For months, researchers have been arguing about just where in the universe the Voyager 1 spacecraft is. They know its position with precision: the NASA mission, launched on September 5, 1977, is now more than 11 billion miles away. But is Voyager still on home turf—within the bubble blown by a wind of fast-moving particles that emanate from our sun? Or has the spacecraft crossed over to a whole new domain, exploring the area between the stars, known as the interstellar medium? Scientists just haven’t been able to agree.
The debate can seem like astronomy semantics: whichever side of the boundary the spacecraft is on, it’s new territory to us. Does being on one side or the other matter?
Some researchers, including a Boston University astrophysicist, argued in a paper in August that Voyager 1 crossed out of the bubble that surrounds our solar system last summer. A model that the researchers developed explained some confusing and ambiguous measurements that the spacecraft had sent back, and suggested it had long since departed. NASA promptly released a statement saying that the paper represented the predictions of just a single model, and that other models suggested the spacecraft was still within the solar bubble.
But now, researchers finally appear to have reached a consensus. A study published in the journal Science on Thursday analyzes the density of electrons surrounding the spacecraft and the Voyager team now concur that the spacecraft has definitely passed beyond the influence of our sun and into the rest of the Milky Way.
“Now that we have new, key data, we believe this is humankind’s historic leap into interstellar space,” Ed Stone, Voyager project scientist said in a statement.
To understand a bit more about what that means, and why it matters, I spoke with Merav Opher, a guest scientist on the NASA mission and the researcher at BU whose model originally suggested that the spacecraft entered the rest of the Milky Way last summer.
Q: What tools does Voyager have to explore these regions of space?
A: One problem is the plasma instrument on Voyager 1 is not working. That instrument can measure the speed of the solar wind, the density, and the temperature. In the interstellar medium, the medium would be colder—much colder and much denser. This is what all the theories and models expect, but we don’t have this instrument working right now. What we do have is magnetic field data, and we do have the radio instrument, and we have the energetic-particle [detector] working.
Q: Without the instrument that measures the solar wind, what was the first evidence Voyager might have traveled beyond it?
A: We’ve been fiercely debating among us what’s happening. What happened is last year, last September, we started to see the Voyager data in a team meeting. ... The data that came from Voyager showed the magnetic field was dead east-west, no change—it’s been like that for years. But there was this event that started around the end of July and then August, from the instrument that measures energetic particles. It saw the particles going away. So there was some leakage; if you think [of the sun’s solar wind creating] a bubble, it’s like someone punched a hole in this bubble and the material from the sun was going away. And at the same time ... the instrument that measured the particles that come from the galaxy had a spike. It was really like: we are not outside the bubble, but somebody opened the window and part of our solar wind disappeared. And at the same time, we let the particles from the galaxy enter and then we closed the window and were back to normal.
A couple days later, the window opened again and particles disappeared again, even more dramatically, as if someone had opened the window even wider. Until August 25 , when the particles from the sun disappeared—they dropped their noise level, they’re gone. So there’s no more solar wind. So, O.K., we left, or something happened. We didn’t have the wind anymore.
There was a room full of people, and there was a vote at the end: Did we cross or didn’t we? The room was split. I don’t even remember what I thought—I changed my mind a couple of times during those days. I do remember that one of the scientists there told me as we were walking away, she said, “We are going to miss the boat; we are going to announce we didn’t cross and we might have crossed. This is a major milestone.”
Q: How did you make up your mind that Voyager had crossed?
A: We put this paper out that explained it, and it’s based on a simulation a Maryland team did, using data from my models. We think we have a consistent picture to describe the magnetic field and the energetic particles, but the Voyager team was uncomfortable with that.
Q: What does this new paper add?
A: It does confirm [Voyager has left]. What this paper is saying is that based on the radio observations ... we were in interstellar space [last summer].
Q: Why does this matter?
A: Well, the most obvious reason is that this is the first man-made object that crossed to the other side, that went into the interstellar medium. We haven’t had anything that is ours [that far]. It’s like landing on the moon. I don’t know when we’ll have [an event] like that happen again.
We left our home; it’s major in that sense. And also in the scientific sense. They will be measuring right there, the medium between the stars, so this is also a first.
Q: Will it be exciting when Voyager 2 crosses this boundary? Will that crossing be more clear, since the spacecraft has more functioning instruments?
A: Yeah, definitely. But my guess is that like anything with Voyager, it’s going to contradict the models, and there will be lots of controversy: Why doesn’t it look like what we expected?