Where does the solar system end? Voyager isn't officially there yet

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By Alan Boyle, Science Editor

After almost a year of debate, NASA has accepted the idea that its Voyager 1 probe has left behind the last wisps of the solar wind and is flying through interstellar space. But has it left the solar system? Um, not exactly.

It depends on what you mean by "solar system": The findings reported Thursday in the journal Science have convinced even the skeptics that the car-sized spacecraft has gone beyond the heliosphere, the huge bubble of electrically charged particles emanating from the sun. But the edge of the heliosphere is not the same as the edge of the solar system.

One of the skeptics, Jonathan McDowell of the Harvard-Smithsonian Center for Astrophysics, said he listened closely to what the Voyager team was saying about the latest findings. "The thing I was really pushing against was the idea that this has 'left the solar system,' which they were careful not to say," he told NBC News. "They were saying that it's left the heliosphere."

Voyager still has to face the solar system's biggest frontier: a huge haze of comets known as the Oort Cloud. The science team at NASA's Jet Propulsion Laboratory estimates that it could be another 200 to 300 years before Voyager enters the Oort Cloud, and another 30,000 years before it comes out the other side. The Oort Cloud may extend outward for 100,000 astronomical units (where 1 AU equals the average Earth-sun distance). That's more than a third of the way to the next star over.

"It's a very fine point, and many people don't realize the Oort Cloud is in interstellar space and it's considered part of the solar system," Veronica McGregor, JPL's news chief, wrote during a Reddit AMA chat session. "We knew many media would make the error, and we tried to make it clear in interviews. ... None of our materials say we've exited the solar system."

So what has changed for Voyager, and what's the big deal?

Outside the solar bubble
Until a little more than a year ago, scientists were in agreement that Earth's most distant spacecraft was traveling through a far-out region where the electrically charged particles continued to sweep outward. Then, on Aug. 25, 2012, Voyager detected a dramatic change: The intensity of the particles coming from the sun dropped off, and the intensity of particles coming in from outside the solar bubble increased.

That led some scientists to declare this April that Voyager was "outside the normal heliosphere." JPL's Ed Stone, chief scientist for the 36-year-long Voyager mission, agreed that the craft was in a transitional zone, but held off from going any further. "It is the consensus of the Voyager science team that Voyager 1 has not yet left the solar system or reached interstellar space," he said at the time.

Voyager's team members stuck to that view in June, and again in August, even though other scientists insisted that the probe had crossed into the interstellar medium. What's different this time is that yet another team of astronomers, led by the University of Iowa's Don Gurnett, made some clever measurements of plasma effects on the spacecraft.

Breaking through
The astronomers tracked a particularly strong storm of charged particles that blasted out from the sun last year and set the plasma around Voyager oscillating in late April and May. The oscillations from the spacecraft's plasma wave instrument were analyzed, leading the team to conclude that the density of electrons surrounding Voyager had risen from the wimpy level associated with the outer edges of the heliosphere to a level about 40 times denser. That's in line with what would be expected if Voyager had broken through into interstellar space.

Gurnett and his colleagues used other measurements to trace Voyager's readings backward, and concluded that the density must have changed in August 2012 — just about the time that the intensity readings changed as well. That would put the edge of the heliosphere at about 11.3 billion miles (18.2 billion kilometers, or almost 122 AU).

Stone was finally convinced: "Now that we have new, key data, we believe this is mankind's historic leap into interstellar space," he said in a NASA news release. "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." 

Some still disagree. Two University of Michigan astrophysicists, George Gloecker and Lennard Fisk, argue that the density measurements could be explained by phenomena occurring inside the heliosphere. Gloecker told Science News that better readings from Voyager 2, a sister probe that isn't yet as far out as Voyager 1, could settle the matter in the next two or three years. "Why rush to conclusions now?" he said.

Surprises lie ahead
Even Tom Krimigis, a member of the Voyager team from Johns Hopkins University, acknowledges that some of the puzzle pieces still don't fit into place: For example, why hasn't there been a change in the orientation of the magnetic field through which Voyager is traveling? And why hasn't the cosmic ray flux evened out in all directions? Both those phenomena were expected to occur when Voyager crossed over into interstellar space, but neither of them has happened yet.

"Initially we thought things were simple," Krimigis told NBC News. "We'd cross the border, and we thought it was going to be flat and boring, and we could all go home. But we found out that things were anything but simple. I can tell you there are still some surprises coming that we haven't published yet."

None of us will be alive when Voyager 1 breaks through the far side of the Oort Cloud — but even though the spacecraft is a long way from the limit of the solar system, Krimigis says Voyager's transition is still well worth celebrating.

"It's like the first Sputnik that went beyond Earth's atmosphere, 56 years ago," he said. "That was a historic moment. This is another historic moment, going from the atmosphere of the sun to the atmosphere of the galaxy. The only difference is the altitude: 600 miles for Sputnik, as opposed to an altitude of 11.3 billion miles."

More about Voyager and the solar system:

In addition to Gurnett, the authors of "In Situ Observations of Interstellar Plasma with Voyager 1" include W.S. Kurth, L.F. Burlaga and N.F. Ness.

Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the NBC News Science Facebook page, following @b0yle on Twitter and adding +Alan Boyle to your Google+ circles. To keep up with NBCNews.com's stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.