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NASA's Voyager at 25 - 2002-08-22

Far beyond the edge of the solar system, heading outbound, is the most distant human-made object in the universe, retreating farther all the time. It is the Voyager 1 spacecraft, launched 25 years ago by the U.S. space agency NASA.

It and the companion Voyager 2 launched two weeks earlier just keep going and going, having returned spectacular images of the outer planets and still gathering data well beyond the solar system's confines.

Lights travels at nearly 300,000 kilometers a second, so imagine how distant Voyager 1 is, as its radio signals, which travel at the same speed, take almost 12 hours to get here. It is about 85 times as far from the sun as we are, and Voyager 2 is 68 times as far.

Together, they have earned a prominent place in the history of exploration.

"Voyager had that sense of exploration, of going where no one has gone before," says Voyager scientist Ed Stone, who has worked on the project for 30 years.

He says no one knew that the twin projectiles would last so long beyond their planned four-year journey to study Jupiter and Saturn. Nevertheless, NASA had its hopes, and the craft did not let it down as they continued flying by Neptune and Uranus and then departed our solar system, still transmitting data.

"These were the most sophisticated spacecraft that had ever been launched deep into space, computer-controlled spacecraft," Mr. Stone says. "Because they were so smart, it took a lot of time to learn how to fly these spacecraft safely. Fortunately, we managed to do that and had very successful Jupiter encounters in 1979."

The Voyagers have sent back much surprising information, revealing how active the outer planets are. Astronomers learned that Jupiter's moon Io has working volcanoes, its atmosphere is stormy, Saturn's rings have kinks, Neptune has the fastest winds of any planet, Neptune's moon Triton spews geysers, and the Uranus moon Miranda has a mixture of old and new surface.

Since the heady days of those discoveries, NASA has reduced its Voyager team from more than 300 to about 12 scientists. These days, project manager Ed Massey and his colleagues are assessing data gathered from more than 10 billion kilometers past the sun, where solar particles still reach.

"We're in contact with the Voyager spacecraft about eight to 10 hours per day," he says. "We are receiving from the spacecraft both health and status data and science data, primarily plasma waves and particles information."

The hope, says Mr. Massey, is that at least one of the two spacecraft will endure long enough to become the first to taste interstellar space.

"The Voyager spacecraft can last until about 2020. Their primary limitation is electrical power," he says. "There is no problem with receiving the signals. Hopefully by the time we run out of power we will have crossed into the area where true interstellar winds exist."

Because of the enormous distances between Earth and the Voyagers, controllers worry that if anything goes wrong on board, it would take a full day to correct it, with radio signals requiring 12 hours each way. So Mr. Massey says ground controllers deal with the issue by operating the spacecraft very carefully, relying on their on-board computers to deal with emergencies.

"The first thing we do is make sure we don't create any problems by being careful in what we send to the spacecraft," he said. "The second thing we do is to have already loaded on board the spacecraft a set of commands that would take care of the emergencies that we can anticipate."

After Voyagers 1 and 2 stop working within the next generation or so, they are expected to proceed ever onward into the Milky Way galaxy. Mounted on their sides are recordings on gold of human sights and sounds. They include music, the sound of a kiss, greetings in several languages, and salutations from then U.N. Secretary-General Kurt Waldheim and U.S. President Jimmy Carter.

The idea was that if a distant civilization intercepts the Voyagers, it would learn something of the species that dispatched them. Therefore, long after the craft end their scientific duty, they might just perform an intergalactic diplomatic task.