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MIT Technology Review

Five things we’ve learned since Voyager 2 left the solar system

About 41 years after launch, the NASA spacecraft joined its twin in leaving the last edges of the solar system’s borders.

November 4, 2019
The heliopauseThe heliopause
The heliopause
NASA/Goddard Space Flight Center/CI Lab

One year ago, NASA’s Voyager 2 probe became just the second human-made object in history to exit the solar system and officially enter interstellar space. Voyager 2 was launched on August 20, 1977—16 days before its twin, Voyager 1, which exited the solar system’s northern hemisphere in 2012 . Voyager 2 was sent on a longer journey that allowed it to make encounters with Uranus and Neptune, and to this day it’s the only spacecraft to have visited these planets up close. It then made for the southern hemisphere of the heliosphere (the outermost region of the solar system, sometimes referred to as “the bubble”), straight for interstellar space.  

On November 5, 2018, Voyager 2 officially left the solar system as it crossed the heliopause, the boundary that marks the end of the heliosphere and the beginning of interstellar space. This happened 119 astronomical units from the sun (one AU is 93 million miles or 149.6 million kilometers, roughly the distance between the sun and Earth).

The spacecraft was able to analyse the makeup of solar winds, the composition and behavior of plasma particles, the interaction of cosmic rays, the structure and direction of magnetic fields, and other traits that define the edges of the solar system. Today, scientists published a bevy of papers in Nature Astronomy that detail the results of what Voyager 2 observed on its way out of the solar system. Here are the five biggest takeaways.

1. The bubble is leaking—both ways.

Voyager 2’s exit from the bubble was not without surprises. According to the data, the bubble was “very leaky,” says Stamatios Krimigis of Johns Hopkins University, the lead author of one of the new papers. Material from the solar bubble was discovered in interstellar space. 

Voyager 1 had actually found signs of a leaky bubble as well. In that instance, however, interstellar material was found streaming into the bubble––the opposite of what Voyager 2 discovered, says Edward Stone of Caltech, the lead author of a different paper. The new findings confirm that the leakiness of the heliopause, spotted in  two very different parts of the heliosphere, is not a rare characteristic of the bubble, although there is still no real explanation for what’s causing it.  

2. The boundary of the bubble is more uniform than we thought.

Before the Voyager missions, scientists predicted that the solar bubble just sort of dissolved into interstellar space as you ventured farther and farther from the sun. Voyager 2 seems to confirm that “in fact, there’s a very very sharp boundary there,” says Donald Gurnett of the University of Iowa, the lead author of this paper. Voyager 2’s plasma wave instrument ended up measuring plasma densities that were very much on par with what Voyager 1 detected. Because solar plasma is so hot (about 1 million °C), and interstellar plasma is incredibly cold (just 10,000 °C), the density of plasma jumps up by a factor between 20 and 50 as you cross the border. “That’s a characteristic of fluids, which oftentimes form very sharp boundaries,” says Gurnett. 

Krimigis was especially surprised that both Voyagers crossed the heliopause at the same relative distances (121 AU and 119 AU, respectively). Previous models heavily predicted that heightened solar activity during Voyager 1’s crossing in 2012 should have pushed the bubble’s boundary farther out. A period of low solar activity should have pulled the heliopause back a bit during Voyager 2’s crossing last year. The fact that both spacecraft left the solar system at pretty much the same distance, at two very different locations, is a source of confusion at the moment.

3. The makeup of the heliopause itself can vary by location.

Voyager 2 also made some observations that don’t square up with a sharp boundary—at least not what we’d expect. The biggest of these is the magnetic field measurements inside and outside the bubble. Astronomers expected the direction of the magnetic field would be very different between the two.  Yet when Voyager 2 crossed this thin surface, “there was essentially no change” in the direction of the field—something Voyager 1 observed as well, says Leonard Burlaga of NASA’s Goddard Space Flight Center, lead author for this paper. At the same time, the magnetic field observations on Voyager 2 suggest it found a thinner and simpler heliopause, filled with less energetic particles, than what Voyager 1 crossed. Again, all this data taken together raises more questions than it can answer.

4. The sun’s influence goes beyond the solar system.

The sun consistently spews out shock waves of plasma called coronal mass ejections (CMEs), which help shape the rest of the solar system. Turns out the sun’s impact goes beyond its own borders. The new Voyager 2 data, like the Voyager 1 data before it, shows how CMEs propagate past the heliopause and lower the amount of cosmic rays beyond the bubble. “This is somewhat similar to what you might find out in the galaxy,”  says Gurnett. Supernovae send shock waves out into the galaxy as well, stirring the interstellar medium, albeit at a much more intense scale than CMEs. “Even the formation of the solar system, most astronomers believe, was triggered by an interstellar shock wave from a supernova,” he says.

If we think about the potential for cosmic rays to promote biological mutations in life on Earth, these findings lend support to the idea that the sun could also have an influence on the evolution of living things on extraterrestrial worlds, in this planetary system and elsewhere. 

5. This was the Voyager program’s final major milestone. 

“When the two Voyagers were launched, the space age was only 20 years old,” says Stone. “It was hard to know at that time that anything could last 40 years.”

Still, the observations of the heliopause really are part of the last hurrah for both spacecraft. Each probe is powered by radioisotopic thermoelectric generators heated by plutonium-238. That material is undergoing natural decay. “We know that somehow, in another five years or so, we may not have enough power to have any scientific instruments on any longer,” says Stone. 

The two missions will continue to learn how the sun’s heliosphere interacts with the interstellar medium and give us clues about other star systems.“We believe every star has these features,” says Stone. “What we learn about this heliosphere will help us learn more about the astrospheres of other stars.” 

Though NASA continues to monitor, communicate with, and collect data from both Voyager probes, converting this data into useful scientific insights is largely the responsibility of scientists based at different institutions throughout the US. There are currently no plans for a successor to the Voyager program (the only other spacecraft headed to such distances, New Horizons, will run out of power at 90 AU), but the success of the missions and the questions they raise will undoubtedly inspire these scientists and engineers to come up with new proposals to study the heliosphere and beyond.