PASADENA, Calif. — A quarter-century after setting out on missions that were to have lasted just four years, NASA's twin Voyager spacecraft have yet to end their travels.

The two probes, launched Aug. 20 and Sept. 5, 1977, are still speeding toward an unexplored region of space where the sun's influence abruptly ends.

"They are showing signs of their old age, but we have no good reason to think they won't last another 20 years or so," said Tim Hogle, the Voyager systems engineer and mission controller at the National Aeronautics and Space Administration's Jet Propulsion Laboratory.

Voyager 1, the most distant of any manmade object, is 7.8 billion miles from Earth; Voyager 2, which was launched first, is 6.3 billion miles away.

Initially, NASA intended the 1-ton robotic probes to visit just Jupiter and Saturn, wrapping up their tour by 1981. Instead, their missions were extended, with Voyager 2 going to Neptune and Uranus. Between 1979 and 1989 the two probes studied 48 moons and four planets — more of the solar system than any other spacecraft.

"Those 10 years probably represent perhaps the greatest mission of discovery in the history of mankind," said Charles Kohlhase, the former Voyager mission design manager.

Passing by Jupiter in 1980, Voyager 1 was swung upward, out of the plane in which the nine planets orbit the sun. Nine years later, Voyager 2 passed by Neptune and was swung downward out of the plane.

Today, the two spacecraft continue to cruise outward from the sun. The heat of decaying plutonium is used to generate about 310 watts of electrical power aboard each probe. Each communicates with Earth using a 23-watt transmitter.

Data takes more than 23 hours to travel between Voyager 1 and Earth and back again; two-way communications with Voyager 2 take just 18.5 hours. About a dozen people keep tabs on the missions, down from more than 300 in the 1980s.

Although the spacecraft left the last of the nine planets behind more than a decade ago, they have another 40,000 years to go before bidding farewell to the last of the bodies that make up our solar system.

The spacecraft will take that long to pass through Oort's cloud, a sphere of cometary nuclei held loosely by the faint gravitational tug of the sun.

Well before then, perhaps in 2003 or 2004, Voyager 1 will reach the "termination shock," or precursor of the boundary that marks the beginning of interstellar space and the end of a region where the sun's gravitational influence holds sway.

On this side of that boundary, called the heliopause, particles from the sun dominate. Beyond, it's the interstellar medium, made up of the particles cast off by dying stars.

Voyager 1, with its twin following close behind, should reach the heliopause within seven to 21 years.

"Outside of that, it's true interstellar space," Hogle said.

The spacecraft each carry a 12-inch, gold-plated copper disk containing sounds and images selected to portray the diversity of life and culture on Earth. They also include printed messages from then-President Carter and U.N. Secretary General Kurt Waldheim.

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