They have working memories a million times smaller than your home computer. They record their scientific data on 8-track tape machines. And they communicate with their aging human inventors back home with a 23-watt whisper.
Even so, the twin explorers, now 33 years into their mission, continue to explore new territory as far as 11 billion miles from Earth. And they still make global news. Scientists announced last month that Voyager 1 had outrun the solar wind, the first manmade object to reach the doorstep to interstellar space.
It's amazing even to Stamatios "Tom" Krimigis, of the Johns Hopkins University's Applied Physics Lab near Laurel. He's one of just two principal investigators of the mission's original 11 still on the job 40 years after Voyager was approved by NASA.
"Needless to say, none of us expected it was going to be operating for so long," said Krimigis, now 72. "We were all praying to get to Neptune [in 1989]. But after that? Who thought we could be with this 33 years [after launch]?"
In all that time, only one instrument, on Voyager 1, has broken down. Nine others on the two craft have been powered down to save dwindling electrical power from their plutonium-powered generators.
But five experiments on each Voyager are still funded and seven are still delivering data. Problems do crop up, but fixes can still be made with radioed instructions that take 12 hours to reach the craft.
"I suspect it's going to outlast me," said Krimigis.
Krimigis is the emeritus head of the Space Department at the lab and the only remaining original member of his Voyager instrument team. He spends most of his time on his duties as principal investigator on another Hopkins instrument aboard the Cassini mission now orbiting Saturn.
Last month, Krimigis and his colleagues on Voyager 1's Low-Energy Charged Particle instrument reported their latest findings to the American Geophysical Union, meeting in San Francisco. Voyager 1 had reached a place in June where the outward flow of charged particles from the sun — the solar wind — stops. It's a bit like where a plume of cigarette smoke stops rising and curls into a cloud.
"Everybody is very excited about this," Krimigis said. "Seeing the end of the outflow of the solar wind after being in the Space Age for, I guess, 54 years now, is quite an event … at least for the aficionados."
Norman F. Ness, professor emeritus at the University of Delaware, has been the principal investigator since 1970 on the Voyager magnetometers. Edward C. Stone, at CalTech, has been project scientist for the Voyager mission since its inception. They're the last of the original leaders.
Ness was 36 when he joined the mission, he said. He was a geophysicist with experience in oil exploration, seismology and magnetic fields. "But space was much more exciting," he said, and NASA was attracting many young scientists.
"Most of the people working in the space business at that time were quite young, because rocketry itself was a young technology. Very few people had any experience in it."
"Voyager was the pinnacle of his career, said Ness, now 77. "There is never going to be a mission in anybody's lifetime, now living, that is ever going to get these observations in hand. So it's once in a lifetime."
The Voyager missions were conceived in the late 1960s. Astronomers realized that the outer planets — Jupiter, Saturn, Uranus and Neptune — were moving into a once-in-175-years' alignment. A spacecraft from Earth would be able to fly efficiently from one to the next, using energy boosts from each planet's gravity along the way. They dubbed it the "Outer Planets Grand Tour," but when NASA ordered two, just in case, they became Voyager 1 and 2.
Krimigis and his colleagues at lab, the University of Maryland, and the University of California San Diego — all in their late 20s and early 30s then — submitted a proposal for instruments to measure the flow of charged particles along the way. NASA accepted it.
Krimigis' challenge was to build and operate devices to measure and report back on the speed and direction of the ions, electrons and other charged particles, most from the solar wind, as they swirled around the planets and raced across interplanetary space.
There was one big hitch. To get a full understanding of the flow of charged-particle environment around the spacecraft, their instruments needed a 360-degree view, sensing particles that were overtaking Voyager from behind, as well as those striking it from the sides and in front.
But Voyager needed to keep its antenna pointed at the Earth at all times, so the spacecraft itself couldn't turn. That meant Krimigis' instrument needed an electric motor and a swivel mechanism that could swing back and forth for more than a decade without seizing up in the cold vacuum of space.
"They said I was crazy," Krimigis said. To spacecraft designers, moving parts spell trouble.
Krimigis argued there was less mission risk in moving one component, than in turning the whole spacecraft. The solution was offered by a California company called Schaeffer Magnetics. Krimigis' team tested the contractor's four-pound motor, ball bearings and dry lubricant.
"We ran it through about a half a million steps [movements], enough to take us to Saturn and then some. And it didn't fail," Krimigis recalled.
It's still working, more than 5 million "steps" later. "I bet that motor is probably the longest, continuously operating device in space, ever," he said.
The two Voyager spacecraft passed Jupiter in 1979; Saturn in 1980 and 1981. Voyager 2, on a different course, passed Uranus in 1986 and Neptune in 1989.
More than 20 years later, they are still seeking the edge of the solar system. Decades of tracking-antenna design improvements have enabled engineers to find the Voyagers' feeble signals at distances far beyond what was possible in the 1970s.
Voyager 1 is nearest to the edge of the "heliosphere," where the million mph flow of charged particles from the sun — the solar wind — meets interstellar space.
Last June, Krimigis' team noticed that solar particles had stopped striking from behind, and started to hit their Voyager 1 instrument from the front. And they were striking at the same speed — 17 kilometers per second — that Voyager 1 was moving away from the sun, like bugs striking the windshield of a moving car.
It suggested the solar particles' speed outward from the sun had fallen to zero. Scientists watched the data for six months and it didn't change. "We're pretty certain this is a steady-state condition," Krimigis said.
Their instrument is still detecting a particle flow perpendicular to Voyager's direction of travel, a mix of solar and interstellar particles. It's not entirely clear what's going on.
"All this stuff should have disappeared, and it's still there," he said. "It's as if we're in some vast region where the solar wind is kind of sloshing around, instead of being in true interstellar space where there is nothing like this."
True interstellar space might remain years and billions of miles away, scientists say.
"Within the next decade we'll be there for sure," said Ness. Voyager will know because all the particle readings from the lab's instrument will drop to zero, or nearly so.
Ness is confident the Voyager craft will make it. "It runs autonomously," he said. "The question is, 'Will NASA be listening?' "
Sometime around 2025, the two craft will fall silent. In 40,000 years, Voyager 1 will sail as Earth's ambassador among the stars of the constellation Camelopardalis — the Giraffe — in the northern sky. Voyager 2 is headed for Sirius, the brightest star in the sky. It should arrive in 296,000 years.