NASA's Voyager 2 spacecraft marks 25 years of discovery

Twenty-five years ago this week, NASA's Voyager 2 probe began its epic, ongoing space odyssey, a voyage of discovery that many consider humanity's greatest feat of pure space exploration.

Zooming past Jupiter, Saturn, Uranus and Neptune, Voyager 2 beamed back a treasure trove of stunning photographs and other priceless data, providing spectacular close-up views of the solar system's four gas giants, their intricate rings and their myriad moons.

The hardy spacecraft is now 6.31 billion miles from the sun, well beyond the current 2.8-billion-mile distance of Pluto, heading toward interstellar space at more than 35,000 mph.

It is so far away it takes radio signals, moving 186,000 miles every second, more than nine hours to cover the vast gulf between Earth and spacecraft. To put that in scale, if Earth were the size of a grain of sand, Voyager 2 would be more than 400 feet away.

Despite 25 years in the harsh environment of space, the spacecraft remains in relatively good health, radioing back a steady stream of data as it searches for the edge of the solar system, the realm where the sun's influence finally gives way to interstellar space.

Scientists say it could take anywhere from seven to 21 more years to reach that unseen boundary. They are hopeful Voyager 2 will survive long enough to mark the transition to interstellar space, chalking up yet another major discovery.

Goes silent in 2020

It isn't a sure thing. Sometime around 2020, when the electrical output of its nuclear generators drops below the threshold needed to power its science instruments, Voyager 2 will no longer be able to phone home. After that, it will sail silently into the depths of space and time on an endless voyage.

On the remote chance aliens might stumble on it eons from now, a gold record called "Sounds of Earth" is mounted on the side of the spacecraft. It's a sampler of human sights and sounds that includes a mother's lullaby, the sound of a kiss and Chuck Berry's rock classic "Johnny B. Goode."