On March 5, 1979, Voyager 1 arrived at Jupiter, followed by Voyager 2 on July 9. Suddenly, NASA's Jet Propulsion Laboratory (JPL) in Pasadena, Calif., was flooded with crystal-clear pictures of Jupiter's turbulent atmosphere and never-before-seen volcanic eruptions on its moon Io.

When the twin spacecraft arrived at Saturn, they matched their previous performances at Jupiter with images of the ringed world's magnificently intricate system and moons, granting Voyager Project Scientist Ed Stone and his colleagues all they had hoped for in a torrent of discoveries.

Voyager 2 would go on to explore Uranus, Neptune and their moons. "Science is about discovering new things about nature, and normally if you discover something once a year you're doing pretty well," Stone says." This was just day after day…of seeing something that no one had seen before."

The Voyagers were the flagships a golden era of planetary exploration in the late 1970s and 1980s. For a generation that came of age after NASA's glory days of Apollo and would come to experience the harsh reality of Challenger's loss in 1986, the Voyager missions represented an exciting path for exploration that was out of reach of manned spaceflight. Even Hollywood picked up on the excitement, making one of the Voyager spacecraft a character in a Star Trek film. And, Voyagers 1 and 2 are still flying, speeding out of the solar system at more than 55,000 kilometers per hour. Both are expected to reach interstellar space by about 2014.

When the twin spacecraft were launched, NASA was taking advantage of a rare alignment of Jupiter, Saturn, Uranus and Neptune that occurs once every 175 years to send probes on a "Grand Tour" of the solar system. The alignment allowed the spacecraft to harness the gravity of each planet and swing from one to the next using relatively minimal amounts of fuel. NASA first demonstrated the technique with its Mariner 10 mission to Venus and Mercury from 1973 to 1975.

Voyager was designed as a four-year mission to Jupiter and Saturn, and the spacecraft were built to last just five years. If the prime mission were successful, however, NASA would then decide whether to continue on to Uranus and Neptune, Stone says.

Voyager was actually a scaled-down version of a much larger plan, first proposed in the mid-1960s, to send four identical spacecraft on an even longer Grand Tour of the outer planets, from Jupiter to Pluto. Two of the Thermoelectric Outer Planets Spacecraft, or TOPS, were to be launched in 1977, with two more following in 1979.

Although NASA submitted a 1973 budget request for this extended Grand Tour in September 1971, then-NASA Administrator James Fletcher learned in December that President Nixon would not support funding for both the space shuttle and the TOPS's Grand Tour. Before 1971 came to a close, Fletcher had agreed to cancel TOPS and replace it with a less expensive pair of spacecraft that would fly only to Jupiter and Saturn.

The Grand Tour was resurrected as Mariner-Jupiter-Saturn 1977, or MJS '77. In March 1977, just months before the launch of both spacecraft, the mission was renamed Voyager. Whereas the Voyager team had hoped for an extended mission to Uranus and Neptune, that was not part of the original flight plan. "Even a four-year journey was considered fairly risky, so rather than committing to a 12-year trip to Neptune, which may not have worked, NASA prudently decided: 'Let's go for a four-year journey to [Jupiter and] Saturn, and then step-wise extend it,'" Stone says.

Voyager 1 completed its prime mission in November 1980 after a flyby of Saturn's moon Titan and behind the planet's rings. The encounter bent the spacecraft's trajectory northward out of the solar system's ecliptic plane and on a course toward interstellar space, forgoing the possibility for continuing on to other outer planets.

If Voyager 1 had failed to fulfill its objectives at Saturn, NASA could have redirected Voyager 2 to complete Voyager 1's mission. Voyager 2 had been launched two weeks before Voyager 1, but was on a longer trajectory designed to take it past Saturn about nine months after Voyager 1 and then on to Uranus and Neptune if Voyager 1 had completed its objectives.

The Voyagers—built on the successes of the Pioneer spacecraft before them and setting the stage for Galileo, Cassini and other interplanetary robotic probes—had a paltry amount of computing power by today's standards. Each spacecraft had three computers with about 8,000 words of memory each, Stone says. That meant the Voyager team had to frequently upload new programs, particularly during encounters when scientists wanted to point cameras at various targets. "The mission was designed to be reprogrammed," Stone says. "What we hadn't anticipated before we launched, because we did one planet at a time, was the reprogramming after Saturn."

On its last planetary encounter, with Neptune on August 25, 1989, Voyager 2 arrived at a spot in space within 100 kilometers of its intended target—after traveling more than seven billion kilometers. The accuracy was equivalent to sinking a golf ball from 3,630 kilometers away. The timing was within a few seconds of what was planned—a critical achievement because the spacecraft's camera and other instruments were programmed to begin work at a specified time, Stone says.

Taking images of the outermost planets required some finesse from the team's programmers. Sunlight was four times dimmer at Uranus and nine times dimmer at Neptune than it was at Saturn; Voyager 2 needed to be reprogrammed so its camera would take longer exposures. But that also meant the probe had to be set to rotate slightly as it flew by what it was photographing. The adjustments allowed the cameras to capture images in dimmer sunlight, but prevent smeared images during long exposures. "We really had to know exactly when and where we had to look, because that's when the maneuver had to be done," Stone says. "It was all timed very precisely. We had to adopt new techniques as we went farther out into the solar system, for these time exposures."

Voyager 2's Grand Tour of the outer solar system revealed a surprising cosmic neighborhood, where moons thought to be frozen and dead were churning with geologic activity, hiding potential subsurface oceans and possibly harboring life. The tour also amazed scientists with the discovery of a tilted magnetic field at Uranus, and, on its flyby of Neptune, with observations of giant Jupiter-like storms on what scientists had thought was a quiescent planet as well as geysers of nitrogen gas and dust on its frozen moon, Triton..

The accumulation of unexpected discoveries humbled the Voyager team, Stone says. "We had a very 'terra-centric' viewpoint before Voyager," he notes. "Our experience with Earth became our benchmark and our expectation, and what Voyager showed time after time was that was too limited a view. We really didn't understand the system, because we thought Earth was sort of typical—and it is not."

Today, the Voyagers are headed into the void. With their cameras shut down and using only essential instruments to ration power from their ever-weakening plutonium batteries, they have reached the solar system's outer boundary, a region called the heliosheath, where solar winds collide with the interstellar medium. Voyager 1, having arced north out of the ecliptic plane first, is farther along—nearly 16.5 billion kilometers from Earth as of July 31. Voyager 2, which left the solar system heading south, is more than 12.8 billion kilometers from home.

A team of 10 full- and part-time flight engineers is in daily contact, although round-trip communication at the speed of light takes about 30 hours for Voyager 1 and 24 hours for Voyager 2. Both spacecraft have enough electrical power and attitude control propellant to continue operating until about 2025. Through Voyager 2's Neptune encounter, the missions cost $865 million. NASA now spends about $5 million annually to maintain both spacecraft, says Ed Massey, Voyager project manager since 1998.

Scientists estimate that in about 40,000 years, each spacecraft will be in the neighborhood of other stars and about two light-years from the sun. Already, their distance gives these probes a unique vantage point—a bird's-eye view of the solar system. And for its last optical hurrah, Voyager 1's camera took one final image from about 6.5 billion kilometers out. The mosaic of 60 frames, snapped on February 14, 1990, captured the sun and six planets. The "Family Portrait," as it became known, showed Earth as a "pale blue dot" floating in a beam of sunlight.

The astronomer Carl Sagan, who had lobbied NASA for years to take the photo, wrote a poetic essay inspired by the image: " Look again at that dot," Sagan wrote. "That's here. That's home. That's us…the only home we've ever known." Onboard the spacecraft are golden records, which contain a trove of information about life on Earth, including images, encyclopedia chapters on human anatomy, and audio recordings of greetings in numerous languages. Championed by Sagan, the time capsule may someday be encountered by alien life.

Massey chuckles when he recalls how he responds to people worried by the prospect of the Voyagers finding ET. "A few of them ask, aren't we telling them where we are? Aren't they going to use that information to attack us?" he says. "My answer is, 'They already know where we are from the I Love Lucy broadcasts.'"