Over the past year, answering the "are we there yet" question has been filled with nos, yeses, and maybes – in no small part because Voyager 1 is exploring, for the first time, a region 11 billion miles away.
But after a year of sorting through evidence Voyager has been gathering on its environment, mission scientists announced Thursday that the craft crossed a poorly understood boundary between sun's sphere of influence, or heliosphere, and interstellar space sometime in August 2012.
"We made it," enthused the mission's project scientist, Ed Stone, during a NASA briefing. "We're out in front of our own bubble."
"It's a remarkable achievement. It's very hard for us to comprehend that the space age started 55 years ago, and in that period we've actually exited the solar system. We're in a truly alien environment," added Gary Zank, director of the Center for Space Plasma and Aeronomic Research at the University of Alabama in Huntsville and a member of the Voyager science team.
NASA launched Voyagers 1 and 2 in 1977 to take a grand tour of the solar system's outer planets, which ended in 1989. The two craft were then tasked with traveling to the edge of the heliosphere and beyond to give scientists a better understanding of the sun's interstellar environment and of the structure and processes within the heliosphere at its outermost reaches.
Voyager 1 is about 11.6 billion miles from Earth, hurtling toward a star known as AC+799 3888. It's 17.6 light-years away in the constellation Camelopardalis, noted Suzanne Dodd, the project's program manager. Voyager 1 is expected to flit past the star at a distance of about 1 light-year in roughly 40,000 years and then begin its own orbit the Milky Way.
Determining Voyager 1's status has required some creative uses of the few instruments that remain active, plus a little help from the sun. The one instrument that would have quickly determined whether this cosmic Elvis had left the building failed in 1980.
One piece of evidence came in the form of cosmic rays the spacecraft measured.
In May, a team led by Bill Webber, a Voyager veteran and professor emeritus at New Mexico State University in Las Cruces, published a study noting a "sudden and decisive change" in the relative abundance of two types of cosmic rays.
One type, formed throughout the galaxy largely by exploding stars, jumped markedly. A second type, formed within the heliosphere from neutral atoms that leak in from interstellar space, fell markedly. This dramatic shift occurred Aug. 25, 2012.
But the clincher has come from indirect measurements of the hot ionized gas, or plasma, that Voyager has encountered since April 2012.
A research group led by Donald Gurnett, a space-plasma physicist at the University of Iowa in Iowa City and member of the Voyager 1 and 2 science teams, reports that Voyager has entered a region of space where the density of plasma is significantly higher than the plasma it encountered as it traveled through the solar system and into the sun's boundary region.
The measured density is remarkably close to the density models have predicted for the cooler plasma in the interstellar medium – the dust and gas found between stars.
To make the measurement, the team used an instrument that picks up radio waves plasmas emit when they are disturbed. The disturbance came in the form of a blast from the sun known as a coronal-mass ejection. These typically send billions of tons of plasma hurtling into space at speeds of a million miles an hour or more. Even at Voyager's distance, solar storms delivered enough material to trigger waves in the plasma surrounding the spacecraft as the material approached. The team used the radio signals from the waves – one set from last fall and from this spring – to calculate the changes in plasma density.
The results are set to appear in Friday's issue of the journal Science.
But as the researchers lacked what they felt would be crucial confirmation of Voyager's break-out: Theory suggested that the craft should detect a roughly 30-degree shift in the orientation of magnetic fields as the magnetic field carried by the sun's plasma yielded to the galaxy's magnetic field.
This led the mission's lead researchers to publish three papers in Science in July that collectively put Voyager in a previously unknown region they dubbed the depletion region, but still within the heliosphere. In effect, they said, we're not there yet.
But that change in magnetic-field orientation hasn't been detected yet because, in all likelihood, there isn't one, at least one large enough to notice, according to a team led by Marc Swisdak, a researcher at the University of Maryland at College Park.
Modeling work Dr. Swisdak and colleagues performed suggested that any change in the orientation magnetic fields will be small, not the easy-to-spot east-west to north-south change researchers had envisioned up to now.
And the model indicated that the fields at the absolute boundary, known as the heliopause, are layered and porous, features that were consistent with the data Voyager 1 was beaming back to Earth. Based on its modeling work the team concluded that Voyager 1 cleared the heliosphere July 28, 2012.
A formal report of the team's effort appeared in Astrophysical Journal Letters on Sept. 1 and was posted online Aug. 15.
While that leaves wiggle room in a precise departure date, the measurements and the modeling study point to a mid-2012 departure for humanity's time capsule to the cosmos.
Voyagers 1 and 2 carry recordings that literally have gone gold. Recorded on gold-plated copper disks are photos and sounds from Earth, in addition to music, outlines of male and female humans, and a schematic of where our solar system is located with respect to the rest of the galaxy.
Mission officials say the craft can still gather science data for another 13 or 14 years before its power sources run too low to keep even a single instrument running. NASA can track the craft for an additional 10 years if they have it continue to feed engineering data back to Earth.
After that, Voyager is on its own.