Confused about Voyager 1's 'exit' from the solar system?

The Voyager 1 probe has reached interstellar space and become the first human-made object to leave the solar system, NASA announced with great fanfare Thursday.

But in a quieter voice, through the probe's Twitter account, the U.S. space agency said that the 36-year-old space probe hadn't really made its exit from the solar system, at least not yet. Huh?

In fact, Voyager 1's departure from the solar system has been announced several times over the past year by various scientists, as other scientists insisted — and some continue to insist — that it hasn't yet reached the edge of the solar system.

Why all the confusion and debate? And more importantly, has Voyager really left our solar system?

To clear that up, here are the answers to some questions you might have.

What is Voyager 1?

Voyager 1 is an unmanned space probe launched from Earth on Sept. 5, 1977.

Along with its sister spacecraft, Voyager 2, launched a few weeks earlier, it was designed to fly close by the giant planets of the outer solar system, collecting data and images.

Voyager 1 made its closest approach to Jupiter on March 5, 1979, and its closest approach to Saturn on Nov. 12, 1980.

It has been continuing its journey toward the edge of our solar system ever since, making scientific measurements along the way. Each year, it has been covering a distance roughly 3.6 times the distance between the Earth and the sun.

Where is Voyager 1 right now relative to the sun?

According to NASA, Voyager 1 is about 19 billion kilometres from our sun. That is:

Four times further from the sun than Neptune, the most distant planet in our solar system.
Three times further from the sun than Pluto, which used to be the most distant planet, but has now been demoted to a dwarf planet.
127 times further from the sun than the Earth.

When Voyager launched from Earth on Sept. 5, 1977, it was just 150 million kilometres from the sun.

This artist's concept puts solar system distances in perspective and shows how the solar system and interstellar space overlap. (NASA/JPL-Caltech )

What exactly did NASA announce this week about its journey?

NASA announced in a news release and at a news conference on Sept. 12 that Voyager 1 was "officially" the first human-made object to reach interstellar space, and that the spacecraft had crossed the boundary into interstellar space in August 2012.

The announcement accompanied the publication of a paper in the journal Science providing details of the data backing up the announcement. The lead author of the paper was University of Iowa scientist Don Gurnett and it was co-authored by scientists at the NASA/Goddard Space Flight Centre and the Catholic University of America.

The paper said Voyager 1 data provided "strong evidence" that the space probe had crossed the heliopause. The heliopause is the boundary where the sun's solar wind — the stream of charged particles emitted by the sun — is no longer strong enough to push back the "interstellar medium" that makes up interstellar space. The researchers wrote that the plasma wave measurements made by Voyager during that were consistent with what they would expect in interstellar space. According to NASA, the heliopause is often considered to be the outer border of the solar system.

However, in its news release, NASA did not say that the Voyager 1 had left the solar system.

In fact, NASA's @NASAVoyager Twitter account tweeted: "I'm in #interstellar space, but I haven't left the solar system. The regions overlap."

Later, though, NASA Science News posted an article with the headline "Voyager 1 has left the solar system."

Why does NASA seem confused about whether Voyager 1 has left the solar system?

NASA says that since the 1960s, most scientists have considered our solar system to extend well beyond the heliopause to the edge of the Oort Cloud, where the sun's gravity loses influence relative to the gravity of other stars. (Meanwhile, interstellar space begins at the edge of the heliopause). NASA estimates it will take 300 years for Voyager to reach the inner edge of the Oort Cloud and possibly 30,000 years to fly beyond the Oort Cloud.

The space agency acknowledges that "informally, of course" solar system means the "planetary neighbourhood around our sun." But it says the two different definitions make the term "solar system" ambiguous.

I feel like I've heard the 'Voyager has now left the solar system' announcement before, possibly several times. Is it just me?

No, but this is the first time NASA, the space agency that launched and runs the Voyager probe, has essentially agreed that Voyager 1 has left the solar system.

Previously:

In June 2012, NASA implied that Voyager might be exiting the solar system. But in July, it said the evidence it had detected had been a false alarm.

On March 20, 2013, a paper published in the journal Geophysical Research Letters by Bill Weber, professor emeritus of astronomy at New Mexico State University, announced that Voyager had exited the heliosphere in August 2012 — the same date that NASA now agrees was Voyager's exit date — and travelled beyond the influence of the sun, based on cosmic ray data from the space probe.

That prompted a rebuttal from NASA, which issued a statement from Voyager project scientist Edward Stone saying, "It is the consensus of the Voyager science team that Voyager 1 has not yet left the solar system or reached interstellar space." He added that NASA scientists expected a change in the magnetic field when the spacecraft crossed into interstellar space, and in the meantime they thought Voyager 1 was in a previously unknown region called a "magnetic highway" between the heliosphere and interstellar space.

On Aug. 16, 2013, a team led by University of Maryland physicist Marc Swisdak, published a paper in Astrophysical Journal Letters containing a scientific model that suggested scientists are wrong to expect a change in the magnetic field direction at the edge of the solar system. He and his colleagues said they therefore agreed that Voyager had left the solar system in the summer of 2012. NASA issued a statement saying that the Swisdak's scientific model was "new and different from other models used so far" and that the other models suggest Voyager 1 was still inside the heliosphere.

Is Voyager 1's exit from the solar system still controversial?

Yes. While Stone and his team say they are now convinced by the plasma wave measurements, other scientists say they want to see more evidence before reaching a conclusion.

Lennard Fisk, a space science professor at the University of Michigan and former NASA associate administrator told The Associated Press that he is still bothered by the lack of a change in the direction of the magnetic field. Harvard University astrophysicist Jonathan McDowell told AP he is "not going to believe it for another year or two."

This is a controversial topic because Voyager 1 has been sending back data that Stone calls "unexpected" and hard to explain scientifically.

"No one has been to interstellar space before, and it's like travelling with guidebooks that are incomplete," he said in a statement. "Still uncertainty is part of exploration. We wouldn't go exploring if we knew exactly what we'd find."