NASA’s Voyager spacecraft may be billions of kilometres away and over 40 years old, but they’re still making significant discoveries, as new research reveals.
A paper published today in the Astronomical Journal describes an entirely new form of electron burst, a discovery made possible by the intrepid Voyager probes. These bursts are happening in the interstellar medium, a region of space in which the density of matter is achingly thin. As the new paper points out, something funky is happening to cosmic ray electrons that are making their way through this remote area: They’re being reflected and boosted to extreme speeds by advancing shock waves produced by the Sun.
By itself, this process, in which shock waves push particles, is nothing new. What is new, however, is that these bursts of electrons are appearing far ahead of the advancing shock wave, and that it’s happening in a supposedly quiet region of space. The new paper was co-authored by astrophysicist Don Gurnett from Iowa University.
Launched in 1977, Voyager 1 and Voyager 2 have done tremendous work for king and country, and they’re still enabling meaningful scientific work after so many years. But instead of studying active volcanoes on Jupiter’s moon Io or taking glorious photos of Saturn’s rings, these probes are now studying the uncharted waters beyond the heliopause — the zone between the hot solar plasma and the cooler interstellar medium at the outer reaches of the solar system.
Voyager 1 is currently 22.6 billion kilometres away, and Voyager 2 is 17.9 billion kilometres away (the probes were launched within 16 days of one another, but they were sent on different trajectories during their respective sojourns through the solar system). Voyager 1 crossed the heliopause boundary in 2012, and Voyager 2 did the same in 2018. They’re currently travelling through a region referred to as the very local interstellar medium (VLISM), according to the study. The Voyager probes are the most distant human-made objects ever.
Some may quibble about the term “interstellar medium” and claim that the Voyager probes are still technically inside the solar system, but Gurnett is adamant that the Voyager probes are indeed travelling through interstellar space, which literally means the “medium between the stars,” as he explained by phone. “We won that argument,” said Gurnett, “but of course I’m biased.” The pressure of gas at the location of the Voyager probes, he said, is equal to the pressure of gas we would expect to see in interstellar space. To him, that means the probes are inside the interstellar medium.
Years ago, before the NASA probes entered this region of space, “we thought it might get downright boring, and that nothing changes out there,” said Gurnett. “But what we found is that it’s not quiet and quiescent at all — the interstellar medium has important things going on!”
As previous research showed, stellar shock waves are travelling into this region of space, the result of coronal mass ejections on the Sun. These highly energetic events propel hot gas and energy into space, hurtling them toward the heliopause and the interstellar medium at tremendous speeds. Even travelling over 1.6 million kilometres per hour, however, it takes more than a year for these shock waves to reach the heliopause and another half year to reach the Voyager probes, explained Gurnett. To get a sense of how far the probes are right now, it takes roughly 20 hours for a Voyager transmission — travelling at the speed of light — to reach Earth.
As the new paper describes, these shock waves are facilitating a previously unseen behaviour in the interstellar medium, namely bursts of electrons appearing far ahead of the advancing shock waves.
“The study is unique in that it looks at several large solar storms that punch through the bubble that the Sun carves out of the interstellar medium and extends far beyond Pluto,” Herbert Funsten, a space scientist at Los Alamos National Laboratory who’s not involved with the new study, explained in an email. “The Voyager spacecraft are in the interstellar medium and are therefore looking into the bubble — and the shocks that cross the bubble boundary — from the outside, providing a unique, quiet observation location that we cannot observe from inside the bubble.”
The Voyager probes detected these spurts of energy with onboard instruments designed to detect cosmic rays (NASA was thinking ahead, and this is exactly the sort of thing the probes were designed to do).
In terms of what’s happening, electrons in the VLISM are bouncing off and being redirected by magnetic field lines in the interstellar plasma, or ionised gas.
“Magnetic field lines in the interstellar medium are almost purely straight lines,” explained Gurnett. “We detected the electron bursts when the shock waves first touched the magnetic field lines running through the Voyager spacecraft — and that’s the mechanism. The shock wave just touches the magnetic field line, and there’s a jump at the shock, which reflects and energises a few of the cosmic ray electrons.”
Indeed, this interaction appears to accelerate the electrons, pushing them ahead of the advancing shock wave. The authors of the study refer to this phenomenon as “interstellar foreshocks.” As a result, the juiced-up electrons move around 670 times faster than the shock waves that originally pushed them toward the heliopause, which means they’re being accelerated to near relativistic speeds. Gurnett compared this phenomenon to a game of pingpong, in which the ball is the electron, and the shock in the magnetic field is the paddle.
Interestingly, the probes also detected the shock waves themselves, which appeared between 13 and 30 days after the electron spikes.
“This is analogous to seeing light reflected from the cloud of a far-away explosion, and then hearing the boom at a later time,” said Funsten. “The time that it takes to see the cloud and hear the boom provides important information about the properties of the interstellar medium and properties of the punch-through of the shock wave into the interstellar medium.”
Astronomers have described shock waves pushing electrons before, but those interactions were at the location of the shock wave. Here, the electron bursts are happening ahead of the shock, which hasn’t been seen before, said Gurnett.
“This is a brand-new mechanism — the shock accelerates the electrons,” he said. “But the shock hasn’t yet reached the spacecraft, so it’s a precursor, which we’re calling the foreshock.”
Funsten said these events are rare, but they’re providing “tantalising clues” about the effects of these shocks on the interstellar medium. However, “more data will be needed to better understand these results,” he said, including more data from Voyager 2, “which hasn’t been in the interstellar medium for long,” as well as NASA’s upcoming IMAP mission (Interstellar Mapping and Acceleration Probe), which is scheduled for launch in 2024.
The new paper could improve our understanding of the complex interactions between shock waves and cosmic radiation, not just in the outskirts of our neighbourhood but around other stars as well, including exploding stars. These findings could also shed new light on the kinds of exposures that astronauts should expect while working in space.
A note to the Voyager probes: Keep doing what you’re doing. You’re awesome.