Both also are unique, because of all the billions and billions of objects shining through the deep black of space, these two were built by human hands.

They are Voyager 1 and Voyager 2, probes sent by NASA on a tour of the outer planets beginning in 1977, now passing 10,000 days of continuous operation. Long since breaching the orbit of Pluto, the twin spacecraft are hurtling on separate trajectories out toward the last reaches of the solar system and into the gap between the stars - going literally where no one has gone before.

"Voyager 1 has entered the final lap on its race to the edge of interstellar space," said Edward Stone, a Voyager project scientist at the California Institute of Technology in Pasadena. Caltech manages NASA's Jet Propulsion Laboratory, which built and continues to operate both probes, these many years after launch.

During the first 12 years of their missions, the Voyagers brought humanity its first close-up look at the gas giants of the solar system, with each discovery leaving ground observers breathless.

In 1979, the Voyagers provided the first detailed images of Jupiter's bands, including a time-lapse movie of both the bands and the planet's famous Great Red Spot in motion during the approach.

Voyager 1 discovered the first active volcanoes on another world, on Jupiter's red moon, Io, and even snapped a photo of one in mid-eruption - something dazzling enough to be featured simultaneously on the covers of National Geographic, Smithsonian, and a host of other publications.

The following year, when the spacecraft reached Saturn, they startled the planetary science community by discovering that the planet's rings were both braided and spoked - and managed by a pair of "shepherd" moons.

Voyager 1 actually plunged through the Cassini Gap in Saturn's rings and headed out of the solar system, while Voyager 2 headed toward a 1986 rendezvous with Uranus. There, it passed by the giant's cloudtops and discovered 10 new moons.

In 1989, Voyager 2 sailed past Neptune, discovering its atmosphere boasted winds traveling at supersonic speeds before heading down and away out of the solar system.

Two years later, Voyager 1 took one of the most amazing photographs in history. From 4 billion miles away, the spacecraft located and captured an image of Earth, looking tiny and precarious against the cosmos - a "Pale Blue Dot," as the late Carl Sagan termed our planet.

Now, some 8.7 billion miles out, Voyager 1 has passed through the last limits of the solar system, a zone astronomers call the termination shock region, where the solar wind - a rapidly moving stream of electrically charged particles - is met and slowed by the pressure of gas that sits between the stars.

For a long time, mission scientists were not sure where the termination shock ended and interstellar space began, because irregularities in the solar wind's speed and intensity affect the termination shock's location.

In December 2004, however, the Voyager 1 instrument package observed sudden increases in the strength of the magnetic field surrounding the spacecraft and in the temperature of the faint surrounding gas. Ever since, the readings have remained high, though fluctuating.

"Voyager's observations over the past few years show the termination shock is far more complicated than anyone thought," said Dr. Eric Christian, discipline scientist for the Sun-Solar System Connection research program at NASA headquarters in Washington.

For the next 15 years, both Voyagers are expected to continue to probe the unexplored reaches of interstellar space, and mission scientists will continue to receive signals from both spacecraft - assuming their funding holds out.

By then, they will have collected nearly half a century of data. Not bad for spacecraft with onboard computers boasting about 10k bytes each of memory.

Carl Sagan, who helped to design Voyager - and maybe even James T. Kirk - would be proud.

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