It’s officially official: After much backing-and-forthing in the scientific community, NASA has reviewed a recent study and agreed with the researchers that Voyager has indeed left the solar system.
In fact, it did so more than a year ago, NASA said in a media release, citing “new and unexpected data” that indicate the probe, launched 36 years ago in 1977, has been cutting through the ionized gas we call plasma of the type that exists “in the space between stars” and is now officially outside the solar bubble. NASA’s Jet Propulsion Laboratory said the probe is in a “transitional region” outside that bubble, based on a report in the journal Science on September 12. It exited in August 2012.
"We have been cautious because we're dealing with one of the most important milestones in the history of exploration," said Voyager Project Scientist Ed Stone of the California Institute of Technology in Pasadena, in a statement from NASA issued on Thursday September 12. "Only now do we have the data—and the analysis—we needed."
It was a massive eruption from the sun that helped scientists figure out that Voyager had indeed crossed the boundary of the heliosphere, the bubble of the sun’s influence that surrounds the solar system. It was an eruption from the sun itself that gave observers the plasma data they needed, Stone said.
The heliosphere, NASA said, becomes inflated when the sun erupts and sends plasma flying to the edges of the solar system. The region beyond, interstellar space, contains the material that has been ejected by exploding stars from millions of years ago. Lacking a plasma detector, the team had to wait for a signature. That came after a sun eruption sent plasma streaming toward the solar system’s edge. It hit the probe a year later, in spring 2013, causing it to “echo like a bell,” as the Associated Press put it. That gave them the data they needed.
There have been a few false alarms on this before, but this appears to be the definitive ruling. In June, NASA suggested that Voyager was still pushing at the furthest boundaries of the heliosphere, refuting the conclusions reached by scientists at the American Geophysical Union the previous March declaring that Voyager had already left.
Then in August another group of scientists published a study in The Astrophysical Journal Letters suggesting that magnetic lines did not need to change direction for the spacecraft to have exited.
Since the latest data that NASA used involved plasma rather than magnetic fields, and that still has some scientists skeptical. But their numbers are dwindling.
“It’s premature to judge,” said Lennard Fisk, a space science professor at the University of Michigan and former NASA associate administrator, to the Associated Press. “Can we wait a little while longer? Maybe this picture will clear up the farther we go.”
But NASA’s scientists were ready enough to declare victory that they held a press conference on September 12 accompanied by the theme from Star Trek.
"Now that we have new, key data, we believe this is mankind's historic leap into interstellar space," said Stone. “The Voyager team needed time to analyze those observations and make sense of them. But we can now answer the question we've all been asking—'Are we there yet?' Yes, we are."
What’s next? Interstellar space, literally where no man—or ’bot—has gone before.
Here, a short history of the 36-year mission (to date) of Voyager 1, from NASA’s Jet Propulsion Lab scientists.