Its data functions get by on only 68 kilobytes of memory, state of the art in 1977. By comparison, the cheaper Apple iPhone comes with 16 gigabytes of memory (a gigabyte is 1 million kilobytes), roughly 240,000 times more powerful.

In 1977, when Voyager was launched, scientists didn't even know how big the solar system was. They suspected the border, the place where the solar magnetic field ends, to lie a few hundred million miles out. As the craft traveled, they reevaluated their theories and began to suspect the border was much, much farther -- between 10 billion and 16 billion miles.

Over the past year, scientists have collected several pieces of evidence to indicate Voyager's passage out of the solar system. But for final proof, they needed to know that the density of plasma surrounding the spacecraft had increased. The instrument that measures plasma density directly failed long ago, so scientists had to wait for the opportunity to measure it another way. That came in April, when bursts of radio waves rattled the plasma, an oscillation picked up by the spacecraft's antenna. The frequency of that vibration indicated a higher-density plasma -- the evidence scientists needed.

At NASA's press conference, the scientists played the realization of Voyager's data as a set of recorded sounds, noisy tones that rise noticeably as the craft enters interstellar space.

At some 12 billion miles from Earth, Voyager 1 has yet to reach the Oort Cloud -- a collection of debris at our solar system's doorstep. By some definitions, the Oort Cloud marks the end of the solar system, so some debate about whether Voyager is or is not outside still exists. But undeniably, it has crossed a boundary, reaching a new milestone in space exploration.

By 2025, its power source is expected to be completely exhausted. It will live out its remaining transmitting years sending lonely signals in an unending trek across interstellar space. Once dead, it will likely reach proximity of a distant star in about 40,000 years, after which it will continue to orbit the center of the galaxy.

The Musical Alien

Partly as a public relations measure, NASA included aboard the Voyager 1 probe a gold-plated record encoded with information for any intelligent aliens who may discover it -- a message in a bottle on the interstellar ocean. The gold-plated disc has printed on it a simple map indicating our position in the universe using 14 known pulsars of varying frequencies as landmarks and diagramed instructions for how to play back the information inscribed in the record, which included a combination of photos, sounds, human greetings in various languages and samples of music.

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