Voyager 2 unravels mysteries at solar system's edge

What a long, strange, enlightening trip it's been for NASA's Voyager 2. Oh solar system, we thought we knew you, but Voyager 2's findings make us revise our visions. (Yes, we're squashed.)

Voyager 2 has been in space for 30 years. It has sent back data that show exotic particles from outside the solar system dominate the outer edge of our solar system. That means it's much more complex out there than we thought.

Last August, Voyager 2 reached the termination shock (83 times farther than the distance between Earth and the sun). That's where the solar wind from our sun slows to subsonic speeds and interacts with and affects the interstellar gas. The spacecraft's mission is to travel toward the outer limits of the heliosphere. That's a bubble (see photo) in space created by the solar wind's multi-directional flow.

Surprisingly, the temperatures at the termination shock are lower than our theories posited. There are also high-velocity particles out there that our models didn't foresee.

"This was totally unexpected," says Stamatios Krimigis of The Johns Hopkins University Applied Physics Laboratory (APL) and principal investigator for Voyager 2's Low-Energy Charged Particles (LECP) instrument, which detected the very highest energy particles. "The environment is totally unlike what the models predicted."

We thought that when the solar wind suddenly slows down at the termination shock, from 217 miles per second to 81 miles per second, the huge amount of energy released heated up the ions and electrons in the solar wind. Not so.

"The temperature was too low to account for the energy loss," says APL's Rob Decker. "We had to ask, where was that energy going?"

They now think most of that energy accelerates particles -- dubbed "pick-up ions" because they were picked up by the solar wind -- from outside the solar system that migrated into the heliosphere.

The heliosheath, beyond the termination shock, is where the slowed solar wind shifts from the interstellar medium and shapes our bullet-like heliosphere.

"Once they are beyond the termination shock, the pick-up ions affect how that medium behaves," says Decker. "They are carrying a lot of energy and therefore play a large role in the dynamics of the flowing plasma, modifying the heliosheath's width and its magnetic field structure, for example. But that's something we're still trying to understand."

Voyager 1 and Voyager 2 crossed the termination shock 10 billion miles apart. Voyager 1 was above and Voyager 2 was below. Researchers thought the environments would be similar. Again, not so. Voyager 2 "sees" much more variability than Voyager 1.

By Jess Zielinski
Photo: NASA/JPL

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