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Stamatios M. Krimigis of the Johns Hopkins University Applied Physics Laboratory is pictured with the Voyager spacecraft’s backup flight unit which was never used. (Jed Kirschbaum, Baltimore Sun/January 17, 2011)

So cool. Thanks to Michael Doyle for posting this on Facebook.

Last June, Krimigis’ team noticed that solar particles had stopped striking from behind, and started to hit their Voyager 1 instrument from the front. And they were striking at the same speed — 17 kilometers per second — that Voyager 1 was moving away from the sun, like bugs striking the windshield of a moving car.

It suggested the solar particles’ speed outward from the sun had fallen to zero. Scientists watched the data for six months and it didn’t change. “We’re pretty certain this is a steady-state condition,” Krimigis said.

Their instrument is still detecting a particle flow perpendicular to Voyager’s direction of travel, a mix of solar and interstellar particles. It’s not entirely clear what’s going on.

“All this stuff should have disappeared, and it’s still there,” he said. “It’s as if we’re in some vast region where the solar wind is kind of sloshing around, instead of being in true interstellar space where there is nothing like this.”

True interstellar space might remain years and billions of miles away, scientists say.

“Within the next decade we’ll be there for sure,” said Ness. Voyager will know because all the particle readings from the lab’s instrument will drop to zero, or nearly so.

Ness is confident the Voyager craft will make it. “It runs autonomously,” he said. “The question is, ‘Will NASA be listening?’ ”

Sometime around 2025, the two craft will fall silent. In 40,000 years, Voyager 1 will sail as Earth’s ambassador among the stars of the constellation Camelopardalis — the Giraffe — in the northern sky. Voyager 2 is headed for Sirius, the brightest star in the sky. It should arrive in 296,000 years.

Voyager 1 at edge of solar system APL instrument still running after 33 years