Thirty-five years ago, Voyager 1 and Voyager 2 began touring the solar system, with Voyager 1 now preparing to exit the bubble and continue its epic journey through interstellar space.
When it becomes the first spacecraft to depart the solar system, it might not command the attention that men on the moon did in 1969, but it will rank high on the list of Space Age achievements.
Then, in about 40,000 years, Voyager 1 will make a flyby of Gliese 445, a star 17.6 light-years from the sun in the constellation of Camelopardalis and in the sky near Polaris, the North Star.
The missions that already have produced astrophysical surprises explain the solar system's structure and the complex interplay of solar winds, cosmic rays and electromagnetic fields. There's hope it will do the same in explaining the makeup of interstellar space.
Voyager 1 continues sending data from 11.3 billion miles away, with each signal traveling 17 hours at light speed to reach Earth. NASA's Jet Propulsion Laboratory, operated by the California Institute of Technology, is analyzing current data detailing the outer layers of the solar system.
But the current focus remains: How close is Voyager 1 to the solar system's edge or has it already burst through?
"We are pressing hard on the analysis of the unprecedented signs Voyager 1 has found in the far reaches of our solar system," said Edward C. Stone, the project scientist based at CalTech. "We're still not sure yet whether Voyager 1 is still inside the bubble that our sun blows around itself or whether it is truly among the stars in interstellar space."
Diane Turnshek, a Carnegie Mellon University astronomer, and Michael Wood-Vasey, a University of Pittsburgh astrophysicist, along with reports from the Jet Propulsion Laboratory, explain Voyager 1's journey, to date, through the solar system.
A description of Voyager 1 and 2's journey requires an understanding of the layers inside the solar system or heliosphere, which is the bubble created by solar winds.
Raindrop shaped as it travels 486,000 mph, the heliosphere is plowing through interstellar space as does a boat through water, creating a wave ahead of it known as the "Bow Shock."
Once Voyager 1, launched Sept. 5, 1977, and Voyager 2, launched Aug. 20, 1977, flew by the last planet, Neptune, and beyond Pluto and other dwarf planets, they passed through the "Termination Shock," a layer where solar-wind intensity diminishes dramatically. That occurred in 2004 for Voyager 1 (which was launched on a faster, shorter trajectory) and 2007 for Voyager 2.
But the Lab's current interest is getting new data explaining the outer reaches inside the bubble.
Both are thought to be in the heliosheath, the layer right before the heliosphere's outer boundary known as the heliopause. There, solar-wind velocities drop nearly to zero as they get canceled out by cosmic rays and gas pressure from interstellar space. Once through the heliopause, which could take years, Voyager 1 will be in the interstellar space of the Milky Way galaxy.
"The interstellar medium is a very different place than our safe, little solar system with its solar winds," Ms. Turnshek said.
There's debate about whether Voyager 1 has exited the solar system.
For two years, the Jet Propulsion Laboratory reports, Voyager has measured increases in cosmic rays, coupled with the slowdown in solar wind velocity, all indicating that it's approaching the heliopause. Dramatic changes in electromagnetic fields represent another clue that it's nearing the edge, where magnetic fields from the solar system and interstellar space interact and even change directions. The lab now is calibrating magnetic-field data to pinpoint whether Voyager 1 is inside or outside the solar system.
"The data are changing in ways that we didn't expect, but Voyager has always surprised us with new discoveries," Mr. Stone, the CalTech project scientist, wrote in a Jet Propulsion Laboratory report.
Ms. Turnshek said Voyager 1 has turned its cameras backward for a different perspective of the solar system. From the edge of the solar system, Earth is but a pale blue dot.
Still "hale and hearty," Voyager 1 and 2 use plutonium 238 as their energy source.
Voyager 1's 23-watt transmitter, which has the electrical power of a refrigerator light bulb, continues sending data to Earth. The Jet Propulsion Laboratory uses radio dishes in California, Spain and Australia, with diameters that would span two football fields, to capture the weakening signals.
But the decline of power from the plutonium's radioactive decay and the ever longer distances from Earth are expected to make Voyager 1's signals undetectable by 2025, Ms. Turnshek said.
Still, the spacecraft will continue traveling toward distant stars.
Each carries a 12-inch, gold-plated copper LP record that contains such natural sounds as whales and dolphins with 115 images portraying the diversity of life and culture on Earth. It also has musical selections from different cultures and eras, ranging from Bach and Beethoven to bagpipes and Chuck Berry's 1958 hit "Johnny B. Goode," along with spoken greetings in 55 languages.
"These are our ambassadors to other worlds," said Ms. Turnshek, adding that the missions continue "paying off on their initial promise. It's like the Energizer bunny after all this time."
Mr. Wood-Vasey, the Pitt astrophysicist, was born the year Voyager 1 and 2 were launched, and he's been noting their progress for much of his life.
"My excitement comes more from being a citizen of the world or country that launched them -- sending probes and pushing the boundaries of what's been unexplored before, which yields new surprises," he said. "We're starting to explore our own backyard."
"I continue to marvel at it," he said. "It's the difference between having an idea and good theories and actually going out there and measuring to check your theories." But "astronomy and big ideas take patience," considering the time it's taken Voyager to reach the solar system's boundary.
"You have to have the willingness to think on decade-long time scales," Mr. Wood-Vasey said. "If you want to have grand visions, grand accomplishments and big ideas, you need to make big, long-term commitments to those ideas."
Eric Fischer, an amateur astronomer from Hampton and member of the Amateur Astronomers Association of Pittsburgh, said his interest in the Voyager missions was sparked by news reports describing the gold-plated recordings on Voyager 1 and 2, along with its instructions to aliens on how to play the records.
"I remember the announced launch on KDKA News about how Voyager will age and be picked up by aliens ... and in other news, milk prices are going up."
He reveled when the spacecrafts passed Jupiter then Saturn, completing their stated missions before passing Uranus and Neptune, using the planets' gravity to sling them deeper into space.
"It's way past its warranty," Mr. Fischer said of Voyager 1. He said he checks the Jet Propulsion Laboratory's website weekly, with progress he described as "extremely exciting."
"We really don't know for sure what's in interstellar space," he said. "It might be nothing, but break through the bubble and start sniffing around. There is no substitute for being there with a sampling tool to grab a piece of space."
Voyager's exit of the solar system represents one of the thresholds in the history of space exploration, perhaps second only to the first moon walk. "Someone said you have to be dead from the neck up not to appreciate this stuff," Mr. Fischer said.
He also provided an earthly perspective on the distances involved.
Consider Pittsburgh to be the sun, with Voyager traveling westward. Voyager would have passed the planets by the time it reached Zanesville, Ohio, 130 miles west of Pittsburgh. On that scale, Voyager 1's current position would be thousands of miles away in outer Mongolia.
"When it was launched," he said, jokingly, "I had hair."
To follow the progress of the Voyagers' missions, go to http://voyager.jpl.nasa.gov/