PASADENA, Calif. -- Thirty-five years after leaving Earth, Voyager 1 is reaching for the stars.

Sooner or later, the workhorse spacecraft will bid adieu to the solar system and enter a new realm of space – the first time a manmade object will have escaped to the other side.

Perhaps no one on Earth will relish the moment more than 76-year-old Ed Stone, who has toiled on the project from the start.

"We're anxious to get outside and find what's out there," he said.

When NASA's Voyager 1 and Voyager 2 first rocketed out of Earth's grip in 1977, no one knew how long they would live. Now, they are the longest-operating spacecraft in history and the most distant, at billions of miles from Earth but in different directions.

Wednesday marks the 35th anniversary of Voyager 1's launch to Jupiter and Saturn. It is now flitting around the fringes of the solar system, which is enveloped in a giant plasma bubble. This hot and turbulent area is created by a stream of charged particles from the sun.

Outside the bubble is a new frontier in the Milky Way – the space between stars. Once it plows through, scientists expect a calmer environment by comparison.

When that would happen is anyone's guess. Voyager 1 is in uncharted celestial territory. One thing is clear: The boundary that separates the solar system and interstellar space is near, but it could take days, months or years to cross that milestone.

Voyager 1 is currently more than 11 billion miles from the sun. Twin Voyager 2, which celebrated its launch anniversary two weeks ago, trails behind at 9 billion miles from the sun.

They're still ticking despite being relics of the early Space Age.

Each only has 68 kilobytes of computer memory. To put that in perspective, the smallest iPod – an 8-gigabyte iPod Nano – is 100,000 times more powerful. Each also has an eight-track tape recorder. Today's spacecraft use digital memory.

The Voyagers' original goal was to tour Jupiter and Saturn, and they sent back postcards of Jupiter's big red spot and Saturn's glittery rings. They also beamed home a torrent of discoveries: erupting volcanoes on the Jupiter moon Io; hints of an ocean below the icy surface of Europa, another Jupiter moon; signs of methane rain on the Saturn moon Titan.

Voyager 2 then journeyed to Uranus and Neptune. It remains the only spacecraft to fly by these two outer planets. Voyager 1 used Saturn as a gravitational slingshot to catapult itself toward the edge of the solar system.

"Time after time, Voyager revealed unexpected – kind of counterintuitive – results, which means we have a lot to learn," said Stone, Voyager's chief scientist and a professor of physics at the California Institute of Technology.

These days, a handful of engineers diligently listen for the Voyagers from a satellite campus not far from the NASA Jet Propulsion Laboratory, which built the spacecraft.

The control room, with its cubicles and carpeting, could be mistaken for an insurance office if not for a blue sign overhead that reads "Mission Controller" and a warning on a computer: "Voyager mission critical hardware. Please do not touch!"

There are no full-time scientists left on the mission, but 20 part-timers analyze the data streamed back. Since the spacecraft are so far out, it takes 17 hours for a radio signal from Voyager 1 to travel to Earth. For Voyager 2, it takes about 13 hours.

Cameras aboard the Voyagers were turned off long ago. The nuclear-powered spacecraft, about the size of a subcompact car, still have five instruments to study magnetic fields, cosmic rays and charged particles from the sun known as solar wind. They also carry gold-plated discs containing multilingual greetings, music and pictures – in the off chance that intelligent species come across them.

Since 2004, Voyager 1 has been exploring a region in the bubble at the solar system's edge where the solar wind dramatically slows and heats up. Over the last several months, scientists have seen changes that suggest Voyager 1 is on the verge of crossing over.

When it does, it will be the first spacecraft to explore between the stars. Space observatories such as the Hubble and Spitzer space telescopes have long peered past the solar system, but they tend to focus on far-away galaxies.

As ambitious as the Voyager mission is, it was scaled down from a plan to send a quartet of spacecraft to Jupiter, Uranus, Neptune and Pluto in what was billed as the "grand tour" of the solar system. But the plan was nixed, and scientists settled for the Voyager mission.

American University space policy expert Howard McCurdy said it turned out to be a boon.

They "took the funds and built spacecraft robust enough to visit all four gas giants and keep communicating" beyond the solar system, McCurdy said.

The double missions so far have cost $983 million in 1977 dollars, which translates to $3.7 billion now. The spacecraft have enough fuel to last until around 2020.

By that time, scientists hope Voyager will already be floating between the stars.

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GALLERY: PHOTOS OF SATURN AND ITS MOONS FROM THE CASSINI SPACECRAFT (All images and captions courtesy NASA/JPL-Caltech/Space Science Institute, unless otherwise noted.)
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  • A crescent Enceladus appears with Saturn's rings in this Cassini spacecraft view of the moon.

  • With giant Saturn hanging in the blackness and sheltering Cassini from the sun's blinding glare, the spacecraft viewed the rings as never before, revealing previously unknown faint rings and even glimpsing its home world.

  • This is an artist's concept of the Saturnian plasma sheet based on data from Cassini magnetospheric imaging instrument. It shows Saturn's embedded 'ring current,' an invisible ring of energetic ions trapped in the planet's magnetic field. Saturn is at the center, with the red 'donut' representing the distribution of dense neutral gas outside Saturn's icy rings. Beyond this region, energetic ions populate the plasma sheet to the dayside magnetopause filling the faintly sketched magnetic flux tubes to higher latitudes and contributing to the ring current. The plasma sheet thins gradually toward the nightside. Image Credit: NASA/JPL/JHUAPL

  • A quartet of Saturn's moons, from tiny to huge, surround and are embedded within the planet's rings in this Cassini composition.

  • This stunning false-color view of Saturn's moon Hyperion reveals crisp details across the strange, tumbling moon's surface. Differences in color could represent differences in the composition of surface materials. The view was obtained during Cassini's very close flyby on Sept. 26, 2005. Hyperion has a notably reddish tint when viewed in natural color. The red color was toned down in this false-color view, and the other hues were enhanced, in order to make more subtle color variations across Hyperion's surface more apparent.

  • The colorful globe of Saturn's largest moon, Titan, passes in front of the planet and its rings in this true color snapshot from NASA's Cassini spacecraft.

  • Vertical structures, among the tallest seen in Saturn's main rings, rise abruptly from the edge of Saturn's B ring to cast long shadows on the ring in this image taken by NASA's Cassini spacecraft two weeks before the planet's August 2009 equinox. Part of the Cassini Division, between the B and the A rings, appears at the top of the image, showing ringlets in the inner division.

  • Flying past Saturn's moon Dione, Cassini captured this view which includes two smaller moons, Epimetheus and Prometheus, near the planet's rings.

  • Data from NASA's Cassini spacecraft show that the sizes and patterns of dunes on Saturn's moon Titan vary as a function of altitude and latitude. Image credit: NASA/JPL-Caltech, and NASA/GSFC/METI/ERSDAC/JAROS and U.S./Japan ASTER Science Team

  • Saturn's small, potato-shaped moon Prometheus appears embedded within the planet's rings near the center of this Cassini spacecraft view while the larger moon Mimas orbits beyond the rings.

  • The line of Saturn's rings disrupts the Cassini spacecraft's view of the moons Tethys and Titan.

  • Although traveling at great speed, the Cassini spacecraft managed to capture this close view of Saturn's small moon Helene during a flyby on March 3, 2010. Saturn's atmosphere makes up the background of this composition.

  • The Cassini spacecraft looks at a brightly illuminated Enceladus and examines the surface of the leading hemisphere of this Saturnian moon.

  • Saturn's third-largest moon Dione can be seen through the haze of its largest moon, Titan, in this view of the two posing before the planet and its rings from NASA's Cassini spacecraft.

  • Saturn's moon Mimas peeks out from behind the night side of the larger moon Dione in this Cassini image captured during the spacecraft's Dec. 12, 2011, flyby of Dione.

  • A quintet of Saturn's moons come together in the Cassini spacecraft's field of view for this portrait.

  • The best view of Saturn's rings in the ultraviolet indicates there is more ice toward the outer part of the rings, than in the inner part, hinting at the origins of the rings and their evolution.

  • NASA's Cassini spacecraft obtained this unprocessed image on Dec. 12, 2011.

  • Recent Cassini images of Saturn's moon Enceladus backlit by the sun show the fountain-like sources of the fine spray of material that towers over the south polar region. The image was taken looking more or less broadside at the 'tiger stripe' fractures observed in earlier Enceladus images. It shows discrete plumes of a variety of apparent sizes above the limb of the moon. The greatly enhanced and colorized image shows the enormous extent of the fainter, larger-scale component of the plume.

  • NASA's Cassini spacecraft obtained this unprocessed image on Dec. 12, 2011.

  • Saturn sits nested in its rings of ice as Cassini once again plunges toward the graceful giant. This natural color mosaic was acquired by the Cassini spacecraft as it soared 39 degrees above the unilluminated side of the rings.