Hurtling through space 31 years after its launch, the Voyager 2 spacecraft has sent back the most detailed view yet of the shock wave that marks the thinning of the solar wind, the charged particles streaming from the sun.

Researchers say the crossing confirms that the heliosphere—the region swept out by the solar wind—is actually lopsided, perhaps due to a tilted magnetic field in local interstellar space.

The shock wave, or heliospheric termination shock, occurs when the supersonic wind thins to the point that it can no longer rebuff the denser haze of charged particles flowing through interstellar space. Instead, the solar wind suddenly collapses in on itself.

Researchers say the phenomenon is sort of like the edge of a stream of tap water after it hits the sink [see image]. Solar wind is swept along by the sun's magnetic field, which means it cascades like a fluid instead of crashing like billiard balls.

Data from Voyager 2, described in a series of papers today in Nature, show that the craft entered the termination shock on August 31, 2007, at a distance from the sun of about eight billion miles (13 billion kilometers) and crossed it the next day.

That's 10 percent closer to the sun than when the craft's sister ship, Voyager 1, passed through the same shock wave in late 2004 heading outbound from the solar system in a different direction.

That far from the sun, the density of solar wind is, at most, a couple of protons and electrons per gallon, astrophysicist J.R. Jokipii of the University of Arizona Lunar and Planetary Laboratory in Tucson says. "It's almost impossible to measure. You have to give it to these experimenters," he says.

Voyager 2 will now follow its twin into the heliosheath, the region of slower-moving wind beyond the termination shock.

Besides confirming earlier research that hinted at the lopsided heliosphere, the crossing provides new details, including the energy and speed of the solar wind, that Voyager 1 could not pick up because its plasma detection instrument had stopped functioning.

According to the new data, the wind downstream of the shock was cooler and faster moving than researchers had anticipated. The interpretation, says Jokippi, who wrote an editorial accompanying the Nature reports, is that the solar wind is imparting energy to neutral atoms from the interstellar gas and causing them to ionize.

These "pickup" ions are then accelerated to speeds of hundreds of miles (kilometers) per second, exerting a strong effect on the structure of the shock, he says.

The twin Voyager craft set out for deep space in 1977 to study Jupiter and Saturn, but after their primary mission was completed, they kept on going. In 10 to 20 years after reaching the termination shock, NASA expects the craft to cross the heliopause, the outer edge of the heliosheath.

That would mean they have exited the solar system and entered the interstellar medium. NASA engineers estimate that both probes' plutonium power packs have the potential to keep them broadcasting data until 2025.

If we're lucky, Jokippi says, they'll let us know what they find.