When the Voyager spacecraft were launched in 1977, their ambitious mission was to undertake the most extensive tour ever of the outer solar system by visiting the planets Jupiter and Saturn. Yet no one had an inkling that, 25 years later, Voyager would remain a pioneer in outer space, becoming the farthest man-made object from Earth, poised to reach the termination shock and, perhaps, be the first spacecraft to leave the confines of our heliosphere.

In some sense, Voyager's history stretches back to the 1920s, when Walter Hohmann demonstrated that the lowest energy path between any two planets is an ellipse that is tangential to the orbits of both the planets. However, rockets designed before 1960 were unable to produce sufficient energy to send spacecraft beyond Jupiter, and calculations put the travel time from Earth to Pluto at 40-50 years, and from Earth to Neptune at 30 years-far too long to be feasible.

Then, in 1961, the fortuitous summer employment of 25-year-old graduate mathematics student, Michael A. Minovitch, at JPL led to a revolution in planetary missions. Minovitch showed that, rather than requiring cancellation, the gravity field of a planet could provide thrust to a spacecraft. He demonstrated that careful design of the trajectory to a target planet could provide a gravity assist to move from that planet to a second planet. Further boosts could provide energy to visit other planets, with gains in speed that would reduce the one-way trip times to each of the planets after the first, and the only energy needed would be that to launch the spacecraft from Earth to the first planet.

Since Jupiter is the largest planet and, consequently, has the strongest gravity field, Minovitch determined that missions to the outer planets via Jupiter would be possible. He illustrated the trajectory for a mission from Earth to Jupiter, Saturn, and Neptune.

About every 175 years, the outer planets, Jupiter, Saturn, Uranus, and Neptune, are aligned geometrically in such a way as to minimize the trip time and energy required to tour all four. In 1965, Gary Flandro, who was at JPL at the time, pointed out that the next such opportunity would occur in 1976, 1977, and 1978 and designed some Grand Tour gravity-assist trajectories that included an Earth-Jupiter-Saturn-Uranus-Neptune mission.

The Voyager spacecraft were not the first to use the gravity-assist technique to visit planets. In 1973 Mariner 10 encountered Venus and then used gravity-assist to continue to Mercury. The Pioneer mission in 1973 was the first to Jupiter, and Pioneer 11 used gravity-assist to become the first spacecraft to reach Saturn, in 1979.

In 1970 the Grand Tour plans were abandoned when budgetary cuts made it too expensive to fund a spacecraft that could visit the four outer planets with sufficient instruments to carry out all the necessary experiments. Instead, funding was approved for a two-spacecraft mission to make close-up studies of Jupiter and its large moon, Io, and Saturn and its large moon, Titan. More than ten thousand trajectories were considered, to maximize the amount of information that could be obtained on the two planetary systems. The new mission was originally called MJS, for Mariner Jupiter/Saturn, but became the Voyager mission about six months before the launch. Although the Voyager mission planned flybys of only Jupiter and Saturn, the trajectory for the second spacecraft, Voyager 2, was designed to allow for an extended flight to Uranus and Neptune. The Grand Tour mission would have cost about 750 million dollars, whereas the Voyager mission would reduce those costs by approximately two thirds.

Thus the Voyager mission was born and two twin spacecraft designed. The Jet propulsion Laboratory, JPL, at the California Institute of Technology in Pasadena, California, manages the Voyager mission for NASA's Office of Space Science in Washington D.C. Voyager 2 was originally conceived as a backup to Voyager 1, but went on to make its own vitally important discoveries.

Voyager 2 actually took off first, on August 20, 1977, followed by Voyager 1 on September 5, 1977. Voyager 2's launch provided a learning curve, so that Voyager 1, whose takeoff was delayed twice, was able to avoid the problems encountered by Voyager 2 and enjoyed a flawless launch. However, Voyager 1's trajectory was shorter, so it reached Jupiter first, on March 5, 1979, and then went on to Saturn on November 12, 1980. Meanwhile, Voyager 2 encountered Jupiter on July 9, 1979, and Saturn on August 25, 1981.

Careful planning, as well as anticipation and correction of possible setbacks ensured the overwhelming success of the Voyager mission. The spacecraft were so extraordinarily successful at taking high resolution images of the atmospheres, satellites, rings, and moons of Jupiter and Saturn and of their magnetospheres, that funding was provided to extend the mission. Instead of following Voyager 1's trajectory, Voyager 2 was placed on a trajectory to Uranus and Neptune. The gravity-assist technique reduced the flight time from Earth to Neptune to 12 years, down from the 30 years originally anticipated. Voyager 1's trajectory, which allowed it to pass particularly close to Saturn's moon, Titan, and behind Saturn's rings, bent its path out of the ecliptic plane, so that Voyager 1 would not encounter more planets. Remote programming of the onboard computers enabled the extended mission, now dubbed the Voyager Neptune Interstellar Mission. Thus the original Grand Tour was reinstated and accomplished.

Voyager 2 encountered Uranus on January 24, 1986, and made its closest approach to Neptune on August 25, 1989, where it was able to achieve all the objectives of the Neptune Interstellar Mission. These included learning as much as possible about Neptune, including color, cloud features, size, mass, density, composition, temperature and variations in temperature, heat balance, wind speeds, and rate of rotation. Voyager 2 was also to study Neptune's large moon, Triton, and small moon, Nereid, to determine their characteristics and whether Triton had an atmosphere, and search for rings and new moons, and their characteristics. In addition, Voyager 2's objectives were to study Neptune's magnetic field and structure and composition of any charged particles in its magnetosphere, to search for lightning, auroras, radio emissions, or other planetary phenomena, and to determine the position in time and rotational pole orientation of the Neptunian system.

The first 12 years of Voyager's momentous exploration had cost NASA $865 million, but the accomplishments were so exceptional that the mission was extended again to the Voyager Interstellar Mission, for which an additional $30 million was set aside for the first two years.

When the Voyager Interstellar Mission began, Voyager 1 was about 40 AU from the sun, and Voyager 2 about 31 AU. Voyager 1 is leaving our solar system at about 3.5 AU per year at an angle of 35 degrees above the ecliptic plane, while Voyager 2 is escaping at approximately 3.1 AU per year at 48 degrees below the ecliptic plane. The spacecraft are still in the termination shock phase of the mission, where they remain in the environment controlled by the solar magnetic field. However, Voyager 1 will soon encounter the termination shock and begin exploration of the heliosheath, the boundary between the termination shock and the heliopause. The final phase will begin when Voyager leaves the heliosphere to begin the first ever examination of interstellar space.

Voyager's original four-year mission expanded to 12 years and then 25, and is still continuing. This remarkable achievement has been accomplished through judicious use of the available electric power and attitude control propellant. At launch, the Radioisotope Thermoelectric Generators (RTGs) supplied about 470 watts of power to the spacecraft. By the beginning of 1997, owing to natural radioactive decay of the plutonium fuel, Voyager 1 was generating 334 watts and Voyager 2 336, both better than had been predicted. By the start of 2001, Voyager 1's power output had dropped to 315 watts and Voyager 2's to 319 watts. To conserve energy, power loads on the spacecraft have to be turned off, which means that certain instruments will have to share power and will eventually no longer be able to function. The first loads to be shut off are the instrument heaters on the scan platform, which will cause the ultraviolet spectrometers to cool and cease to function. Thereafter, shutdown of gyro operations will put an end to the magnetometer experiments and affect the Sun Sensor and the High Gain Antenna. Both spacecraft should be able to continue operating until about 2020, whereupon this exceptional mission will end its epic journey.

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