While the four-planet mission was known to be possible, it was deemed to be too expensive to build a spacecraft that could go the distance, carry the instruments needed and last long enough to accomplish such a long mission. Thus, the Voyagers were funded to conduct intensive flyby studies of Jupiter and Saturn only. More than 10,000 trajectories were studied before choosing the two that would allow close flybys of Jupiter and its large moon Io, and Saturn and its large moon Titan; the chosen flight path for Voyager 2 also preserved the option to continue on to Uranus and Neptune.

From the NASA Kennedy Space Center at Cape Canaveral, Florida, Voyager 2 was launched first, on August 20, 1977; Voyager 1 was launched on a faster, shorter trajectory on September 5, 1977. Both spacecraft were delivered to space aboard Titan-Centaur expendable rockets.

The prime Voyager mission to Jupiter and Saturn brought Voyager 1 to Jupiter on March 5, 1979, and Saturn on November 12, 1980, followed by Voyager 2 to Jupiter on July 9, 1979, and Saturn on August 25, 1981.

Voyager 1's trajectory, designed to send the spacecraft closely past the large moon Titan and behind Saturn's rings, bent the spacecraft's path inexorably northward out of the ecliptic plane -- the plane in which most of the planets orbit the Sun. Voyager 2 was aimed to fly by Saturn at a point that would automatically send the spacecraft in the direction of Uranus.

After Voyager 2's successful Saturn encounter, it was shown that Voyager 2 would likely be able to fly on to Uranus with all instruments operating. NASA provided additional funding to continue operating the two spacecraft and authorized JPL to conduct a Uranus flyby. Subsequently, NASA also authorized the Neptune leg of the mission, which was renamed the Voyager Neptune Interstellar Mission.

Voyager 2 encountered Uranus on January 24, 1986, returning detailed photos and other data on the planet, its moons, magnetic field and dark rings. Voyager 1, meanwhile, continues to press outward, conducting studies of interplanetary space. Eventually, its instruments may be the first of any spacecraft to sense the heliopause -- the boundary between the end of the Sun's magnetic influence and the beginning of interstellar space.

Following Voyager 2's closest approach to Neptune on August 25, 1989, the spacecraft flew southward, below the ecliptic plane and onto a course that will take it, too, to interstellar space. Reflecting the Voyagers' new transplanetary destinations, the project is now known as the Voyager Interstellar Mission.

Voyager 1 is now leaving the solar system, rising above the ecliptic plane at an angle of about 35 degrees at a rate of about 520 million kilometers (about 320 million miles) a year. Voyager 2 is also headed out of the solar system, diving below the ecliptic plane at an angle of about 48 degrees and a rate of about 470 million kilometers (about 290 million miles) a year.

Both spacecraft will continue to study ultraviolet sources among the stars, and the fields and particles instruments aboard the Voyagers will continue to search for the boundary between the Sun's influence and interstellar space. The Voyagers are expected to return valuable data for two or three more decades. Communications will be maintained until the Voyagers' nuclear power sources can no longer supply enough electrical energy to power critical subsystems.

The cost of the Voyager 1 and 2 missions -- including launch, mission operations from launch through the Neptune encounter and the spacecraft's nuclear batteries (provided by the Department of Energy) -- is $865 million. NASA budgeted an additional $30 million to fund the Voyager Interstellar Mission for two years following the Neptune encounter.

The Voyagers travel too far from the Sun to use solar panels; instead, they were equipped with power sources called radioisotope thermoelectric generators (RTGs). These devices, used on other deep space missions, convert the heat produced from the natural radioactive decay of plutonium into electricity to power the spacecraft instruments, computers, radio and other systems.

The spacecraft are controlled and their data returned through the Deep Space Network (DSN), a global spacecraft tracking system operated by JPL for NASA. DSN antenna complexes are located in California's Mojave Desert; near Madrid, Spain; and in Tidbinbilla, near Canberra, Australia. The Voyager project manager for the Interstellar Mission is George P. Textor of JPL. The Voyager project scientist is Dr. Edward C. Stone of the California Institute of Technology. The assistant project scientist for the Jupiter flyby was Dr. Arthur L. Lane, followed by Dr. Ellis D. Miner for the Saturn, Uranus and Neptune encounters. Both are with JPL.

Saturn is the second largest planet in the solar system. Ittakes 29.5 Earth years to complete one orbit of the Sun, and itsday was clocked at 10 hours, 39 minutes. Saturn is known to haveat least 17 moons and a complex ring system. Like Jupiter,Saturn is mostly hydrogen and helium. Its hazy yellow hue wasfound to be marked by broad atmospheric banding similar to butmuch fainter than that found on Jupiter. Close scrutiny byVoyager's imaging systems revealed long-lived ovals and otheratmospheric features generally smaller than those on Jupiter.

Perhaps the greatest surprises and the most puzzles werefound by the Voyagers in Saturn's rings. It is thought that therings formed from larger moons that were shattered by impacts ofcomets and meteoroids. The resulting dust and boulder- tohouse-size particles have accumulated in a broad plane around theplanet varying in density.

The irregular shapes of Saturn's eight smallest moonsindicates that they too are fragments of larger bodies. Unexpected structure such as kinks and spokes were found in addition tothin rings and broad, diffuse rings not observed from Earth. Much of the elaborate structure of some of the rings is due tothe gravitational effects of nearby satellites. This phenomenonis most obviously demonstrated by the relationship between theF-ring and two small moons that "shepherd" the ring material. The variation in the separation of the moons from the ring maythe ring's kinked appearance. Shepherding moons were also foundby Voyager 2 at Uranus.

Radial, spoke-like features in the broad B-ring were foundby the Voyagers. The features are believed to be composed offine, dust-size particles. The spokes were observed to form anddissipate in time-lapse images taken by the Voyagers. Whileelectrostatic charging may create spokes by levitating dustparticles above the ring, the exact cause of the formation of thespokes is not well understood.

Winds blow at extremely high speeds on Saturn -- up to 1,800kilometers per hour (1,100 miles per hour). Their primarilyeasterly direction indicates that the winds are not confined tothe top cloud layer but must extend at least 2,000 kilometers(1,200 miles) downward into the atmosphere. The characteristictemperature of the atmosphere is 95 kelvins.

Saturn holds a wide assortment of satellites in its orbit,ranging from Phoebe, a small moon that travels in a retrogradeorbit and is probably a captured asteroid, to Titan, theplanet-sized moon with a thick nitrogen-methane atmosphere. Titan's surface temperature and pressure are 94 kelvins (-292Fahrenheit) and 1.5 atmospheres. Photochemistry converts someatmospheric methane to other organic molecules, such as ethane,that is thought to accumulate in lakes or oceans. Other morecomplex hydrocarbons form the haze particles that eventually fallto the surface, coating it with a thick layer of organic matter. The chemistry in Titan's atmosphere may strongly resemble thatwhich occurred on Earth before life evolved.

The most active surface of any moon seen in the Saturnsystem was that of Enceladus. The bright surface of this moon,marked by faults and valleys, showed evidence of tectonicallyinduced change. Voyager 1 found the moon Mimas scarred with acrater so huge that the impact that caused it nearly broke thesatellite apart.

Saturn's magnetic field is smaller than Jupiter's, extendingonly one or two million kilometers. The axis of the field isalmost perfectly aligned with the rotation axis of the planet.

Uranus is the third largest planet in the solar system. Itorbits the Sun at a distance of about 2.8 billion kilometers (1.7billion miles) and completes one orbit every 84 years. Thelength of a day on Uranus as measured by Voyager 2 is 17 hours,14 minutes.

Uranus is distinguished by the fact that it is tipped on itsside. Its unusual position is thought to be the result of acollision with a planet-sized body early in the solar system'shistory. Given its odd orientation, with its polar regionsexposed to sunlight or darkness for long periods, scientists werenot sure what to expect at Uranus.

Voyager 2 found that one of the most striking influences ofthis sideways position is its effect on the tail of the magneticfield, which is itself tilted 60 degrees from the planet's axisof rotation. The magnetotail was shown to be twisted by theplanet's rotation into a long corkscrew shape behind the planet.

The presence of a magnetic field at Uranus was not knownuntil Voyager's arrival. The intensity of the field is roughlycomparable to that of Earth's, though it varies much more frompoint to point because of its large offset from the center ofUranus. The peculiar orientation of the magnetic field suggeststhat the field is generated at an intermediate depth in theinterior where the pressure is high enough for water to becomeelectrically conducting.

Radiation belts at Uranus were found to be of an intensitysimilar to those at Saturn. The intensity of radiation withinthe belts is such that irradiation would quickly darken (within100,000 years) any methane trapped in the icy surfaces of theinner moons and ring particles. This may have contributed to thedarkened surfaces of the moons and ring particles, which arealmost uniformly gray in color.

A high layer of haze was detected around the sunlit pole,which also was found to radiate large amounts of ultravioletlight, a phenomenon dubbed "dayglow." The average temperature isabout 60 kelvins (-350 degrees Fahrenheit). Surprisingly, theilluminated and dark poles, and most of the planet, show nearlythe same temperature at the cloud tops.

Voyager found 10 new moons, bringing the total number to 15. Most of the new moons are small, with the largest measuring about150 kilometers (about 90 miles) in diameter.

The moon Miranda, innermost of the five large moons, wasrevealed to be one of the strangest bodies yet seen in the solarsystem. Detailed images from Voyager's flyby of the moon showedhuge fault canyons as deep as 20 kilometers (12 miles), terracedlayers, and a mixture of old and young surfaces. One theoryholds that Miranda may be a reaggregration of material from anearlier time when the moon was fractured by an violent impact.

The five large moons appear to be ice-rock conglomerateslike the satellites of Saturn. Titania is marked by huge faultsystems and canyons indicating some degree of geologic, probablytectonic, activity in its history. Ariel has the brightest andpossibly youngest surface of all the Uranian moons and alsoappears to have undergone geologic activity that led to manyfault valleys and what seem to be extensive flows of icymaterial. Little geologic activity has occurred on Umbriel orOberon, judging by their old and dark surfaces.

All nine previously known rings were studied by thespacecraft and showed the Uranian rings to be distinctlydifferent from those at Jupiter and Saturn. The ring system maybe relatively young and did not form at the same time as Uranus. Particles that make up the rings may be remnants of a moon thatwas broken by a high-velocity impact or torn up by gravitationaleffects.

Neptune orbits the Sun every 165 years. It is the smallestof our solar system's gas giants. Neptune is now known to haveeight moons, six of which were found by Voyager. The length of aNeptunian day has been determined to be 16 hours, 6.7 minutes.

Even though Neptune receives only three percent as muchsunlight as Jupiter does, it is a dynamic planet and surprisinglyshowed several large, dark spots reminiscent of Jupiter'shurricane-like storms. The largest spot, dubbed the Great DarkSpot, is about the size of Earth and is similar to the Great RedSpot on Jupiter. A small, irregularly shaped, eastward-movingcloud was observed "scooting" around Neptune every 16 hours orso; this "scooter," as Voyager scientists called it, could be acloud plume rising above a deeper cloud deck.

Long, bright clouds, similar to cirrus clouds on Earth, wereseen high in Neptune's atmosphere. At low northern latitudes,Voyager captured images of cloud streaks casting their shadows oncloud decks below.

The strongest winds on any planet were measured on Neptune. Most of the winds there blow westward, or opposite to therotation of the planet. Near the Great Dark Spot, winds blow upto 2,000 kilometers (1,200 miles) an hour.

The magnetic field of Neptune, like that of Uranus, turnedout to be highly tilted -- 47 degrees from the rotation axis andoffset at least 0.55 radii (about 13,500 kilometers or 8,500miles) from the physical center. Comparing the magnetic fieldsof the two planets, scientists think the extreme orientation maybe characteristic of flows in the interiors of both Uranus andNeptune -- and not the result in Uranus's case of that planet'ssideways orientation, or of any possible field reversals ateither planet. Voyager's studies of radio waves caused by themagnetic field revealed the length of a Neptunian day. Thespacecraft also detected auroras, but much weaker than those onEarth and other planets.

Triton, the largest of the moons of Neptune, was shown to benot only the most intriguing satellite of the Neptunian system,but one of the most interesting in all the solar system. Itshows evidence of a remarkable geologic history, and Voyager 2images showed active geyser-like eruptions spewing invisiblenitrogen gas and dark dust particles several kilometers into thetenuous atmosphere. Triton's relatively high density andretrograde orbit offer strong evidence that Triton is not anoriginal member of Neptune's family but is a captured object. Ifthat is the case, tidal heating could have melted Triton in itsoriginally eccentric orbit, and the moon might even have beenliquid for as long as one billion years after its capture byNeptune.

An extremely thin atmosphere extends about 800 kilometer(500 miles) above Triton's surface. Nitrogen ice particles mayform thin clouds a few kilometers above the surface. Theatmospheric pressure at the surface is about 14 microbars,1/70,000th the surface pressure on Earth. The surfacetemperature is about 38 kelvins (-391 degrees Fahrenheit) thecoldest temperature of any body known in the solar system.

The new moons found at Neptune by Voyager are all small andremain close to Neptune's equatorial plane. Names for the newmoons were selected from mythology's water deities by theInternational Astronomical Union, they are: Naiad, Thalassa,Despina, Galatea, Larissa, Proteus.

Voyager 2 solved many of the questions scientists had aboutNeptune's rings. Searches for "ring arcs," or partial rings,showed that Neptune's rings actually are complete, but are sodiffuse and the material in them so fine that they could not befully resolved from Earth. From the outermost in, the rings have been designated Adams, Plateau, Le Verrier and Galle.

As the Voyagers cruise gracefully in the solar wind, their fields, particles and waves instruments are studying the space around them. In May 1993, scientists concluded that the plasma wave experiment was picking up radio emissions that originate at the heliopause -- the outer edge of our solar system.

The heliopause is the outermost boundary of the solar wind,where the interstellar medium restricts the outward flow of thesolar wind and confines it within a magnetic bubble called theheliosphere. The solar wind is made up of electrically chargedatomic particles, composed primarily of ionized hydrogen, thatstream outward from the Sun.

Exactly where the heliopause is has been one of the greatunanswered questions in space physics. By studying the radioemissions, scientists now theorize the heliopause exists some 90 to120 astronomical units (AU) from the Sun. (One AU is equal to 150million kilometers (93 million miles), or the distance from theEarth to the Sun.

The Voyagers have also become space-based ultravioletobservatories and their unique location in the universe givesastronomers the best vantage point they have ever had for lookingat celestial objects that emit ultraviolet radiation.

The cameras on the spacecraft have been turned off and theultraviolet instrument is the only experiment on the scan platformthat is still functioning. Voyager scientists expect to continueto receive data from the ultraviolet spectrometers at least untilthe year 2000. At that time, there not be enough electrical power for the heaters to keep the ultraviolet instrument warm enough to operate.

Yet there are several other fields and particle instruments that can continue to send back data as long as the spacecraft stay alive. They include: the cosmic ray subsystem, the low-energycharge particle instrument, the magnetometer, the plasma subsystem,the plasma wave subsystem and the planetary radio astronomy instrument. Barring any catastrophic events, JPL should be able to retrieve this information for least the next 20 and perhaps even the next 30 years.