Last week NASA celebrated a quarter-century of exploration by the twin robots Voyagers 1 and 2. These two veteran explorers must now run on a few watts of power trickling out of small nuclear thermal units and are operated by computers that were outclassed more than a decade ago by the simplest household appliance. Nonetheless, these two spacecraft forever altered what we know about our solar system.

Now they are on a mission to explore interstellar space.

A third Voyager was built but never had a chance to explore. Instead, it now greets millions of visitors at the National Air and Space Museum in Washington, D.C., surrounded by murals depicting all of the wonders its siblings revealed.

A human generation after they were launched, both Voyagers are still performing their original mission, even if they do so at a great distance from Earth. Their task is now to head for the heliopause — the region in space where the Sun's influence ends and true interstellar space begins. We're not quite sure where this is. Voyager 1 is likely to reach it first — yet it only has enough power to operate until 2020. The heliopause could be as close as seven years' travel time or as distant as 21. Hopefully it will reach this important milestone and still be able to radio the news home.

Voyager 1 is now the most distant object of human origin and is 7.9 billion miles (12.6 billion km) away — and speeding along at 38,542 mph (17,230 kph) — relative to the sun. Voyager 2 is closer, at a distance of 6.2 billion miles (10 billion km), and is moving (relative to the sun) at 35,161 mph (15,719 kph). Very soon Voyager 1 will have the distinction of being one "light-day" away from Earth. (Illustration courtesy SpaceRef)

Voyager 1 visited Jupiter and then Saturn. In order to get a close look at Saturn's moon Titan, its trajectory was altered. Alas, this also sent it flying out of the solar system. Voyager 2 encountered Jupiter and Saturn, and then went on to visit Uranus and Neptune, gaining more speed via a gravitational assist with each encounter.

Voyagers 1 and 2 were launched outward from Earth at the same time that yet another "Voyager" was set to debut on a movie soundstage. In the film "Star Trek: The Motion Picture," a hastily arranged mission of the newly refitted Enterprise encounters "Voyager 6" — its name panel obscured so as to cause its alien discoverers to call it "V'Ger".

During the course of Voyager 6's travels it fell into a black hole and emerged in another part of the universe where a machine civilization augmented it so as to allow it to carry out its original mission: to learn all that it could and then sent that information back to its creator.

During the V'Ger encounter, Captain Kirk repaired a communications link on V'Ger so as to allow it to complete its mission and communicate with Earth.

Earlier this year, at a distance of 7.8 billion miles (12.5 billion km) NASA activated a backup Sun sensor and star tracker on Voyager 1. They were activated when the primary systems became unreliable. These were backup systems that had not been used in the 25-year lifetime of the mission. As such, they were almost brand new when activated.

These repairs made certain that the spacecraft would continue to be able to send back news of discoveries to its creators for decades to come.

When you count up the number of worlds seen close-up for the first time, and the number of new moons discovered, these two machines surely rank among the most prolific explorers ever constructed. They truly went where no one went before. (Photo courtesy NASA)

P.S. Both Voyagers will one day come close to other stars. To be certain that any alien civilization finding these relics would know from whence they came, each was included with a precursor to the CD: a large golden record of sounds and pictures. Hundreds of languages and cultures were represented — including the voice of a child speaking in English. "Hello from the children of the planet Earth" said a young Nick Sagan, son of astronomer Carl Sagan. Years later, as an adult, Nick Sagan became a staff writer for Star Trek: Voyager at Paramount Pictures.

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