Voyager 2 was launched on Aug. 20, 1977, and its twin Voyager 1 blasted off a few weeks later, on Sept 5. Both spacecraft were tasked mainly with studying Jupiter, Saturn and their moons.
The spacecraft are also carrying so-called "golden records" containing the distilled essences of humanity, such as various musical offerings and greetings to the universe in 55 different languages. The goal is to teach alien civilizations a little about us, should they ever pluck the Voyagers out of the void. (SETI)
In their early years, the Voyagers made a series of important discoveries about the giant planets. For example, the mission detected active volcanoes on Jupiter's moon Io – the first time such features were found beyond Earth. The spacecraft also found evidence of a liquid-water ocean beneath the icy surface of Jupiter's moon Europa.
"Each of these discoveries changed the way we thought of other worlds," Stone said in a statement.
The Voyagers made it past Saturn, with the spacecraft examining Neptune and Uranus as well. And then they just kept on going, zooming toward the edge of the solar system in different directions and different planes.
Voyager 1 is now about 11 billion miles (17.7 billion kilometers) from Earth, while Voyager 2 is about 9 billion miles (14.5 billion km) away, Stone said. Voyager 1 is the most far-flung human-made object in the universe. [NASA's 10 Greatest Science Missions]
While the Voyagers have left the planets well behind, they're not beyond the solar system yet.
They're still within a huge bubble called the heliosphere, which is made of solar plasma and solar magnetic fields. This gigantic structure is about three times wider than the orbit of Pluto, researchers said.
Specifically, the Voyagers are plying the heliosphere's outer shell, a turbulent region called the heliosheath.
"We're smelling, we're touching the ionized matter in the heliosheath," said Merav Opher of Boston University, a Voyager guest investigator.
The Voyagers are helping scientists better understand the mysterious heliosphere. For example, measurements from the spacecraft revealed that the structure is distorted and asymmetric, yanked out of shape by the interstellar magnetic field, researchers said.
And in June 2010, Voyager 1 measured the outward velocity of the solar wind -- the million-mile-per-hour stream of charged particles coming from the sun -- to be zero in its location in the heliosheath. That surprising reading hasn't changed since.
Researchers don’t think the solar wind has stopped out there; they believe it may have just turned a corner. So they've recently started ordering Voyager 1 to do a series of acrobatic maneuvers, to point its instruments in different directions so the craft can pick up and track the puzzling solar breeze.
Breaking free
The heliosheath looks to be about 3 to 4 billion miles (4.8 to 6.4 billion km) thick, and the spacecraft are already well into it. Based on their speed, they should be out in about five years, Stone said.
That time frame is manageable. The Voyagers' radioisotope thermoelectric generators -- which convert the heat emitted by plutonium's radioactive decay into electricity -- can power their instruments until at least 2020. And the spacecraft have enough hydrazine fuel left to perform maneuvers for another 60 years, researchers said.
Of course, there are no signposts marking the start of interstellar space, where the Voyagers will escape the sun's wind and magnetic field only to be buffeted by those of other, far-flung stars. So astronomers will probably have a hard time knowing when the historic moment occurs.
"We are starting to talk about what we expect to see," Stone said. "I suspect, like in the past, we will be surprised, and we may in fact have a debate for a year or two before we finally decide, 'We have crossed the boundary.'"
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