Late last night, right before I turned off my computer, an email popped into my inbox from Carolyn Porco of the Cassini Imaging Team. It was truly historic news — a first for humanity. Voyager I, the Little Probe that Could, has officially crossed the heliopause and is now traversing interstellar space.
This morning I woke up, checked the news, and found that nerds all over the world are having a collective freakout. And why not? As Ms. Porco says, “Such an event happens for the first time in human history only once.” And we are living through it. This is really, really, really cool stuff!
A NASA artist’s conception of Voyager 1 passing into interstellar space. The brownish haze represents interstellar plasma, ionized gas thrown off by kazillions of stars.
As I wrote back in July, there’s been quite a bit of scientific head-scratching and debate over whether or not Voyager actually passed into interstellar space a year ago. Some indicators showed very strongly that it had, but other indicators gave either inconclusive evidence or evidence that no one knew how to interpret, because it was contrary to all expectations.
But an article published yesterday in the journal Science, by Don Gurnett and the plasma wave science team at the University of Iowa, has put the debate to rest. Here’s the very dry abstract, which somehow manages to suck all the amazement and joy out of the event. I’ll try to reinsert the coolness factor afterwards:
Launched over 35 years ago, Voyagers 1 and 2 are on an epic journey outward from the Sun to reach the boundary between the solar plasma and the much cooler interstellar medium. The boundary, called the heliopause, is expected to be marked by a large increase in plasma density, from about 0.002 cm−3 in the outer heliosphere, to about 0.1 cm−3 in the interstellar medium. On 9 April 2013, the Voyager 1 plasma wave instrument began detecting locally generated electron plasma oscillations at a frequency of about 2.6 kHz. This oscillation frequency corresponds to an electron density of about 0.08 cm−3, very close to the value expected in the interstellar medium. These and other observations provide strong evidence that Voyager 1 has crossed the heliopause into the nearby interstellar plasma.
For the “other observations,” you can check my earlier post, which explains it in much juicier terms. Here, I’m going to focus on the one observation that convinced the astronomy world that Elvis has indeed left the building.
By “solar plasma,” the authors mean the ionized gas that our Sun shoots out at sonic speeds in all directions. This gas is also called the solar wind, which works better for my visually-oriented mind. The solar wind creates a sphere around our solar system, where it pushes back against the far cooler and denser interstellar plasma. The sphere is called a heliosphere, and its turbulent outer rim — the area where the solar wind stutters to a halt due to the interstellar plasma pushing back against it — is called the heliosheath. The very edge of the heliosheath, where the solar wind no longer has any effect and interstellar plasma reigns supreme, is the heliopause. Crossing the heliopause means being officially in interstellar space.
So, here’s the cool part. The interstellar plasma is known to be much denser than our solar plasma, and as of yesterday, we know that Voyager 1 is in the dense stuff. But Voyager is old, and its plasma sensor isn’t working anymore. So how do we know it’s in dense plasma now? Because it has a plasma wave instrument on board, and that wave instrument detected something very, very different in April of this year. The plasma around the spacecraft suddenly began vibrating “like a violin string,” according to NASA. It was vibrating because our Sun blew out a coronal mass ejection — a giant plume of solar wind and magnetic fields — in March 2012. It took 13 months for this extra strong wind to reach Voyager’s position, and when it did, the plasma all around Voyager sang. And Voyager heard it.
The pitch of these wave oscillations allowed the plasma wave science team to determine the density of the plasma around Voyager. The result: it was more than 40 times denser than what Voyager had sailed through in the outer layers of the heliosphere. As of April 2013, Voyager was in interstellar space.
But wait, it gets cooler! Since the team now knew what to look for, they started combing through Voyager’s earlier plasma wave records, and found a fainter vibration event in October/November 2012.
When the team plotted these two events on a graph, the different oscillations indicated a less dense plasma in the earlier event. Not only that, but the “tone” of the oscillations rose at a steady level through both events, indicating a steadily increasing density of plasma as Voyager traveled.
With two points on a graph, the team could draw a line through them and trace backwards to the point where the plasma density would indicate that Voyager had crossed the heliopause. Their result: August 2012. And if you read my earlier post, you know that August 2012 is also the date when Voyager stopped detecting the solar wind.
NASA, bless its wonderful publicly-funded heart, has put a short video out showing these vibrations and how the graphing worked. Best of all, the video contains the actual recording that Voyager made of the plasma singing. It’s mind-boggling to listen to that and realize that we are hearing the sound of our Sun exciting ionized gas that is fifteen billion kilometers away.
Voyager is no longer a solar system probe. It is a starship. Long may it sail!