing two that would allow close flybys of Jupiter and its moon Io, Saturn and its large moon Titan. For Voyager II, engineers chose a path that would allow the option to continue onto Uranus and Neptune, if at that time Voyager II seemed capable to handle the mission add-on.

From NASA Kennedy Space Centre at Cape Canaveral, Florida, Voyager II was launched first, on August 20, 1977. Fifteen days later, Voyager I was launched on a shorter trajectory on September 5, 1997. Both spacecrafts were launched into space aboard Titan-Centaur expendable rockets.

Voyager I's trajectory, designed to give a close flyby of the large moon Titan and behind Saturn's rings, was bent northward out of the elliptic plane. The main difference between the two Voyager trajectories was that Voyager II's was aimed to fly by Saturn at a point that would automatically send the spacecraft in the direction of Uranus. Voyager I started its flyby of Jupiter on March 5, 1979 and Saturn on November 12, 1980, followed by Voyager II starting its flyby of Jupiter on July 9,1979 and Saturn on August 25, 1981.

After Voyager II's successful Saturn encounter, NASA officials deemed it was possible for Voyager II to continue with the mission and fly onto Uranus since all the instruments were still operating. Following the continuation onto Uranus, NASA also authorized the Neptune leg of the mission, which was later renamed to the Voyager Neptune Interstellar Mission.

Voyager II encountered Uranus on January 24, 1986, and captured numerous detailed photos and other valuable information about the planet, its moons, magnetic field and dark rings. Meanwhile, Voyager I continued to travel outwards, still conducting studies of interplanetary space. After many more years, its instruments may be the first of any spacecraft to sense the heliopause.

Following Voyager 2's closest approach to Neptune on August 25, 1989, the spacecraft travelled southward, below the elliptic plane and joined Voyager 1 in its interstellar space. Once again, the mission name was changed to the Voyager Interstellar Mission.

The total cost for the Voyager I and II missions (including launch, mission operation from launch through the Neptune encounter and the spacecraft's nuclear batteries) was $865 million. This isn't including the additional $30 million to fund the Voyager Interstellar Mission for two years following the Neptune encounter.