Spacecraft Voyagers 1 and 2 have outlived their "warranty" by at least five years and have traveled nearly 4 billion miles since they left Earth 10 years ago this week.

When the first of the robot spacecraft was launched on Aug. 20, 1977, they were scheduled to fly by only Jupiter and Saturn. They were so successful and durable that scientists at the Jet Propulsion Laboratory in California sent them on to overtime duty.

At Jupiter, they revealed active volcanoes on its moon, Io; sent back detailed photos of the planet's atmosphere and its Great Red Spot, and detected lightning bolts in Jupiter's upper atmosphere and a faint ring system encircling it.

At Saturn, they revealed remarkable details of its famous rings, and at Uranus, Voyager 1 returned stunning photos of the planet's five large moons. One, Miranda, exhibits the strangest geography of any body investigated in the solar system, including an ice cliff twice the size of Mount Everest.

The Voyagers discovered 19 previously unknown moons as they took their "grand tour."

Voyager 2 is hurtling toward an Aug. 24, 1989, rendezvous with Neptune and its moon, Triton, after which the pair will continue outward in search of the heliopause -- the outer boundary of the sun's influence.

"The only game in town for this decade is the Voyager mission," said mission planner Charles Kohlhase. Scientists hope to track the Voyagers for 30 more years, to study the gas among the stars.