Published online 10 December 2007 | Nature | doi:10.1038/news.2007.365

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30-year-mission crosses the boundary out of the solar system.

Voyager 2 has followed Voyager 1 through the termination shock boundary. NASA

SAN FRANCISCO – Like a soft orange that fell just a bit too hard from its tree, the giant bubble that protects the solar system from interstellar space is squashed on one side, new data show.

On 30 August, NASA’s Voyager 2 spacecraft — which has been sailing through space since 1977 — crossed the ‘termination shock’, the boundary between the bubble in space dominated by the solar wind coming from the Sun and the transition region beyond that lies between Earth and interstellar space.

Voyager 2’s twin, Voyager 1, crossed this same boundary in December 2004. But Voyager 2 did it while almost 1 billion miles closer to the Sun, suggesting that something — such as an interstellar magnetic field — is compressing the bubble of the solar wind on that side. The twin Voyagers headed out of the solar system in different directions, with Voyager 1 taking a northern path and Voyager 2 a southern one.

“Now both spacecraft are in the final frontier of the solar system,” says project scientist Edward Stone, of the California Institute of Technology in Pasadena. “We’ve reached another major milestone in our 30 years of discovery.” He and other Voyager scientists presented their findings on Monday at a meeting of the American Geophysical Union in San Francisco.

More than one instrument aboard Voyager 2 captured its transition out of the solar wind bubble, across the termination shock. In particular, key data came from an instrument that measures the velocity, temperature and density of the solar wind. When Voyager 1 crossed the same boundary back in 2004, its identical instrument wasn’t working, leading to much confusion and several years of debate over when exactly it had made the crossing.1,2

But because its instruments were functioning correctly this August, Voyager 2 actually captured the crossing at least five times, as the termination shock washed back and forth over the spacecraft like a wave on a beach.

Surprisingly, the charged particles on the other side were much cooler than thought, says John Richardson of the Massachusetts Institute of Technology in Cambridge, Massachusetts. Researchers had expected the particles to be on the order of 1 million degrees, but instead they were a mere 100,000 to 200,000 degrees, he says. Some of that energy may have been dissipated by accelerating other speeding particles known as cosmic rays, he says.

Voyager scientists intend to keep taking data as long as NASA keeps funding the missions. Stone says he expects that within a decade, both will move out of the transition region and into true interstellar space — becoming the first manmade objects to fully exit the solar system. Radioactive generators aboard each spacecraft, powering their electrical systems, may allow them to transmit through the transition and beyond.

Voyager 1 is currently nearly 10 billion miles from the Sun, travelling nearly a million miles a day. Voyager 2 is nearly 8 billion miles away, and moving just slightly slower.

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Next June, NASA plans to launch its orbiting Interstellar Boundary Explorer mission, which will study particles coming from the termination shock. It would, says Stone, provide a three-dimensional picture of this region to help flesh out the two data points the Voyagers have provided so far.