Voyager 1 has also found oddities in the heliosheath's magnetic field.
"There are 'potholes' where the magnetic field almost goes away, and 'speed bumps' where it's stronger," Stone said.
Nobody knows why. "This kind of variation is not found in the solar wind itself," he said.
Leonard Burlaga of NASA's Goddard Space Flight Center in Greenbelt, Maryland, said, "The most recent surprise is that at times, trains of closely spaced and overlapping magnetic holes or magnetic humps are observed for days at a time, indicating a very 'rough road.'
"Neither the physical nature nor the 3-D geometry of these features is understood," Burlaga said by email.
That's not to say the heliosheath's "rough road" is actually shaking the probe.
"Only our sensitive instruments know that this is happening," Stone says.
Another surprise is that the termination shock (the boundary between the solar system proper and the heliosheath) doesn't appear to be a great source of cosmic rayswhich contradicts 30 years of scientific theory.
Most cosmic rays are extremely high-energy particles that bombard our solar system from interstellar space. The heliosheath deflects most of them harmlessly away from Earth.
But not all of the rays appear to come from outside our system.
The inner edge of the heliosheath seems to produce low-energy cosmic rays when it traps interstellar particles between magnetic boundaries and bounces them ever faster, back and forthlike a table tennis paddle hitting a bouncing Ping-Pong ball ever closer to the table.
Scientists had been expecting Voyager 1 to pass through the source of these particles when it first crossed the termination shock. But the spacecraft didn't encounter any more of them there than it had been seeing all along.
"So we didn't find the source," Stone said. "It's somewhere else."
Another surprise appears to be looming from Voyager 2, which is already seeing signs of the heliosheath, even though the craft is about 10 percent closer to the sun than Voyager 1 was when it started encountering the same signs.
This means that the heliosheath appears to press inward in the direction in which Voyager 2 is headed (the two probes are on slightly different paths).
"That's telling us something about what's outside [our solar system]," Stone said.
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