Not content with simply being the man-made object to travel farthest from Earth, NASA’s Voyager 1 spacecraft recently entered a bizarre new region at the solar system’s edge that has physicists baffled. Their theories don’t predict anything like it.
Launched 36 years ago, Voyager 1 and its twin Voyager 2 made an unprecedented tour of the outer planets, returning spectacular data from their journey. The first Voyager sped out of the solar system in 1980 and it has since been edging closer and closer to interstellar space. The probe is currently out more than 120 times the distance between the Earth and the sun.
Scientists initially thought that Voyager’s transition into this new realm, where effects from the rest of the galaxy become more pronounced, would be gradual and unexciting. But it’s proven to be far more complicated than anything researchers had imagined, with the spacecraft now encountering a strange region that scientists are struggling to make sense of.
“The models that have been thought to predict what should happen are all incorrect,” said physicist Stamatios Krimigis of the Johns Hopkins University Applied Physics Laboratory, who is lead author of one of three new papers on Voyager appearing in Science on June 27. “We essentially have absolutely no reliable roadmap of what to expect at this point.”
The sun produces a plasma of charged particles called the solar wind, which get blown supersonically from its atmosphere at more than 1 million km/h. Some of these ions are thrown outward by as much as 10 percent the speed of light. These particles also carry the solar magnetic field.
Eventually, this wind is thought to hit the interstellar medium – a completely different flow of particles expelled from the deadly explosions of massive stars. The extremely energetic ions created in these bursts are known as galactic cosmic rays and they are mostly blocked from coming into the solar system by the solar wind. The galaxy also has its own magnetic field, which is thought to be at a significant angle to the sun’s field.
Researchers know that Voyager 1 entered the edge of the solar wind in 2003, when the spacecraft’s instruments indicated that particles around it were moving subsonically, having slowed down after traveling far from the sun. Then, about a year ago, everything got really quiet around the probe. Voyager 1’s instruments indicated at the solar wind suddenly dropped by a factor of 1,000, to the point where it was virtually undetectable. This transition happened extremely fast, taking roughly a few days.
At the same time, the measurements of galactic cosmic rays increased significantly, which would be “just as we expected if we were outside the solar wind,” said physicist Ed Stone of Caltech, Voyager’s project scientist and lead author of one of the Science papers. It looked almost as if Voyager 1 had left the sun’s influence.
So what’s the problem? Well, if the solar wind was completely gone, galactic cosmic rays should be streaming in from all directions. Instead, Voyager found them coming preferentially from one direction. Furthermore, even though the solar particles had dropped off, the probe hasn’t measured any real change in the magnetic fields around it. That’s hard to explain because the galaxy’s magnetic field is thought to be inclined 60 degrees from the sun’s field.
No one is entirely sure what’s going on.
“It’s a huge surprise,” said astronomer Merav Opher of Boston University, who was not involved in the work. While the new observations are fascinating, they are likely something that theorists will debate about for some time, she added.
“In some sense we have touched the intergalactic medium,” Opher said, “but we’re still inside the sun’s house.”
Extending this analogy, it’s almost as if Voyager thought it was going outside but instead found itself standing in the foyer of the sun’s home with an open door that allows wind to blow in from the galaxy. Not only were scientists not expecting this foyer to exist, they have no idea how long the probe will stay inside of it. Stone speculated that the probe could travel some months or years before it reaches interstellar space.
“But it could happen any day,” he added. “We don’t have a model to tell us that.” Even then, Stone said, Voyager would not have really left the solar system but merely the region where the solar wind dominates.
For his part, Krimigis didn’t even want to speculate on what Voyager might encounter next because theorists’ models have so far not worked extremely well.
“I’m convinced that nature is far more imaginative than we are,” he said.